Democritus was a Greek philosopher who theorized that all matter consisted of tiny particles that he called atoms, a name derived from two Greek words meaning indivisible. He proposed the earliest views on the shapes and connectivity of atoms. He reasoned that the solidness of the material corresponded to the shape of the atoms involved. Using analogies from our sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts. Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets.

The physicist J.J Thompson, through his work on cathode rays in 1897, discovered the electron, and concluded that they were a component of every atom. Thus he overturned the belief that atoms are the indivisible, ultimate particles of matter. Thomson postulated that the low mass, negatively charged electrons were distributed throughout the atom, possibly rotating in rings, with their charge balanced by the presence of a uniform sea of positive charge. This later became known as the plum pudding model.

In 1909, Hans Geiger and Ernest Madsen, under the direction of physicis, bombarded a sheet of gold foil with alpha rays—by then known to be positively charged helium atoms—and discovered that a small percentage of these particles were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive charge of a heavy gold atom and most of its mass was concentrated in a nucleus at the center of the atom—the Rutherford model.

While experimenting with the products of radioactive decay, in 1913 radiochemist Fredrick Soddy discovered that there appeared to be more than one type of atom at each position on the periodic table. The term isotope was coined by Margaret Todd as a suitable name for different atoms that belong to the same element. J.J. Thomson created a technique for separating atom types through his work on ionized gases, which subsequently led to the discovery of stable isotopes.

Meanwhile, in 1913, physicist Niels Bohr suggested that the electrons were confined into clearly defined, quantized orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states. An electron must absorb or emit specific amounts of energy to transition between these fixed orbits. When the light from a heated material was passed through a prism, it produced a multi-colored spectrum. The appearance of fixed lines in this spectrum was successfully explained by these orbital transitions.

Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons. As the chemical properties of the elements were known to largely repeat themselves according to the periodic law, in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of electron shells about the nucleus.

The Stern - Gelach experiment of 1922 provided further evidence of the quantum nature of the atom. When a beam of silver atoms was passed through a specially shaped magnetic field, the beam was split based on the direction of an atom's angular momentum, or spin. As this direction is random, the beam could be expected to spread into a line. Instead, the beam was split into two parts, depending on whether the atomic spin was oriented up or down.

In 1924, Louis de Broglie proposed that all particles behave to an extent like waves. In 1926, Erwin Shrochdinger used this idea to develop a mathematical model of the atom that described the electrons as three-dimensional waveforms rather than point particles. A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at the same time; this became known as the uncertainty principle, formulated by Werner Heisenberg in 1926. In this concept, for a given accuracy in measuring a position one could only obtain a range of probable values for momentum, and vice versa. This model was able to explain observations of atomic behavior that previous models could not, such as certain structural and spectral patterns of atoms larger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described atomical mass zones around the nucleus where a given electron is most likely to be observed.

The development of the mass spectrometer allowed the exact mass of atoms to be measured. The device uses a magnet to bend the trajectory of a beam of ions, and the amount of deflection is determined by the ratio of an atom's mass to its charge. The chemist Francis William Aston used this instrument to show that isotopes had different masses. The atomic mass of these isotopes varied by integer amounts, called the whole number rule. The explanation for these different isotopes awaited the discovery of the neutron, a neutral-charged particle with a mass similar to the proton , by the physicist James Chadwick in 1932. Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus.

In 1938, the German chemist Otto Hahn, a student of Rutherford, directed neutrons onto uranium atoms expecting to get transuranium elements. Instead, his chemical experiments showed barium as a product. A year later, Lise Meitner and her nephew Otto Frisch verified that Hahn's result were the first experimental nuclear fission. In 1944, Hahn received the Nobel prize in chemistry. Despite Hahn's efforts, the contributions of Meitner and Frisch were not recognized.

In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies. Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks. Standard models of nuclear physics were developed that successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions.

Kyle Ingram

Examine the history of the development of the atom. We have already discussed the
major players in the development of the atom. Pick out at least 8 major players who
helped aid in the development of the concept of the atom. Then explain each
experiment in a concise explanation. Then explain how each one aided in the
development of the next or how their concept assisted in the further development of the current quantum model of the atom.

J.J. Thomson
Beginning in 1895 physicist J. J. Thomson theorized that cathode rays produced in Crookes' tubes must be composed of what he called "corpuscles", a single type of negatively charged particle. In 1897, applying his own vacuum technique to the study of these then-mysterious rays, Thomson made a convincing argument for composition based on sub-atomic particles, "this matter being the substance from which all the chemical elements are built up".
To make sense of this theory he proposed a "plum pudding model" of the atom, which was debated for several years and disproved by his former student, Ernest Rutherford. Thomson also showed a stream of channel rays could be separated into two or more parts through exposure to electrical and magnetic fields, leading eventually to the invention of the mass spectrograph and discovery of isotopes by another of Thomson's students, Francis W. Aston. Thomson won the Nobel Prize in Physics in 1906, and his son, physicist George Paget Thomson, won the same honor in 1937.
THOMSON’S FIRST CATHODE RAY EXPERIMENT
Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.
His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.
He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.
THOMSON’S CATHODE RAY SECOND EXPERIMENT
Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.
Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.
For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.
As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.
THOMSON’S THIRD EXPERIMENT
The third experiment was a brilliant piece of scientific deduction and shows how a series of experiments can gradually uncover truths.
Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis.
He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.
Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand time smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from with the atoms themselves, a very bold and innovative idea.
LATER DEVELOPMENTS
Thomson came up with the initial idea for the structure of the atom, postulating that it consisted of these negatively charged particles swimming in a sea of positive charge. His pupil, Rutherford, developed the idea and came up with the theory that the atom consisted of a positively charged nucleus surrounded by orbiting tiny negative particles, which he called electrons.
Quantum physics has shown things to be a little more complex than this but all quantum physicists owe their legacy to Thomson. Although atoms were known about, as apparently indivisible elementary particles, he was the first to postulate that they had a complicated internal structure.
Thomson’s greatest gift to physics was not his experiments, but the next generation of great scientists who studied under him, including Rutherford, Oppenheimer and Aston. These great minds were inspired by him, marking him out as one of the grandfathers of modern physics.





John Dalton
It was in the early 1800s that John Dalton, an observer of weather and discoverer of color blindness among other things, came up with his atomic theory. Let's set the stage for Dalton's work. Less than twenty years earlier, in the 1780's, Lavoisier ushered in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy. By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition ( also called the Law of Definite Proportions). In 1803 Dalton noted that oxygen and carbon combined to make two compounds. Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.
The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.
So that, briefly, is Dalton's theory. With modifications, it has stood up pretty well to the criteria that we talked about earlier. It did not convince everyone right away however. Although a number of chemists were quickly convinced of the truth of the theory, it took about a half century for the opposition to die down, or perhaps I should say die off.
Let me point out again the difference between a model of atoms and a theory of atoms. A model focuses on describing what the atoms are like, whereas the theory not only talks about what the atoms are like but how they interact with one another and so forth. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by what kind of element they were. We will use that model of an atom for now, but we will modify it considerably in a later lesson.

Ernest Rutherford
The Discovery of Radioactivity (Ernest Rutherford)- In 1899 Ernest Rutherford studied the absorption of radioactivity by thin sheets of metal foil and found two components: alpha (a) radiation, which is absorbed by a few thousandths of a centimeter of metal foil, and beta (b) radiation, which can pass through 100 times as much foil before it was absorbed. Shortly thereafter, a third form of radiation, named gamma (g) rays, was discovered that can penetrate as much as several centimeters of lead.
The Gold Foil Experiment (Ernest Rutherford)- Rutherford began his graduate work by studying the effect of x-rays on various materials. Shortly after the discovery of radioactivity, he turned to the study of the -particles emitted by uranium metal and its compounds.
Before he could study the effect of -particles on matter, Rutherford had to develop a way of counting individual -particles. He found that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an -particle. Rutherford and his assistant, Hans Geiger, would sit in the dark until his eyes became sensitive enough. They would then try to count the flashes of light given off by the ZnS screen. (It is not surprising that Geiger was motivated to develop the electronic radioactivity counter that carries his name.)
Rutherford found that a narrow beam of -particles was broadened when it passed through a thin film of mica or metal. He therefore had Geiger measure the angle through which these -particles were scattered by a thin piece of metal foil. Because it is unusually ductile, gold can be made into a foil that is only 0.00004 cm thick. When this foil was bombarded with -particles, Geiger found that the scattering was small, on the order of one degree.
These results were consistent with Rutherford's expectations. He knew that the -particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the -particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the -particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.
One day, Geiger suggested that a research project should be given to Ernest Marsden, who was working in Rutherford's laboratory. Rutherford responded, "Why not let him see whether any -particles can be scattered through a large angle?" When this experiment was done, Marsden found that a small fraction (perhaps 1 in 20,000) of the -particles were scattered through angles larger than 90o (see Figure 6.7a). Many years later, reflecting on his reaction to these results, Rutherford said: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you."
Rutherford concluded that there was only one way to explain these results. He assumed that the positive charge and the mass of an atom are concentrated in a small fraction of the total volume and then derived mathematical equations for the scattering that would occur. These equations predicted that the number of -particles scattered through a given angle should be proportional to the thickness of the foil and the square of the charge on the nucleus, and inversely proportional to the velocity with which the -particles moved raised to the fourth power. In a series of experiments, Geiger and Marsden verified each of these predictions.
When he published the results of these experiments in 1911, Rutherford proposed a model for the structure of the atom that is still accepted today. He concluded that all of the positive charge and essentially all of the mass of the atom is concentrated in an infinitesimally small fraction of the total volume of the atom, which he called the nucleus (from the Latin for little nut).
Most of the -particles were able to pass through the gold foil without encountering anything large enough to significantly deflect their path. A small fraction of the -particles came close to the nucleus of a gold atom as they passed through the foil. When this happened, the force of repulsion between the positively charged -particle and the nucleus deflected the -particle by a small angle. Occasionally, an -particle traveled along a path that would eventually lead to a direct collision with the nucleus of one of the 2000 or so atoms it had to pass through. When this happened, repulsion between the nucleus and the -particle deflected the -particle through an angle of 90o or more.
By carefully measuring the fraction of the -particles deflected through large angles, Rutherford was able to estimate the size of the nucleus. According to his calculations, the radius of the nucleus is at least 10,000 times smaller than the radius of the atom. The vast majority of the volume of an atom is therefore empty space.
Naming the Proton (Ernest Rutherford)- Shortly after the World War I, in 1920, Rutherford proposed the name proton for the positively charged particles in the nucleus of an atom.
Proposing the Neutron (Ernest Rutherford)- At the same time that Rutherford proposed the name proton for the positively charged particle in the nucleus of an atom, he proposed that the nucleus also contained a neutral particle, eventually named the neutron. It was not until 1932, however, that James Chadwick was able to prove that these neutral particles exist.


Pierre and Marie Curie
Pierre and Marie Curie are best known for their pioneering work in the study of radioactivity, which led to their discovery in 1898 of the elements radium and polonium. He discovered the phenomenon of piezoelectricity, whereby changes in the volume of certain crystals excite small electric potentials. Along with work on crystal symmetry, Pierre Curie studied the magnetic properties of materials and constructed a torsion balance with a tolerance of 0.01 mg. He discovered that the magnetic susceptibility of paramagnetic materials is inversely proportional to the absolute temperature (Weiss-Curie's law) and that there exists a critical temperature above which the magnetic properties disappear (Curie temperature). Marie measured the strength of the radiation emitted from uranium compounds and found it proportional to the uranium content, constant over a long period of time, and uninfluenced by external conditions. She detected a similar immutable radiation in the compounds of thorium. While checking these results, she made the unexpected discovery that uranium pitchblende and the mineral chalcolite emitted about four times as much radiation as could be expected from their uranium content. In 1898 she therefore drew the revolutionary conclusion that pitchblende contains a small amount of an unknown radiating element.

Niels Bohr
A model of the atom, first described by Niels Bohr, that explains the emission and absorption of radiation as transitions between stationary electronic states in which the electron orbits the nucleus at a definite distance. The Bohr model violates the Heisenberg uncertainty principle, since it postulates definite paths and momenta for electrons as they move around the nucleus. Modern theories usually use atomic orbitals to describe the behavior of electrons in atoms. The Bohr Model is probably familar as the "planetary model" of the atom illustrated in the adjacent figure that, for example, is used as a symbol for atomic energy (a bit of a misnomer, since the energy in "atomic energy" is actually the energy of the nucleus, rather than the entire atom). In the Bohr Model the neutrons and protons (symbolized by red and blue balls in the adjacent image) occupy a dense central region called the nucleus, and the electrons orbit the nucleus much like planets orbiting the Sun (but the orbits are not confined to a plane as is approximately true in the Solar System). The adjacent image is not to scale since in the realistic case the radius of the nucleus is about 100,000 times smaller than the radius of the entire atom, and as far as we can tell electrons are point particles without a physical extent. This similarity between a planetary model and the Bohr Model of the atom ultimately arises because the attractive gravitational force in a solar system and the attractive Coulomb (electrical) force between the positively charged nucleus and the negatively charged electrons in an atom are mathematically of the same form. (The form is the same, but the intrinsic strength of the Coulomb interaction is much larger than that of the gravitational interaction; in addition, there are positive and negative electrical charges so the Coulomb interaction can be either attractive or repulsive, but gravitation is always attractive in our present Universe.)


Henry Mosley
Before Moseley and his created law, atomic numbers had been thought of as a semi-arbitrary ordering number, vaguely increasing with atomic weight but not strictly defined by it. Moseley redefined the idea of atomic numbers from its previous status as an ad hoc numerical tag to help sorting the elements, in particular in the Periodic Table, into a real and objective whole-number quantity that was experimentally measurable. Furthermore, as noted by Bohr, Moseley's law provided a reasonably complete experimental set of data that supported the conception by Ernest Rutherford and Antonius Van den Broek of the atom, with a positively-charged nucleus surrounded by negatively-charged electrons in which the atomic number is understood to be the exactly physical number of positive charges in the central atomic nuclei of the elements. Simple modification of Rydberg's and Bohr's formulas were found to give theoretical justification for Moseley's empirically-derived law for determining atomic numbers. The atomic number is one of the most important things of an atom. It has helped us understand so much about current atoms. His discovery was a very important and useful discovery.

James Chadwick
In 1932, English Physicist James Chadwick, after a decade-long struggle to track down this tricky particle (all the methods available at the time were used only to detect charged particles), performed tests on a new type of radiation which had been baffling physicists for years, and which had previously been mistaken for “gamma rays” (a form of radiation consisting of high-energy photons).
The test, to simplify as much as possible, went like this:
A sample of Beryllium was bombarded with alpha particles (another type of naturally occurring radiation which are technically just ionized helium nuclei), which causes it to emit this mysterious radiation. It was then discovered by Irene Joliot-Curie (daughter of Marie and Pierre Curie) and her husband Frederic Joliot-Curie that this radiation, upon striking a proton-rich surface (paraffin was the preferred example), would discharge some of the protons, which could then be detected using a Geiger counter (a device that measures radiation).
This was the premise, and from here, Chadwick simply had to play detective and put all the pieces of the puzzle together. For instance, he could tell that the mysterious radiation in question was neutral due to the fact that it was not affected by proximity to a magnetic field, and, unlike standard gamma radiation, did not invoke the photoelectric effect (when photons, such as gamma rays, strike certain surfaces, they discharge electrons, which can be simply measured), but rather discharged protons, which meant that the particles had to be more massive than previously expected.
In the end, Chadwick finally solved the puzzle and officially discovered the neutron in 1932, thus vindicating Rutherford’s original theory (not that Rutherford needed any more accomplishments in his already prolific scientific career). For his efforts, Chadwick received the Nobel Prize in 1935.




Democritus
The ancient philosopher, Heraclitus, maintained that everything is in a state of flux. Nothing escapes change of some sort (it is impossible to step into the same river). On the other hand, Parmenides argued that everything is what it is, so that it cannot become what is not (change is impossible because a substance would have to transition through nothing to become something else, which is a logical contradiction). Thus, change is incompatible with being so that only the permanent aspects of the Universe could be considered real.
An ingenious escape was proposed in the fifth century B.C. by Democritus. He hypothesized that all matter (plus space and time) is composed of tiny indestructible units, called atoms. This idea seems motivated by the question of how finely one can go on cutting up matter. While Democritus performed no experiments and had only the flimsiest evidence for postulating the existence of atoms, his theory was kept alive by the Roman poet Lucretius which survived the Dark Ages to be discovered in 1417. The atoms in Democritus theory themselves remain unchanged, but move about in space to combine in various ways to form all macroscopic objects. Early atomic theory stated that the characteristics of an object are determined by the shape of its atoms. So, for example, sweet things are made of smooth atoms, bitter things are made of sharp atoms.
In this manner permanence and flux are reconciled and the field of atomic physics was born. Although Democritus' ideas were to solve a philosophical dilemma, the fact that there is some underlying, elemental substance to the Universe is a primary driver in modern physics, the search for the ultimate subatomic particle.

Dustin George

Democritus:
A Greek philosopher who theorized that all matter consisted of tiny particles that he called atoms, a name derived from two Greek words meaning indivisible. He proposed the earliest views on the shapes and connectivity of atoms. He reasoned that the solidness of the material corresponded to the shape of the atoms involved. Thus, iron atoms are solid and strong with hooks that lock them into a solid; water atoms are smooth and slippery; salt atoms, because of their taste, are sharp and pointed; and air atoms are light and whirling, pervading all other materials. Democritus was the main proponent of this view. Using analogies from our sense experiences, he gave a picture or an image of an atom that distinguished them from each other by their shape, their size, and the arrangement of their parts. Moreover, connections were explained by material links in which single atoms were supplied with attachments: some with hooks and eyes others with balls and sockets.
His contributions broke open the world of the atom.

Joseph Proust:
A French chemist and apothecary who came up with the Law of Definite Proportions which states that a chemical compound always contains exactly the same proportion of elements by mass. Proust studied copper carbonate, the two tin oxides,and the two iron sulfides to prove this law. He did this by making artificial copper carbonate and comparing it to natural copper carbonate. With this he showed that each had the same proportion of weights between the three elements involved (Cu, C, O). Between the two types of the other compounds, Proust showed that no intermediate indeterminate compounds exist between them.
His law was adapted as part of Dalton's Theory.

John Dalton:
An English chemist, meteorologist, and physicist who theorized that:
1.Elements are made of tiny particles called atoms.
2.The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative atomic weights.
3.All atoms of a given element are identical.
4.Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms.
5.Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.
He reached this theory after a series of experiments that included the analysis of a number of pure compounds and the study of gases.
John Dalton contributed an early understanding of the behavior of atoms that has had to be refined a little bit sense his stating it.

J.J. Thomson:
A British physicist who discovered the electron in 1897, the first subatomic particle discovered. Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field . By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was able to measure the particle's mass. This showed that cathode rays were matter, but he found that the particles were about 2000 times lighter than the mass of the lightest atom, hydrogen. He concluded that the rays were composed of very light negatively charged particles which he called "corpuscles".
His experiments were the first to introduce a subatomic particle, opening the way for more.

Ernest Rutherford :
A New Zealand-British chemist and physicist who discovered Protons. He discovered this through an experiment where a beam of alpha particles, generated by the radioactive decay of radium, was directed normally onto a sheet of very thin gold foil. The gold foil was surrounded by a circular sheet of zinc sulfide (ZnS) which was used as a detector: the ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees; measuring the pattern of scattered particles was expected to provide information about the distribution of charge within the atom. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. Through this experiment he was able to say that there had to be a nucleus where a large amount of the atom's charge and mass is concentrated into a very physically-small (as compared with the size of the atom) region. He also theorized about the existence of neutrons.
His discovery of the proton opened the way for a neutrally charged particle to balance it.

James Chadwick:
An Englishman who discovered the neutron. His experiment was based on the 1930 discovery that Beryllium, when bombarded by alpha particles, emitted a very energetic stream of radiation. This stream was originally thought to be gamma radiation. However, further investigations into the properties of the radiation revealed contradictory results. Like gamma rays, these rays were extremely penetrating and since they were not deflected upon passing through a magnetic field, neutral. However, unlike gamma rays, these rays did not discharge charged electroscopes (the photoelectric effect). Irene Curie and her husband discovered that when a beam of this radiation hit a substance rich in protons, for example paraffin, protons were knocked loose which could be easily detected by a Geiger counter.
In 1932, Chadwick proposed that this particle was Rutherford's neutron. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton.
His discovery completed the knowledge of the main subatomic particles in the atom today.

Niels Bohr:
A Danish physicist who made fundamental contributions to understanding atomic structure, he is most credited with the Bohr Atomic Model. He published this model introducing the theory of electrons traveling in orbits around the atom's nucleus, with the chemical properties of the element being largely determined by the number of electrons in the outer orbits. He reached this model by adapting Rutherford's nuclear structure to Max Planck's quantum theory.
His model of the atom is still used as a basic model today and started the process toward the Quantum Model.

Carl David Anderson :
An American physicist who discovered the positron. He began investigations into cosmic rays during the course of which he encountered unexpected particle tracks in his cloud chamber photographs that he correctly interpreted as having been created by a particle with the same mass as the electron, but with opposite electrical charge. This discovery, announced in 1932 and later confirmed by others, validated Paul Dirac's theoretical prediction of the existence of the positron. Anderson obtained the first direct proof that positrons existed by shooting gamma rays produced by the natural radioactive nuclide ThC'' (208Tl) into other materials, resulting in creation of positron-electron pairs.
His contributions proved the existance of antimatter and adds to the understanding of how atoms behave.

Lindsey Kunze

Examine the history of the development of the atom. We have already discussed the major players in the development of the atom. Pick out at least 8 major players who helped aid in the development of the concept of the atom. Then explain each experiment in a concise explanation. Then explain how each one aided in the development of the next or how their concept assisted in the further development of the current quantum model of the atom.

Democritus: He opposed the atomic theory, but in doing so he summarized its principal doctrines. Thus he attributed to Leucippus the ideas that the atoms are "infinite in number and imperceptible because of the minuteness of their size. They move about in empty space (for there is empty space) and by joining together they produce perceptible objects, which are destroyed when the atoms separate. according to Democritus, the atom was the irreducibly minimal quantity of matter. The concept of the infinite divisibility of matter was flatly contradicted by the atomic theory, since within the interior of the atom there could be no physical parts or unoccupied space. Every atom was exactly like every other atom as a piece of corporeal stuff. But the atoms differed in shape, and since their contours showed an infinite variety and could be oriented in any direction and arranged in any order, the atoms could enter into countless combinations. In their solid interior there was no motion, while they themselves could move about in empty space. Thus, for the atomic theory, the physical universe had two basic ingredients: impenetrable atoms and penetrable space. For Democritus, space was infinite in extent, and the atoms were infinite in number. By their very nature the atoms were endowed with a motion that was eternal and not initiated by any outside force. Since the atoms were not created at any time in the past and would never disintegrate at any time in the future, the total quantity of matter in the universe remained constant: this fundamental principle of Democritus's atomic theory implies the conservation of matter, the sum total of which in the universe neither increases nor diminishes. Though Democritus's conception of the atom has been modified in several essential respects in modern times, his atomic theory remains the foundation of modern science. According to Democritus, the atom was the irreducibly minimal quantity of matter. The concept of the infinite divisibility of matter was flatly contradicted by the atomic theory, since within the interior of the atom there could be no physical parts or unoccupied space. Every atom was exactly like every other atom as a piece of corporeal stuff. But the atoms differed in shape, and since their contours showed an infinite variety and could be oriented in any direction and arranged in any order, the atoms could enter into countless combinations. In their solid interior there was no motion, while they themselves could move about in empty space. Thus, for the atomic theory, the physical universe had two basic ingredients: impenetrable atoms and penetrable space. For Democritus, space was infinite in extent, and the atoms were infinite in number. By their very nature the atoms were endowed with a motion that was eternal and not initiated by any outside force. Since the atoms were not created at any time in the past and would never disintegrate at any time in the future, the total quantity of matter in the universe remained constant: this fundamental principle of Democritus's atomic theory implies the conservation of matter, the sum total of which in the universe neither increases nor diminishes. Though Democritus's conception of the atom has been modified in several essential respects in modern times, his atomic theory remains the foundation of modern science.

John Dalton: Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory, stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances (remember they cannot be separated into elements by phase changes) because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.

Dmitri Mendeleev: Mendeleev's original work covered a wide range, from questions in applied chemistry to the most general problems of chemical and physical theory. His name is best known for his work on the Periodic Law. Various chemists had traced numerical sequences among the atomic weights of some of the elements and noted connections between them and the properties of the different substances; but it was left to him to give a full expression to the generalization, and to treat it not merely as a system of classifying the elements according to certain observed facts, but as a "law of nature" which could be relied upon to predict new facts and to disclose errors in what were supposed to be old facts. Thus in 1871 he was led by certain gaps in his tables to assert the existence of three new elements so far unknown to the chemist, and to assign them definite properties. These three he called ekaboron, ekaaluminium, and ekasilicon; and his prophecy was completely vindicated within fifteen years by the discovery of gallium in 1871, scandium in 1879, and germanium in 1886. Again, in several cases he ventured to question the correctness of the "accepted atomic weights", on the ground that they did not correspond with the Periodic Law, and here also he was justified by subsequent investigation. In 1902, in an attempt at a chemical conception of the ether, he put forward the hypothesis that there are in existence two elements of smaller atomic weight than hydrogen, and that the lighter of these is a chemically inert, exceedingly mobile, all-penetrating and all-pervading gas, which constitutes the "aether." Mendeleev also devoted much study to the nature of such "indefinite" compounds as solutions, which he looked upon as homogeneous liquid systems of unstable dissociating compounds of the solvent with the substance dissolved, holding the opinion that they are merely an instance of ordinary definite or atomic compounds, subject to John Dalton's laws. In another department of physical chemistry he investigated the expansion of liquids with heat, and devised a formula for its expression similar to Gay-Lussac's law of the uniformity of the expansion of gases, while so far back as 1861 he anticipated T. Andrews's conception of the critical temperature of gases by defining the absolute boiling-point of a substance as the temperature at which cohesion and heat of vaporization become equal to zero and the liquid changes to vapor, irrespective of the pressure and volume. Mendeleev wrote largely on chemical topics, his most widely known book probably being The Principles of Chemistry, which was written in 1868-70, and has gone through many subsequent editions in various languages. For his work on the Periodic Law he was awarded in 1882, at the same time as L. Meyer, the Davy medal of the Royal Society, and in 1905 he received its Copley medal. He died at St. Petersburg on the 2nd of February 1907.

Pierre and Marie Curie: Pierre and Marie Curie are best known for their pioneering work in the study of radioactivity, which led to their discovery in 1898 of the elements radium and polonium. He discovered the phenomenon of piezoelectricity, whereby changes in the volume of certain crystals excite small electric potentials. Along with work on crystal symmetry, Pierre Curie studied the magnetic properties of materials and constructed a torsion balance with a tolerance of 0.01 mg. He discovered that the magnetic susceptibility of paramagnetic materials is inversely proportional to the absolute temperature (Weiss-Curie's law) and that there exists a critical temperature above which the magnetic properties disappear (Curie temperature). Marie measured the strength of the radiation emitted from uranium compounds and found it proportional to the uranium content, constant over a long period of time, and uninfluenced by external conditions. She detected a similar immutable radiation in the compounds of thorium. While checking these results, she made the unexpected discovery that uranium pitchblende and the mineral chalcolite emitted about four times as much radiation as could be expected from their uranium content. In 1898 she therefore drew the revolutionary conclusion that pitchblende contains a small amount of an unknown radiating element.

Ernest Rutherford: In 1899 Ernest Rutherford studied the absorption of radioactivity by thin sheets of metal foil and found two components: alpha (a) radiation, which is absorbed by a few thousandths of a centimeter of metal foil, and beta (b) radiation, which can pass through 100 times as much foil before it was absorbed. Shortly thereafter, a third form of radiation, named gamma (g) rays, was discovered that can penetrate as much as several centimeters of lead. The three kinds of radiation also differ in the way they are affected by electric and magnetic fields. Rutherford began his graduate work by studying the effect of x-rays on various materials. Shortly after the discovery of radioactivity, he turned to the study of the -particles emitted by uranium metal and its compounds. Before he could study the effect of -particles on matter, Rutherford had to develop a way of counting individual -particles. He found that a screen coated with zinc sulfide emitted a flash of light each time it was hit by an -particle. Rutherford and his assistant, Hans Geiger, would sit in the dark until his eyes became sensitive enough. They would then try to count the flashes of light given off by the ZnS screen. (It is not surprising that Geiger was motivated to develop the electronic radioactivity counter that carries his name.) Rutherford found that a narrow beam of -particles was broadened when it passed through a thin film of mica or metal. He therefore had Geiger measure the angle through which these -particles were scattered by a thin piece of metal foil. Because it is unusually ductile, gold can be made into a foil that is only 0.00004 cm thick. When this foil was bombarded with -particles, Geiger found that the scattering was small, on the order of one degree.
These results were consistent with Rutherford's expectations. He knew that the -particle had a considerable mass and moved quite rapidly. He therefore anticipated that virtually all of the -particles would be able to penetrate the metal foil, although they would be scattered slightly by collisions with the atoms through which they passed. In other words, Rutherford expected the -particles to pass through the metal foil the way a rifle bullet would penetrate a bag of sand.One day, Geiger suggested that a research project should be given to Ernest Marsden, who was working in Rutherford's laboratory. Rutherford responded, "Why not let him see whether any -particles can be scattered through a large angle?" When this experiment was done, Marsden found that a small fraction (perhaps 1 in 20,000) of the -particles were scattered through angles larger than 90o (see Figure 6.7a). Many years later, reflecting on his reaction to these results, Rutherford said: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." Rutherford concluded that there was only one way to explain these results. He assumed that the positive charge and the mass of an atom are concentrated in a small fraction of the total volume and then derived mathematical equations for the scattering that would occur. These equations predicted that the number of -particles scattered through a given angle should be proportional to the thickness of the foil and the square of the charge on the nucleus, and inversely proportional to the velocity with which the -particles moved raised to the fourth power. In a series of experiments, Geiger and Marsden verified each of these predictions.When he published the results of these experiments in 1911, Rutherford proposed a model for the structure of the atom that is still accepted today. He concluded that all of the positive charge and essentially all of the mass of the atom is concentrated in an infinitesimally small fraction of the total volume of the atom, which he called the nucleus (from the Latin for little nut). Most of the -particles were able to pass through the gold foil without encountering anything large enough to significantly deflect their path. A small fraction of the particles came close to the nucleus of a gold atom as they passed through the foil. When this happened, the force of repulsion between the positively charged -particle and the nucleus deflected the -particle by a small angle. Occasionally, an -particle traveled along a path that would eventually lead to a direct collision with the nucleus of one of the 2000 or so atoms it had to pass through. When this happened, repulsion between the nucleus and the -particle deflected the -particle through an angle of 90o or more. By carefully measuring the fraction of the -particles deflected through large angles, Rutherford was able to estimate the size of the nucleus. According to his calculations, the radius of the nucleus is at least 10,000 times smaller than the radius of the atom. The vast majority of the volume of an atom is therefore empty space.
Naming the Proton (Ernest Rutherford)
Shortly after the World War I, in 1920, Rutherford proposed the name proton for the positively charged particles in the nucleus of an atom.
Proposing the Neutron (Ernest Rutherfor)
At the same time that Rutherford proposed the name proton for the positively charged particle in the nucleus of an atom, he proposed that the nucleus also contained a neutral particle, eventually named the neutron. It was not until 1932, however, that James Chadwickwas able to prove that these neutral particles exist.


J.J. Thomson: He discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. Thomson interpreted the deflection of the rays by electrically charged plates and magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very large value for the charge-to-mass ratio. Later he estimated the value of the charge itself. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. Thomson's last important experimental program focused on determining the nature of positively charged particles. Here his techniques led to the development of the mass spectrograph. His assistant, Francis Aston, developed Thomson's instrument further and with the improved version was able to discover isotopes—atoms of the same element with different atomic weights—in a large number of nonradioactive elements.

Henry Mosley: British physicist who first established the atomic numbers of the elements by studying their X-ray spectra. This led to a complete classification of the elements, and also provided an experimental basis for an understanding of the structure of the atom. he made discoveries which were of fundamental importance to the development of both physics and chemistry. When he joined Rutherford's group in 1910, Rutherford was researching the phenomena associated with natural radioactivity. Moseley at first helped Rutherford in this work, but when reports of the diffraction of X-rays by Max von Laue (1879-1960) reached him in 1912, Moseley persuaded Rutherford to allow him to study X-ray spectra. He received instruction in X-ray diffraction from Lawrence Bragg (1890-1971) and in 1913 Moseley introduced X-ray spectroscopy to determine the X-ray spectra of the elements. In a series of brilliant investigations, Moseley allowed the X-rays produced from various substances used as a target in an X-ray tube to be diffracted by a crystal of potassium ferrocyanide. The glancing angles were measured accurately and the position of the diffracted beams determined to obtain the wavelengths and frequencies of the X-rays emitted. Moseley examined metals from aluminum to gold and he found that their X-ray spectra were similar but with a deviation that changed regularly through the series. He found that a graph of the square root of the frequency of each radiation against the number representing the element's position in the periodic table gave a straight line. He called this number the atomic number of the element, which has since been shown to be the positive charge on the nucleus and thus the number of protons in the nucleus. Since the atom is electrically neutral, the atomic number is also the number of electrons surrounding the nucleus. It was as a direct result of this work that atomic numbers were placed on a sound experimental foundation. Moseley found that when the elements are arranged in the periodic table according to their atomic numbers, all irregularity caused in the older system of grouping elements by their atomic weight disappeared. Now that the elements were numbered, the rare earth elements could be sorted out, a process that Moseley began at Oxford towards the end of his life. The numbering system also enabled Moseley to predict that several more elements would be discovered, namely those with atomic numbers of 43, 61, 72, 75, 87 and 91. These were all found in due course. Although the number of elements which Moseley was able to examine was limited, the equation relating the square root of the frequency to the atomic number has been found to hold in all cases. The equation is known as Moseley's Law. It has enabled scientists to identify a total of 105 elements in a continuous series of atomic numbers. Any further elements that might be produced by nuclear reactions can only have greater atomic numbers. In 1913 and 1914, the young physicist published his findings in two remarkable papers in the Philosophical Magazine and entitled them The High-Frequency Spectra of the Elements. Moseley's discovery told how many electrons were present in any element, and tied in nicely with the quantum theory of the hydrogen atom which was published in 1913 by Niels Bohr (1885-1962). Moseley's fundamental discovery was a milestone in our knowledge of the constitution of the atom, and we are left to ponder on what this great brain might have discovered had he not been so tragically killed at so young an age.

James Chadwick: James Chadwick proved the existence of neutrons, the elementary particle without any electrical charge and a fundamental building block of the atom's nucleus. One of the most important scientific discoveries of the twentieth century, it effectively solved the jigsaw puzzle of the atom, and earned Chadwick the 1935 Nobel Prize for Physics. Following Chadwick's breakthrough, he and other scientists began experimenting on all types of materials with neutrons, leading to the discovery of nuclear fission when uranium is bombarded with neutrons, and the eventual development of nuclear weapons and nuclear power production. Chadwick proofs the existence of the neutron in 1932 what accelerated the research in nuclear physics immensely. Chadwick's own research focused on radioactivity. In 1919 Rutherford developed a theory on the atom's nucleus and theorized on the existence of a neuron, a non charged particle within the atom's nucleus. But they and other researchers were finding that the neutron did not seem to be the only particle in the nucleus. As they studied atomic disintegration, they kept seeing that the atomic number (number of protons in the nucleus, equivalent to the positive charge of the atom) was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. So in the helium example, there would be four protons and two electrons in the nucleus to yield a mass of 4 but a charge of only 2. Rutherford also put out the idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron. There was no evidence for any of these ideas. Chadwick kept the problem in the back of his mind while working on other things. Experiments in Europe caught his eye, especially those of Frederic and Irene Joliot-Curie. They used a different method for tracking particle radiation. Chadwick repeated their experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to determine that the neutron did exist and that its mass was about 0.1 percent more than the proton's. He published his findings with characteristic modesty in a first paper entitled "Possible Existence of Neutron." In 1935 he received the Nobel Prize for his discovery. His findings were quickly accepted and Werner Heisenberg then showed that the neutron could not be a proton-electron pairing, but had to be its own unique particle -- the third piece of the atom to be found. This new idea dramatically changed the picture of the atom and accelerated discoveries in atomic physics. Physicists soon found that the neutron made an ideal "bullet" for bombarding other nuclei. Unlike charged particles, it was not repelled by similarly-charged particles and could smash right into the nucleus. Before long, neutron bombardment was applied to the uranium atom, splitting its nucleus and releasing the huge amounts of energy predicted by Einstein's equation E = mc2.

Kayly Adams
Question 1B?

Ernest Rutherford: Known as the father of nuclear physics. he discovered the concept of radioactive half life, proved that radioactivity involved the transmutation of one chemical element to another, and also differentiated and named alpha and beta radiation. He was awarded the Nobel Prize in Chemistry in 1908 "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances". he postulated that atoms have their positive charge concentrated in a very small nucleus, and thereby pioneered the Rutherford model, or planetary, model of the atom, through his discovery and interpretation of Rutherford scattering in his gold foil experiment. He is widely credited with first splitting the atom in 1917, and leading the first experiment to "split the nucleus" in a controlled manner by two students under his direction, one being John Cockcroft..

John Cockcroft: A British physicist, he shared the Nobel Prize in Physics for splitting the atomic nucleus with Ernest Walton, and was instrumental in the development of nuclear power. In 1928, he began to work on the acceleration of protons with Ernest Walton. In 1932, they bombarded lithium with high energy protons and succeeded in transmuting it into helium and other chemical elements. This was one of the earliest experiments to change the atomic nucleus of one element to a different nucleus by artificial means. In 1951, Cockcroft, along with Walton, was awarded the Nobel Prize in Physics for his work in the use of accelerated particles to study the atomic nucleus.

Ernest Walton: He was an Irish physicist and Nobel laureate for his work with John Cockcroft with atom-smashing experiments done at Cambridge University in the early 1930s. Walton is the only Irishman to have won a Nobel Prize in science.Walton and John Cockcroft collaborated to build an apparatus that split the nuclei of lithium atoms by bombarding them with a stream of protons accelerated inside a high-voltage tube (700 kilovolts). The splitting of the lithium nuclei produced helium nuclei. This was experimental verification of theories about atomic structure that had been proposed earlier by Rutherford, George Gamow, and others.

Erwin Schrodinger: He was an Austrian theoretical physicist who was one of the fathers of quantum mechanics, and is famous for a number of important contributions to physics. Especially his experiment, Schrodinger equation, for which he received the Noble Prize in Physics for in 1933. In 1935, after much correspondence with his friend ALBERT EINSTEIN, he proposed the Schrodinger's Cat Thought Experiment.

Examine the history of the development of the atom. We have already discussed the major players in the development of the atom. Pick out at least 8 major players who helped aid in the development of the concept of the atom. Then explain each experiment in a concise explanation. Then explain how each one aided in the development of the next or how their concept assisted in the further development of the current quantum model of the atom.


Democritus- The material cause of all things that exist is the coming together of atoms and void. Atoms are too small to be perceived by the senses. They are eternal and have many different shapes, and they can cluster together to create things that are perceivable. Differences in shape, arrangement, and position of atoms produce different things. By aggregation they provide bulky objects that we can perceive with our sight and other senses. We see changes in things because of the rearrangement of atoms, but atoms themselves are eternal. Words such as ‘nothing’, ‘the void’, and ‘the infinite’ describe space. Individual atoms are describable as ‘not nothing’, ‘being’, and ‘the compact’. There is no void in atoms, so they cannot be divided. I hold the same view as Leucippus regarding atoms and space: atoms are always in motion in space.



John Dalton- He proposed the Atomic Theory in 1803 which stated that (1) all matter was composed of small indivisible particles termed atoms, (2) atoms of a given element possess unique characteristics and weight, and (3) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules). Dalton's theory was presented in New System of Chemical Philosophy (1808-1827). This work identified chemical elements as a specific type of atom, therefore rejecting Newton's theory of chemical affinities. Instead, Dalton inferred proportions of elements in compounds by taking ratios of the weights of reactants, setting the atomic weight of hydrogen to be identically one. Following Richter, he proposed that chemical elements combine in integral ratios. Despite the importance of the work as the first view of atoms as physically real entities and introduction of a system of chemical symbols, New System of Chemical Philosophy devoted almost as much space to the caloric theory as to atomism.

Dmitiri Mendeleev- He is best known for his work on the periodic table; arranging the 63 known elements into a Periodic Table based on atomic mass, which he published in Principles of Chemistry in 1869. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error. This organization surpassed attempts at classification by Beguyer de Chancourtois and Newlands and was published a year before the work of Lothar Meyer. Mendeleev provided for variance from strict atomic weight order, left space for new elements, and predicted three yet-to-be-discovered elements including eke-silicon and eke-boron. His table did not include any of the Noble Gases, however, which had not yet been discovered. The original table has been modified and corrected several times, notably by Mosley, but it had accommodated the discovery of isotopes, rare gases, etc. Mendeleev anticipated Andrews' concept (1869) of the critical temperature of gases. He also investigated the thermal expansion of liquids, and studied the nature and origin of petroleum. He was considered one of the greatest teachers of his time. In 1890 he resigned his professorship and in 1893 became director of the bureau of weights and measures in St. Petersburg, where he remained until his death in 1907.

J.J. Thompson- At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as " cathode rays". His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter. Thomson's speculation was not unambiguously supported by his experiments. It took more experimental work by Thomson and others to sort out the confusion. The atom is now known to contain other particles as well. Yet Thomson's bold suggestion that cathode rays were material constituents of atoms turned out to be correct. The rays are made up of electrons: very small, negatively charged particles that are indeed fundamental parts of every atom. Modern ideas and technologies based on the electron, leading to television and the computer and much else, evolved through many difficult steps.

Earnest Rutherford- Rutherford, Ernest (1871-1937): Born in New Zealand, Rutherford studied under J. J. Thomson at the Cavendish Laboratory in England. His work constituted a notable landmark in the history of atomic research as he developed Bacquerel's discovery of Radioactivity into an exact and documented proof that the atoms of the heavier elements, which had been thought to be immutable, actually disintegrate (decay) into various forms of radiation.Rutherford was the first to establish the theory of the nuclear atom and to carry out a transmutation reaction (1919) (formation of hydrogen and and oxygen isotope by bombardment of nitrogen with alpha particles). Uranium emanations were shown to consist of three types of rays, alpha (helium nuclei) of low penetrating power, beta (electrons), and gamma, of exceedingly short wavelength and great energy.Ernest Rutherford also discovered the half-life of radioactive elements and applied this to studies of age determination of rocks by measuring the decay period of radium to lead-206. He also used alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.

H.G.J. Moseley- Henry Moseley's research career lasted only forty months before tragically ending with his death on a Gallipoli battlefield in World War I. But in his classic study of the x-ray spectra of elements, he established the truly scientific basis of the Periodic Table by arranging chemical elements in the order of their atomic numbers. During this time period the first coherent theory of the structure of the atom was just then being developed by Rutherford and his research group, which, besides Moseley, included Niels Bohr, Hans Geiger, Kasimir Fajans, and others. The nature of x rays was also receiving new interest because of the discovery by the German physicist Max von Laue in 1912 that they were diffracted by their passage through crystals and therefore possessed a wave nature. Succeeding experiments by William L. Bragg the same year showed that similar results could be obtained by the reflection of x rays from the face of a crystal. Moseley persuaded Rutherford to allow him and a colleague, C. S. Darwin, to further study the nature of x rays. Their work demonstrated that the spectral line of platinum, which they were using as the anticathode in their x-ray tube, was characteristic of that element alone. Moseley returned to Oxford, and despite the experimental deficiencies of his laboratories, measured the x-ray spectral lines of nearly all the elements from aluminum to gold. The results of his study showed a clear and simple progression of the elements that was based on the number of protons in the atomic nucleus, rather than the order based on atomic weights that was then the basis of the Periodic Table.

Niels Bohr- Bohr began to work on the problem of the atom's structure. Ernest Rutherford had recently suggested the atom had a miniature, dense nucleus surrounded by a cloud of nearly weightless electrons. There were a few problems with the model, however. For example, according to classical physics, the electrons orbiting the nucleus should lose energy until they spiral down into the center, collapsing the atom. Bohr proposed adding to the model the new idea of quanta put forth by Max Planck in 1901. That way, electrons existed at set levels of energy, that is, at fixed distances from the nucleus. If the atom absorbed energy, the electron jumped to a level further from the nucleus; if it radiated energy, it fell to a level closer to the nucleus. His model was a huge leap forward in making theory fit the experimental evidence that other physicists had found over the years. A few inaccuracies remained to be ironed out by others over the next few years, but his essential idea was proved correct. He received the Nobel Prize for this work in 1922, and it's what he's most famous for. But he was only 37 at the time, and he didn't stop there. Among other things, he put forth the theory of the nucleus as a liquid drop, and the idea of "complementarity" -- that things may have a dual nature (as the electron is both particle and wave) but we can only experience one aspect at a time. After Hitler took power in Germany, Bohr was deeply concerned for his colleagues there, and offered a place for many escaping Jewish scientists to live and work. He later donated his gold Nobel medal to the Finnish war effort. In 1939 Bohr visited the United States with the news from Lise Meitner (who had escaped German-occupied Austria) that German scientists were working on splitting the atom. This spurred the United States to launch the Manhattan Project to develop the atomic bomb. Shortly after Bohr's return home, the German army occupied Denmark. Three years later Bohr's family fled to Sweden in a fishing boat. Then Bohr and his son Aage left Sweden traveling in the empty bomb rack of a British military plane. They ultimately went to the United States, where both joined the government's team of physicists working on atomic bomb at Los Alamos. Bohr had qualms about the consequences of the bomb. He angered Winston Churchill by wanting to share information with the Soviet Union and supporting postwar arms control. Bohr went on to organize the Atoms for Peace Conference in Geneva in 1955.

Schrodinger- In 1920 he took up an academic position as assistant to Msx Wein, followed by positions at Stuttgart (extraordinary professor), Breslau (ordinary professor), and at the University of Zurich (replacing von Laue) where he settled for six years. In later years Schrödinger looked back to his Zurich period with great pleasure - it was here that he enjoyed so much the contact and friendship of many of his colleagues, among whom were Hermann Weyl and Peter Debye. It was also his most fruitful period, being actively engaged in a variety of subjects of theoretical physics. His papers at that time dealt with specific heats of solids, with problems of thermodynamics (he was greatly interested in Boltzmann's probability theory) and of atomic spectra; in addition, he indulged in physiological studies of colour (as a result of his contacts with Kohlrausch and Exner, and of Helmholtz's lectures). His great discovery, Schrödinger's wave equation, was made at the end of this epoch-during the first half of 1926.It came as a result of his dissatisfaction with the quantum condition in Bohr's orbit theory and his belief that atomic spectra should really be determined by some kind of eigenvalue problem. Shrodinger viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.For this work he shared with Dirac the Nobel Prize for 1933.

Question 1B

Niels Bohr- A model of the atom, first described by Niels Bohr, that explains the emission and absorption of radiation as transitions between stationary electronic states in which the electron orbits the nucleus at a definite distance. The Bohr model violates the Heisenberg uncertainty principle, since it postulates definite paths and momenta for electrons as they move around the nucleus. Modern theories usually use atomic orbitals to describe the behavior of electrons in atoms.

Ernest Rutherford- Ernest Rutherford explains his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (mostly gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'. His model didn't make any headway in the explanation of the electrons but just kinda helped in the understanding of the past and present atomic structures.

William Crookes- This fascinating scientist discovered the phenomenon upon which depends the action of the well-known little instrument, the Crookes radiometer, in which a system of vanes, each blackened on one side and polished on the other, is set in rotation when exposed to radiant energy. He did not, however, provide the true explanation of this apparent attraction and repulsion resulting from radiation. Of more fundamental importance were his researches on the passage of the electrical discharge through rarefied gases. He found that as the attenuation of the gas was made greater the dark space round the negative electrode extended, while rays, now known as cathode rays, proceed from the electrode. He investigated the properties of the rays, showing that they travel in straight lines, cause phosphorescence in objects upon which they impinge, and by their impact produce great heat. He believed that he had discovered a fourth state of matter, which he called "radiant matter". But his theoretical views on the nature of "radiant matter" proved to be mistaken. He believed the rays to consist of streams of particles of ordinary molecular magnitude. It remained for J. J. Thomson to discover their subatomic nature, and to prove that cathode rays consist of streams of negative electrons, that is, of negatively electrified particles whose mass is only 1/1,800 that of the atom of hydrogen.

J.J. Thompson-Thomson's work suggested that the atom was not an "indivisible" particle as John Dalton had suggested but, a jigsaw puzzle made of smaller pieces.
Thomson's notion of the electron came from his work with a nineteenth century scientific curiosity: the cathode ray tube. For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen; however, no one could explain why. Thomson found that the mysterious glowing stream would bend toward a positively charged electric plate. Thomson theorized, and was later proven correct, that the stream was in fact made up of small particles, pieces of atoms that carried a negative charge. These particles were later named electrons. The present day atom has electrons in it so that has not changed. His discovery played a huge part in the constructing of the present day model.

John Dalton-a study of Dalton's own laboratory notebooks, discovered in the rooms of the Lit & Phil, concluded that so far from Dalton being led by his search for an explanation of the law of multiple proportions to the idea that chemical combination consists in the interaction of atoms of definite and characteristic weight, the idea of atoms arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases. Dalton proceeded to print his first published table of relative atomic weights. Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton provided no indication in this first paper how he had arrived at these numbers. However, in his laboratory notebook under the date 6 September 1803 there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time. His discoveries led to the discovery of a few elements his atomic theory has also played a major role in the new discoveries.


H.G.J Mosley-Before Moseley and his created law, atomic numbers had been thought of as a semi-arbitrary ordering number, vaguely increasing with atomic weight but not strictly defined by it. Moseley redefined the idea of atomic numbers from its previous status as an ad hoc numerical tag to help sorting the elements, in particular in the Periodic Table, into a real and objective whole-number quantity that was experimentally measurable. Furthermore, as noted by Bohr, Moseley's law provided a reasonably complete experimental set of data that supported the conception by Ernest Rutherford and Antonius Van den Broek of the atom, with a positively-charged nucleus surrounded by negatively-charged electrons in which the atomic number is understood to be the exactly physical number of positive charges in the central atomic nuclei of the elements. Simple modification of Rydberg's and Bohr's formulas were found to give theoretical justification for Moseley's empirically-derived law for determining atomic numbers. The atomic number is one of the most important things of an atom. It has helped us understand so much about current atoms. His dicovery was a very important and useful discovery.

Dmitri Mendeleev- Developed the first periodic table when trying to classify elements not by accidental or instinctive reasons, but by a set principle. He believed it should be numerical in nature to eliminate any margin of arbitrariness. The trend of increasing atomic mass allowed him to discover a periodicity of elemental properties. The first model used vertical columns and showed that there were some missing places where there could be undiscovered elements.

James Chadwick- discovered a third type of subatomic particle, which he named the neutron. Neutrons help stabilize the protons in the atom's nucleus. Because the nucleus is so tightly packed together, the positively charged protons would tend to repel each other normally. Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms and they are about the same size as protons. However, neutrons do not have any electrical charge; they are electrically neutral.
Atoms are electrically neutral because the number of protons + charges is equal to the number of electrons - charges and thus the two cancel out. As the atom gets larger, the number of protons increases, and so does the number of electrons in the neutral state of the atom. The illustration linked below compares the two simplest atoms, hydrogen and helium. The present day model still consists of the protons. His discovery is still a very important factor today and for the present day atom.

gatorshelton

Examine the history of the development of the atom. We have already discussed the
major players in the development of the atom. Pick out at least 8 major players who
helped aid in the development of the concept of the atom. Then explain each
experiment in a concise explanation. Then explain how each one aided in the
development of the next or how their concept assisted in the further development of the current quantum model of the atom.


Democritus:The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is." But their exact position on weight of atoms is disputed.

Issac Newton: Newton laid the foundation for differential and integral calculus. His work on optics and gravitation make him one of the greatest scientists the world has known. Newton's greatest achievement was his work in physics and celestial mechanics, which culminated in the theory of universal gravitation. By 1666 Newton had early versions of his three laws of motion. He had also discovered the law giving the centrifugal force on a body moving uniformly in a circular path. However he did not have a correct understanding of the mechanics of circular motion.



john dalton:John Dalton developed the first useful atomic theory of matter around 1803. In the course of his studies on meteorology, Dalton concluded that evaporated water exists in air as an independent gas. He wondered how water and air could occupy the same space at the same time, when obviously solid bodies can't. If the water and air were composed of discrete particles, Dalton reasoned, evaporation might be viewed as a mixing of water particles with air particles. He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different.

J.J. Thomson: J J Thomson experimented on cathode rays. In Britain, physicists had argued these rays were particles, but German physicists disagreed, thinking they were a type of electromagnetic radiation. Thomson showed that cathode rays were particles with a negative electric charge and much smaller than an atom. He also thought all atoms contained them. These particles were later named electrons.



micheal faraday:Faraday's introduction of the concept of lines of force was rejected by most of the mathematical physicists of Europe, since they assumed that electric charges attract and repel one another, by action at a distance, making such lines unnecessary. Faraday had demonstrated the phenomenon of electromagnetism in a series of experiments, however. This experimental necessity probably led the physicist James Clerk Maxwell to accept the concept of lines of force and put Faraday's ideas into mathematical form, thus giving birth to modern field theory.



Dmitri Mendeleev: Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.


albert einstein:Einstein's contributions to physics began in 1905 with three major results: the explanation of Brownian motion in terms of molecules; the explanation of the photoelectric effect in terms of the quantum; and the special theory of relativity that links time to space and energy to matter. From 1907 to 1915 Einstein developed general relativity, a theory of gravity more accurate than Newton's; it became the basis of theoretical cosmology. In failed efforts in the 1930s to refute the interpretation of quantum theory in terms of probability, Einstein contributed to the theoretical basis for what is sometimes called teleportation of photons (which Einstein called "spooky action at a distance"). His last major effort was an attempt to unify electromagnetism and gravity into a single unified field theory, still an active problem of physics.

niels bohr:Niels Bohr was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. He was also part of the team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Niels Bohr, grew up to be an important physicist, who like his father received the Nobel prize, in 1975. Bohr is widely considered to be the one of the greatest physicists of the 20th century.

question 1B

Examine the history of the development of the atom. We have already discussed the major players in the development of the atom. Pick out at least 8 major players who helped aid in the development of the concept of the atom. Then explain each experiment in a concise explanation. Then explain how each one aided in the development of the next or how their concept assisted in the further development of the current quantum model of the atom.

Democritus: Started the idea that matter was composed of the tiny particles called "atoms".The theory of Democritus and Leucippus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; that between atoms lies empty space; that atoms are indestructible; have always been, and always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Of the mass of atoms, Democritus said "The more any indivisible exceeds, the heavier it is." But their exact position on weight of atoms is disputed. These were the things that composed all matter and the only differences were the size, shape, and weight. There was much speculation and without a clear way to prove the idea, they largely ignored his ramblings because they didn't have the technology to further study it. They didn’t look at this again for about 2000 years.

John Dalton: Credited with developing the atomic theory. His theory is as follows. 1. Matter is composed of small particles called atoms. 2. All atoms of an element are identical, but are different from those of any other element. 3. Atoms are neither created nor destroyed. 4. Atoms always combine in whole number multiples of each other. Dalton proceeded to print his first published table of relative atomic weights. Six elements appear in this table, namely hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen conventionally assumed to weigh 1. Dalton provided no indication in this first paper how he had arrived at these numbers. However, in his laboratory notebook under the date 6 September 1803[4] there appears a list in which he sets out the relative weights of the atoms of a number of elements, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

Dmitri Mendeleev: Developed the first periodic table when trying to classify elements not by accidental or instinctive reasons, but by a set principle. He believed it should be numerical in nature to eliminate any margin of arbitrariness. The trend of increasing atomic mass allowed him to discover a periodicity of elemental properties. The first model used vertical columns and showed that there were some missing places where there could be undiscovered elements. On 6 March 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both atomic weight and valence. This presentation stated that 1.The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties. 2.Elements which are similar in regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs). 3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F. 4.The elements which are the most widely diffused have small atomic weights. 5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body. 6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminum and silicon, whose atomic weights would be between 65 and 75. 7. The atomic weight of an element may sometimes be amended by knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here Mendeleev seems to be wrong as the "atomic mass" of tellurium (127.6) remains higher than that of iodine (126.9) as displayed on modern periodic tables, but this is due to the way atomic masses are calculated, based on a weighted average of all of an element's common isotopes, not just the one-to-one proton/neutron-ratio version of the element to which Mendeleev was referring. 8. Certain characteristic properties of elements can be foretold from their atomic weights

Pierre and Marie Curie: These two worked in the discovery of radioactivity by following the notes of Henri Becquerel. In 1896 Henri Becquerel discovered that uranium salts emitted rays that resembled X-rays in their penetrating power. He demonstrated that this radiation, unlike phosphorescence, did not depend on an external source of energy, but seemed to arise spontaneously from uranium itself. Becquerel had, in fact, discovered radioactivity. Curie decided to look into uranium rays as a possible field of research for a thesis. She used a clever technique to investigate samples. Fifteen years earlier, her husband and his brother had invented the electrometer, a sensitive device for measuring electrical charge. Using the Curie electrometer, she discovered that uranium rays caused the air around a sample to conduct electricity. Using this technique, her first result was the finding that the activity of the uranium compounds depended only on the quantity of uranium present. She had shown that the radiation was not the outcome of some interaction of molecules, but must come from the atom itself. In scientific terms, this was the most important single piece of work that she conducted.

Ernest Rutherford: Determined that radiation was emitted from two different components of uranium. He unsuccessfully attempted to separate the two by using prisms of glass, aluminum, and paraffin wax. Using two positively charged plates, he identified the components as positive particles and lighter mass negative particles. During the investigation of radioactivity he coined the terms alpha and beta in 1899 to describe the two distinct types of radiation emitted by thorium and uranium. These rays were differentiated on the basis of penetrating power. From 1900 to 1903 he was joined at McGill by the young Frederick Soddy and they collaborated on research into the transmutation of elements. Rutherford had demonstrated that radioactivity was the spontaneous disintegration of atoms. He noticed that a sample of radioactive material invariably took the same amount of time for half the sample to decay—its "half-life"—and created a practical application using this constant rate of decay as a clock, which could then be used to help determine the age of the Earth, which turned out to be much older than most of the scientists at the time believed. In 1903, Rutherford realized that a type of radiation from radium discovered (but not named) by French chemist Paul Villard in 1900 must represent something different from alpha rays and beta rays, due to its very much greater penetrating power. Rutherford gave this third type of radiation its name also: the gamma ray.

J.J. Thomson: Until 1897, scientists believed atoms were indivisible; the ultimate particles of matter, but Thomson proved them wrong when he discovered that atoms contained particles known as electrons. Thomson discovered this through his explorations on the properties of cathode rays. Thomson found that the rays could be deflected by an electric field (in addition to magnetic fields, which was already known). By comparing the deflection of a beam of cathode rays by electric and magnetic fields he was able to measure the particle's mass. This showed that cathode rays were matter, but he found that the particles were about 2000 times lighter than the mass of the lightest atom, hydrogen. He concluded that the rays were composed of very light negatively charged particles which he called "corpuscles". Thomson believed that the corpuscles emerged from the atoms of the trace gas inside his cathode ray tubes. He thus concluded that atoms were divisible, and that the corpuscles were their building blocks. To explain the overall neutral charge of the atom, he proposed that the corpuscles were distributed in a uniform sea of positive charge; this was the plum pudding model as the electrons were embedded in the positive charge like plums in a plum pudding (although in Thomson's model they were not stationary).

Henry Mosley: Before Moseley's discovery, the atomic numbers (or elemental number) of an element had been thought of as a semi-arbitrary sequential number, based on the sequence of atomic masses, but modified somewhat where chemists found this to be desirable, such as by the great Russian chemist, Dmitri Ivanovich Mendeleev. In his invention of the Periodic Table of the Elements, Mendeleev had interchanged the orders of a few pairs of elements in order to put them in more appropriate places in this table of the elements. For example, the metals cobalt and nickel had been assigned the atomic numbers 27 and 28, respectively, based on their known chemical and physical properties, even though they have nearly the same atomic masses. In fact, the atomic mass of cobalt is slightly larger than that of nickel, which would have placed them in backwards order if they had been placed in the Periodic Table blindly according to atomic mass. Moseley's experiments in X-ray crystallography showed directly from their physics that cobalt and nickel have the different atomic numbers, 27 and 28, and that they are placed in the Periodic Table correctly by Moseley's objective measurements of their atomic numbers. Hence, Moseley's discovery demonstrated that the atomic numbers of elements are not just rather arbitrary numbers based on chemistry and the intuition of chemists, but rather, they have a firm experimental basis from the physics of their X-ray spectra.

James Chadwick: He solved the problem of the extra nuclear mass when he identified the neutron. This occurred while studying the radiation resulting from bombarding of beryllium with alpha particles. He noted a particle with approximately the same mass as a proton being released. He determined that, since the particle was not bent by electrical fields and was highly penetrating, it was electrically neutral. In 1932, Chadwick discovered a previously unknown particle in the atomic nucleus. This particle became known as the neutron because of its lack of electric charge. Chadwick's discovery was crucial for the fission of uranium 235. Unlike positively charged alpha particles, which are repelled by the electrical forces present in the nuclei of other atoms, neutrons do not need to overcome any Coulomb barrier and can therefore penetrate and split the nuclei of even the heaviest elements. For this discovery he was awarded the Hughes Medal of the Royal Society in 1932 and the Nobel Prize for Physics in 1935. Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory. His discovery particularly inspired Enrico Fermi, Italian physicist and Nobel laureate, to discover nuclear reactions brought by slowed neutrons, and led Lise Meitner, Otto Hahn and Fritz Strassmann, German radio chemists in Berlin, to the revolutionary discovery of “nuclear fission”.