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 atoms were the things that composed all matter and the only differences were the size, shape, and weight. In that day and age though 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. The idea was ignored for about the next 2000 years.

John Dalton: Credited with developing the first coherent 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. During chemical reactions, atoms are neither created or destroyed, simply rearranged. 4. Atoms always combine in whole number multiples of each other. His theory also helped the concept of atomic weights. Dalton assumed that elements combined in one to one ratios, and therefore was able to tabulate a set of relative weights.

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.

Pierre and Marie Curie: These two worked in the discovery of radioactivity by following the notes of Henri Bequerel. They discovered that some of the elements were disintigrating over time and giving of an energy they called radiation. This theorized that atoms could be made of smaller particles, and have a structure that could be analyzed.

Ernest Rutherford: Determined that radiation was emitted from two different components of uranium. He unsuccessfully attempted to seperate 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.

J.J. Thomson: While performing experiments with cathode ray tubes, Thomson discovered that this ray was composed of particles. By using plates at different angles, he was able to indicate positively charged particles and negatively charged particles. He conclude that negatively charged particles were subatomic particles that were part of every atom. He theorized tat since atoms were neutral then there must be some positive charges. Based on his conclusions, Thomson assumed that atoms were spheres with positive charge with negatively charged "corpuscles" imbedded in it.

Henry Mosley: Bombarded a number of elements with electrons to get the frequency of the resulting x-rays. The relationship led Mosley to believe that there is a fundamental quantity in the atom, that increases by regular steps as we pass from one element to the next. Since the atomic weight of the elements does not increase in regular steps he concluded that the "quantity can only be the charge on the central positive nucleus" of the Rutherford atom.

James Chadwick: He solved the problem of the extra nuclear mass when he identified the neutron. This occured 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.