Descriptions and data are given of a number of mass comparisons (Ti-O, Cu-O, Cu-Ti, Cr-Ag, Mn-Ag, Fe-Ag, Cu-Zn, Cu-Mo, Cu-Pt-Ir, Cu-Au, Zn-Au, Pt-Au). These, together with other recent data, are critically analyzed in order to obtain values for the packing fraction curve. A curve is shown in which packing fractions are plotted against atomic number (instead of mass number), and the rule is suggested that the packing fraction of an element with odd atomic number is algebraically larger than the average of the two adjacent elements with even atomic number. This implies that elements with even atomic number are more stable than odd-numbered elements and, also, supports the theory that protons in the nucleus are associated together in groups of two.

• Received 15 May 1942

DOI:https://doi.org/10.1103/PhysRev.62.19