Thomson's model marks the moment when the development of atomic theory passed from chemists to physicists. While [[atomic theory]] had beenwas widely accepted by chemists by the end of the 19th century, physicists remained skeptical because the atomic model lacked any properties which concerned their field, such as [[electric charge]], [[magnetic moment]], volume, or absolute mass. Before Thomson's model, atoms were simply the basic units of weight by which the chemical elements react. Thomson himself was a physicist and his atomic model was a byproduct of his investigations of [[cathode ray]]s, by which he discovered the negatively charged particles now known as [[electron]]s. Thomson hypothesized that the quantity, arrangement, and motions of electrons in the atom could explain its physical and chemical properties, such as emission spectra, valencies, reactivity, and ionization. He was on the right track, though his approach was based on classical mechanics and he did not have the insight to incorporate quantized energy into it.
