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The idea of the "atom" didn't come into question until a philosopher named Democritus came along. He proposed his theory which states, "The universe is composed of two elements: the atoms and the void in which they exist and move." He also claims that each atom differs in shape, size, & mass. He creates the very first concept of the atomic model, which is all but a sphere...
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After Democritus, the idea of the "atom" resurfaces when English chemist, John Dalton, applies his thought into this long-forgotten concept. His theory states, "Atoms make up all matter, and they are 'indivisible' & 'indestructible'." His atomic model introduces the possibility of "compounds", which are a combination of two or more different atoms.
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The "atom" receives its next major update, when English Physicist, J.J. Thomson, discovers a negative force just outside of the nucleus, known as the "neutron". The discovery was made when J.J. experimented with a cathode ray tube and found the Rays to have a negative charge. His atomic model became known as the "plum pudding model".
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Perhaps one of the most important contributors to the atomic model, this New Zealand physicist is known for being the "father of nuclear physics". Opposing Thomson's "plum pudding" model, Rutherford claims that the atom is nothing but empty space (with small electrons floating in place), and with a small charged nucleus in the middle of it all.
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One of the last major updates to the atomic model was applied by Danish Physicist, Niels Bohr. He theorized that energy is only transferred in well defined quantities, which means that instead of electrons simply floating in place (as suggested by Rutherford), they maintain an orbit around the nucleus. Today, we still use Bohr's model.
