Describe the contributions of the following scientists to our knowledge of atomic structure: J. J. Thomson, R. A. Millikan, Ernest Rutherford, and James Chadwick

See explanation.

Explanation:

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Henceforth, each contribution is listed:

- J. J. Thomson: he discovered the electron in 1897 as well as the plum pudding model of the atom in 1904 before the electrons were included into the atomic model. His model, makes up of an atom that is constituted of electrons surrounded by protons to balance the electrons' negative charges. Electrons were assumed to be located into the atom. In this model the atom was considered as a "cloud" of protons.

- Robert Millikan: he found the elemental charge of the electron by performing the oil-drop experiment in which he placed droplets of oil between two electrodes in order to balance the gravitational force with the upward forces, consequently, mechanical equilibrium was studied to determine the charge.

- Ernest Rutherford: he discovered that atoms makes up of a charged nucleus surrounded by wide empty spaces, which are indeed tiny electrons. It has been widely known as the Rutherford model or planetary model of the atom.

- James Chadwick: he found that neutrons have the same mass as protons. Therefore, he concluded that the atom is a nucleus containing both protons and neutrons with surrounding electrons, with a tiny mass compared to that of the nucleus.

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J. J. Thomson discovered and identified the electron, which was the first subatomic particle to be discovered. R. A. Millikan determined the unit charge of the electron in 1909 with his oil drop experiment. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. Ernest Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny and heavy nucleus. James Chadwick discovered the Neutron in atoms.