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In 427 B.C Plato introduced the first atomic theory. It broke down mathematically into triangles, such that the form elements had the following shape

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In 400 B.C. Democritus proposed the idea that everything was made of smaller indivisible atoms.

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Between 1772 and 1794, through a series of experiments found that the total mass of products and reactants in a chemical reactions is always the same. This led to the theory of the law of conservation of mass.

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John Dalton discovered the ISSM in early 19th century. He came up with this theory when he was working with gas.

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JJ Thomson came up with this model in 1903 with a positive medium filled with negative electrons inside.

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In 1909 Millikan began a series of experiments to determine the electric charge carried by a single electron. He used water drops to measure the charge.

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In 1911 Rutherford envisioned the atom to be like a solar system. The electrons will be circulating the nucleus

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In 1913, Bohr developed a new model of the atom from the work of Rutherford. The electrons circulate a positively charged nucleus.

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In 1925 Pauli added something to his principle stated that no two electrons occupy the same orbital line and have to have opposite spins

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In 1925 Hund made a rule that stated, every orbital in a subshell is singly occupied with one electron before any one orbital is doubly occupied, and all electrons in singly occupied orbitals have the same spin.

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Erwin Schrodinger made this model in 1926 to help visualize the most probable location of electrons

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In 1932 Chadwick interpreted a radiation with a neutral charge. This led to the discovery of the neutrons.