HEISENBERG has discussed the hypothesis that the nucleus of an atom is composed of neutrons and protons only, the neutron being regarded as a fundamental entity and not as a combination of an electron and a proton. On this view, the nucleus is formed of n neutrons and p protons, and p is also the number of planetary electrons required to form an electrically neutral atom. Thus p, the charge number, is identical with Moseley's atomic number, Z, which determines the position of the element in the periodic table. The mass number, A, is the sum of n and p. For example, the nucleus of an ordinary hydrogen atom consists of a single proton, while that of the hydrogen isotope of mass 2 consists of one proton and one neutron. The helium nucleus or Î±-particle, which may form a constituent of heavier nuclei, is composed of two protons and two neutrons.
NATURE, 131, 97, 398; 1933.