ELEMENTS AND THEIR ISOTOPES

Abstract

THE Ritchie Lecture for 1945 of the Royal Society of Edinburgh was delivered by Dr. N. Feather (then at the Cavendish Laboratory, Cambridge, and now professor of physics at the University of Edinburgh), who took as his subject “The Number of the Elements”. The lecture has now been published (Proc. Roy. 8oc. Edin., A, 62, Pt. 2, 211 ; 1946). Prof. Feather commenced by tracing the development of the idea of the chemical element from its earliest beginnings until, in the last quarter of the nineteenth century, Mendeléeff's Periodic Table became accepted as fundamental. Further elucidation of the Periodic Table had to wait for the emergence of the nuclear model of the atom as described by Rutherford in 1911. This led to the Bohr quantum theory, Moseley's work, Soddy's idea of isotopy and Aston's mass analysis. Physicists found that the nucleus could be adequately specified by two parameters, Z and A : the former, the atomic number, is a measure of the positive nuclear charge, and the latter, the mass number, the nearest integer to the number expressing the mass of the corresponding neutral atom in terms of chemical oxygen as 16. The elements were defined and distinguished by their Z-values. By 1932 nearly all the different atom types had been identified and classified according to their Z- and A-values. In 1932, Chadwick discovered the neutron (Z = 0, A = 1), and the nuclear picture was much simplified by regarding the nucleus as made up of heavy particles, neutrons and protons (Z = 1, A = 1). The mass defect, or 1/c2 times the energy of binding of the nucleus, can be estimated as the difference in mass between that of the actual nucleus and of its constituent protons .and neutrons taken separately. It is important to note that the mass defect increases approximately linearly with A ; and that this, together with other known facts, leads to the conclusion that intra-nuclear forces show saturation with the ?-particle as the quasi-saturated unit.