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Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. Therefore, changes in matter were a result of dissociations or combinations of the atoms as they moved throughout the void. Discovered atoms around 400 BCE. He model was the atomos.

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John Dalton believed that all matter is composed of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties. Atoms of different elements differ in size, mass, and other properties. Atoms cannot be subdivided, created, or destroyed. Dalton discovered this by discovered that oxygen combined with either one or two volumes of nitric oxide in closed vessels over water. His model is the billiard ball model. He made these discoveries in 1803.

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Mendeleev discovered that, when all the known chemical elements were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of properties within groups of elements. He is accredited with making the first periodic table of elements. He made his discovery in February of 1869. His model was called the modern periodic table

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JJ Thomson discovered that all atoms contain tiny negatively charged subatomic particles, or electrons. This led to the discovery of electrons in 1897. The model that he used is called the plum pudding model.

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Ernest Rutherford postulated the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. He did an experiment in 1911 in which his experiment was the gold foil experiment which consisted of a Alfa particle beam to see if the particles would pass through the foil. His atomic model was called the Rutherford nuclear atom.

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Niels Bohr believed that electrons orbit the nucleus at fixed energy levels. Orbits further from the nucleus exist at higher energy levels. When electrons return to a lower energy level, they emit energy in the form of light. He made these discoveries in 1913. His model is called The Bohr Model.

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Werner Heisenberg (not from the show “Breaking Bad”) contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle, which states that a particle's position and momentum cannot both be known exactly. He made these discoveries in 1923, and his atomic model was called the strong force.

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Erwin Schrodinger is accredited with the experiment “Schrodinger’s Cat” in which a cat is in a box with a vial of poison and tell whether the vial breaks with an atom inside the box decays. He made these discoveries in 1926 and his model is called the quantum mechanical model of the atom.

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James Chadwick is accredited with making the discoveries in the domain of nuclear science: he proved the existence of neutrons, elementary particles devoid of any electrical charge. He did this by bombarding Beryllium with alpha particles from the natural radioactive decay of Polonium. The resulting radiation showed high penetration through a lead shield, which could not be explained via the particles known at that time. He did this in 1932 and his model is called the Chadwick Atomic Model.