The large-angle scattering of α-particles by light nuclei

Abstract

1. The hypothesis of the nuclear structure of the atom was first put forward by Sir Ernest Rutherford in 1911 to explain the phenomena of large-angle scattering of α-particles. It was found that when a beam of α-particles passed through matter, the number of particles scattered at large angles far exceeded the number that would be expected on probability considerations, if these large deflections were made up of a number of successive small deflections. The facts could only be explained on the assumption that these deflections were single deflections, occurring as the result of a very close encounter between the α-particle and the atom. The experimental results could be explained quantitatively on the assumption that the mass of the atom was concentrated in a nucleus of small volume at the centre, with a positive charge approximately equal numerically to half the atomic weight. The force was an inverse-square electrostatic one and the electrons outside contributed practically nothing to the scattering owing to their small mass. The nuclear theory of the atom soon received very strong support from different sources of evidence. The extensive series of experiments carried out by Geiger and Marsden on the scattering of α-particles by gold and silver between the angles of 5° and 150° furnished results in perfect agreement with the theory, and showed that, within the limit of experimental error, the inverse-square law held between the α-particle and the nucleus down to distances of 3 x 10 -12 cm. At the same time, Bohr, combining the idea of nuclear structure of the atom with the new hypothesis of stationary states, suggested by the quantum theory, showed how the simpler series spectra could be completely and accurately explained on the inverse-square law. The pioneer work of Moseley on X-ray spectra gave strong support to the idea first proposed by Van den Broek that the nuclear charge, instead of being quite equal to one-half the atomic charge, was indeed equal to the atomic number or number of the place occupied by an element in the periodic table.