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-"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].
Democritus proposed the earliest views on the shapes and connectivity of atoms. They reasoned that the solidness of the material corresponded to the shape of the atoms involved. 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 Democritean atom is an inert solid (merely excluding other bodies from its volume) that interacts with other atoms mechanically. In contrast, modern, quantum-mechanical atoms interact via electric and magnetic force fields and are far from inert. His discover is what led us to believe that everything is made up of atoms. He was the one to plant the idea of the atom there.

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.

Ernest Rutherford- Ernest Rutherford publishes 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 (notably 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.

Niels Bohr- In atomic physics, the Bohr model, devised by Niels Bohr, depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. This was an improvement on the earlier cubic model (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr model is a quantum physics-based modification of the Rutherford model, many sources combine the two, referring to the Rutherford–Bohr model.
Introduced by Niels Bohr in 1913, the model's key success lay in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been known experimentally, it did not gain a theoretical underpinning until the Bohr model was introduced. Not only did the Bohr model explain the reason for the structure of the Rydberg formula, it also provided a justification for its empirical results in terms of fundamental physical constants. This model broke away from previous models and explained things a lot better. It also helped solve the Rydberg Formula which was a huge help in further discoveries.

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.

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.

H.G.J Mosley-Before Moseley and his 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's discovery showed that atomic numbers were not arbitrarily assigned, but rather, they have a strong physical basis. 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 (new from 1911) 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 (later discovered and called protons) in the central atomic nuclei of the elements. [Moseley mentioned the two scientists above in his research paper, but he did not actually mention Bohr, who was rather new on the scene then.] 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.

Nagaoka-In 1904, Nagaoka developed an early, incorrect "planetary model" of the atom. The model was based around an analogy to the explanation of the stability of the Saturn rings (the rings are stable because the planet they orbit is very, very massive). The model made two predictions:
a very massive nucleus (in analogy to a very massive planet)
electrons revolving around the nucleus, bound by electrostatic forces (in analogy to the rings revolving around Saturn, bound by gravitational forces).
Both predictions were successfully confirmed by Rutherford (who mentions Nagaoka's model in his 1911 paper in which the nucleus is proposed). However, other details of the model were incorrect (in particular, charged rings would be unstable due to repulsive disruption, which is not the case with Saturn's rings), and Nagaoka himself abandoned it in 1908. His model didn't help much it just proved some things about previous atomic structures were correct. But it did help in that it showed us what wasn't correct. His model proved some things that couldn't be true about atoms and therfore saved us time from having to expirement to find out.