Question #1- Todd Thornal 2nd Period

Democritus- Democritus stated that matter was composed of small particles called "atoms" that could be divided no further. These atoms were all composed of the same primary matter with the only differences between them being their size, shape and weight. The differences in these characteristics explained the differences in the properties of the matter around us.

John Dalton- credited with developing the first coherent atomic theory that states that matter is composed of small particles called atoms, all atoms of an element are identical, but are different from those of any other element, atoms are neither created nor destroyed, but are simply rearranged, and atoms always combine in whole number multiples of each other. Dalton's theory also helped further the concept of atomic weights.

Mendeleev- Mendeleev arranged the elements in order of increasing atomic weight and he discovered that there existed a periodicity of the elemental properties. He used this to create a table in which that elements with similar properties were vertically aligned with each other.

J.J. Thomson- performed an experiment with a cathode ray and was able to determine the mass to charge ratio of these particles by varying the potential on the plates. He found negatively charged particles and said that they were subatomic particles that were part of every atom and that the atom must also contain some positive charge. Thomson proposed that an atom was composed of a positive charge and was credited with discovering protons

Rutherford- Said that the atom was composed of positive and negative charges and made a experiment that allowed alpha particles to be shot at a very thin piece of gold foil while monitered the trajectory. He observed some particles were deflected to a great degree. This force could only be caused by a large concentration of positive charges within the atom. This charge was located at the center of the atom and later became known as the nucleus. Rutherford is credited for discovering the electon and is known for his famous Gold Foil experiment.

Bohr- made fundamental contributions to understanding atomic structure and quantum mechanics and came up with the Bohr model of the atom that depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus.

Mosely- Set up the periodic table based on atomic number instead of Mendeleev's version with atomic weight. Mosely's version of the periodic table helped scientist later make more discoveries of the atom.

Chadawick- proved the existence of neutrons by using particles emitted through radioactivity to discover the neutrally charged partical.

Through many outstanding discoveries these scientist have been able to come up with many different models of the atom and have been able to find out much more information about the atom.

Sources:

http://www.neoam.cc.ok.us/~rjones/Pages/online1014/chemistry/chapter_8/pages/history_of_atom.html