Alex Warren

Democritus: Started the idea that matter was composed of the tiny particles called "atoms". The theory of Democritus and Leucippus said that everything is composed of "atoms"; between atoms lies empty space; that atoms are indestructible; always will be, in motion; that there are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. Democritus said "The more any indivisible exceeds, the heavier it is." The exact position and weight of atoms is never the same. atoms composed all matter and the only changes were the size, shape, and weight. There was much speculation and without a clear way to prove the idea. people ignored Democritus because they didn’t have the technology to test anything at that time.

John Dalton: developed the atomic theory. 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 made a table of relative atomic weights. Six elements appear in this table, hydrogen, oxygen, nitrogen, carbon, sulfur, and phosphorus, with the atom of hydrogen usually assumed to weigh 1. Dalton did not tell people how he got these numbers. However, in his notebook under the date September 6 1803 there appears a list in which he sets out the relative weights of the atoms of a number of elements.

Dmitri Mendeleev: Developed the first periodic table when trying to classify elements by a set principle. He believed it should be numerical. The trend of increasing atomic mass allowed him to discover a periodicity of essential properties. The first model used vertical columns and showed that there were some missing places where there could be undiscovered elements. On March 6 1869, Mendeleev made a formal presentation, 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 or which increase regularly. 3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valances, as well as to their unique chemical properties; as is obvious among other series. 4. The elements which are the most widely diffused have small atomic weights. 5. The figure of the atomic weight determines the character of the element. 6. We must expect the discovery of many yet unknown elements. 7. The atomic weight of an element may sometimes be altered by knowledge of those of its nearby elements. 8. Certain specific properties of elements can be expected from their atomic weights

Pierre and Marie Curie: These two worked in the discovery of radiation by following the notes of Henri Becquerel. In 1896 Henri Becquerel discovered that uranium salts released rays that resembled X-rays in their strong power. He proved that this radiation did not depend on an external source of energy, but seemed to rise freely from uranium itself. Becquerel had, in fact, discovered radiation. 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 be influenced only on the quantity of uranium present. She had shown that the radiation was not the outcome of some contact of molecules, but must come from the atom itself.

Ernest Rutherford: Determined that radiation was emitted from two different components of uranium. He ineffectively attempted to separate the two by using prisms of glass. Using two positively charged plates, he identified the components as positive particles and lighter mass negative particles. During the research of radiation he created the terms alpha and beta in 1899 to describe the two distinct types of radiation released by thorium and uranium. These rays were separated on the basis of strong power. From 1900 to 1903 he was joined at McGill by Frederick Soddy and they work together on research into the change of elements. Rutherford had demonstrated that radiation was the spontaneous collapse of atoms. He noticed that a sample of radioactive material usually took the same amount of time for half the sample to decay and created a practical presentation 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 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: Thomson 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 bent by an electric field. By comparing the bend 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 determined that the rays were composed of very light negatively charged particles which he called "corpuscles". Thomson believed that the corpuscles appeared 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 unchanging sea of positive charge; this was the plum pudding model as the electrons were fixed in the positive charge like plums in a plum pudding.

Henry Mosley: Before Moseley's discovery, the atomic numbers of an element had been thought of as a semi-random consecutive number, based on the order of atomic masses, but revised somewhat where chemists found this to be wanted. 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. Later, Moseley's discovery demonstrated that the atomic numbers of elements are not just rather random numbers based on chemistry and the insight of chemists, but rather, they have a firm trial 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 happened while studying the radiation resulting from bombarding of beryllium with alpha particles. He distinguished a particle with about 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 vital for the separation of uranium. Unlike positively charged alpha particles neutrons do not need to overcome any column barrier and can consequently 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 lab. His discovery mainly inspired Enrico Fermi to discover nuclear reactions brought by slowed neutrons, and led Lise Meitner, Otto Hahn and Fritz Strassmann, to the new discovery of “nuclear fission”.