Rutherford’s model of atomic structure left an important problem unsolved. It was known that hydrogen, the simplest atom, contained only one proton, and that the helium atom contained two protons. Therefore, the ratio of the mass of a helium atom to a hydrogen atom should be 2:1. (Because electrons are much lighter than protons, their contribution to the atomic mass can be ignored). However, in reality the ratio is 4:1.
Rutherford and other researchers had proposed that there should be another type of subatomic particle in the nucleus, a fact that the English physicist James Chadwick proved in 1932.
When Chadwick bombarded a thin sheet of beryllium with alpha particles, the metal emitted very high-energy radiation, similar to gamma rays.
Later experiments showed that these rays actually consisted of a third type of subatomic particle, which Chadwick called neutrons because they were shown to be electrically neutral particles with a mass slightly greater than the mass of protons. The mystery of the mass ratio could now be explained. In the helium nucleus there are two protons and two neutrons, while in the hydrogen nucleus there is only one proton and no neutrons; thus the ratio is 4:1.
James Chadwick (1891-1972). British physicist. In 1935 he received the Nobel Prize in physics for demonstrating the existence of neutrons. The electron, proton and neutron are the fundamental components of the atom that are important for chemistry. The table shows the charge and mass values of these three elementary particles.