Contemporary Advances in Physics, XXVI, The Nucleus, First Part

O M E thirty years have now elapsed since the atom-nucleus was first imagined. Before it could be conceived men had to discover and measure negative electrons, and evolve the idea that these corpuscles normally reside in' atoms, which in that case must comprise positive charges as well. Since an electron is less than one onethousandth as massive as the lightest kind of atom, it is natural to suppose that the positive charges within an atom are linked with the main mass thereof. From this it is but a step to the notion of a heavy positive nucleus serving as central sun of the atom, with electrons revolving around it after the fashion of planets. This step was taken in 1904 (by Rutherford, and on the other side of the world by Nagaoka). A few years later, the picture was made more precise by assigning a definite number of circling electrons to every kind of atom-- that is to say, to the atoms of all the elements; this at first was rather vaguely estimated at about half of the atomic weight of the element in question; then in 1915 it was chosen equal to the atomic number (customarily called Z) which marks the place of the element in the periodic table. Everything since discovered has justified this choice. It necessarily fixes the positive charge of the nucleus, which must exactly balance the total of the charges on the Z electrons, since the 288 CONTEMPORARY ADVANCES IN PHYSICS 289 atom as a whole is neutral; to the atom-nuclei of the Zth element of the periodic table it therefore assigns the positive charge Ze.