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Democritus was the first one to propose atomic theory. He theorized that everything is composed of atoms, there is space between the atoms, there are an infinite number of atoms, they are always in motion, they have different sizes and shapes, and they are indivisible.

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The electron was discovered in 1897 by J. J. Thompson. He discovered the electron by using a cathode ray tube, which was a glass vacuum tube with an anode at one end and a cathode at the other end. He then applied high electrical voltage to each side and he found that negatively charged ray of particles were coming out of the cathode. These particles were electrons.

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This model was proposed by J.J. Thompson in 1904 soon after he discovered the existence of the electron. This model has a positively charged space with negatively charged electrons spread throughout. It has no nucleus or neutrons.

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Discovery of the Charge of an Electron (1910)

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Discovery of the Charge of an Electron (1910)

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The charge of an electron is negative. This was discovered by Robert Millikan. In 1910 Robert Millikan succeeded in precisely determining the magnitude of the electron's charge. Small electrically charged drops of oil were suspended between two metal plates where they were subjected to the downward force of gravity and the upward attraction of an electrical field.

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This model was proposed by Ernest Rutherford in 1911. He made it after performing the gold foil experiment which showed the existence of a nucleus. The gold foil experiment was when Rutherford and his colleagues shot alpha particles at a thin piece of gold foil, and every so often the alpha particles would hit another particle (the nucleus) and be deflected.

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Niels Bohr invented the Bohr Atomic Model in 1913. He proposed a model of the atom that is similar to the model of the solar system. The Bohr model works well for very simple atoms such as hydrogen but not for more complex atoms.

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The Quantum Mechanical Atomic Model was invented by Erwin Schrodinger in 1926. The quantum mechanical model of the atom uses complex shapes of orbitals, volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty.

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In May 1932 James Chadwick discovered the Neutron. James Chadwick used scattering data to calculate the mass of this neutral particle in order to discover it.