Electron spin

Abstract

Modern theories of atomic structure may be said to have begun with the experimental establishment of the existence of the electron just before the opening of the present century. It was shown to be a negatively charged particle of mass only 1/1838 that of the hydrogen 1838 atom. Lenard in 1902 showed that cathode rays could be made to penetrate a thin aluminium window which led him to conclude that swift cathode rays could pass freely through thousands of atoms, assuming for the size of the atom the magnitude deduced from the kinetic theory of gases. Thus it appeared that the greater part of an atom must be empty. As a result of experiments on the scattering of ? particles, Rutherford in 1911 suggested a model of the atom as consisting of a positively charged nucleus, very small compared with the size of the complete atom, surrounded by a distribution of electrons whose number was equal to the nuclear charge (if expressed in units of electron charge). This model may be compared with a planetary system with the central nucleus corresponding to the sun. It was later shown that for an atom of atomic number IT (corresponding to its position in Mendelejeff's periodic table), the nucleus would have positive charge Ne where -e = -4.802 x 10

-10 e.s.u. is the charge on an electron.