J. J. Thomson. Image provided by Edgar Fahs Smith Collection, University of Pennsylvania Library.		The atomic model and quantum mechanics While atoms were thought to be indivisible, that all changed when J(oseph) J(ohn) Thomson (1856–1940) discovered the electron in 1897. Ernest Rutherford (1871–1937) showed that the electrons occupied a region of space surrounding the tiny nucleus, later shown to be made of protons and neutrons. Figuring out just how those electrons behaved required the development of quantum mechanics, a theory in which electrons are treated as wavelike and described using probability and statistical mechanics.