The magnetic susceptibility of the alkali metals

Abstract

In a recent paper, Miss L. Crow found the alkali metals to be diamagnetic: they seemed in this respect to resemble copper, silver and gold, which have in the vapour state one Bohr magneton per atom, but are diamagnetic when solidified. In view of the statement made by Miss Crow that the susceptibility of the glass containers used was negligibly small, it seemed to us necessary to repeat her work. The following paper gives the results of our investigation. In a series of experiments, Gerlach and Stern, and also Taylor, showed that for elements of the first column the vapour molecules possess one Bohr magneton m 0 = 5550/N gauss cm., N being Avogadro’s number. When placed in a magnetic field H the velocity of the valence electron increases, and the energy of the molecule changes by the amount ( m 0 H) + ( e 2 H 2 /8 mc 2 )Σ r 2 , m being the mass of the electrons, e its charge, c the velocity of light, and r the distance from the nucleus. The second member, the diamagnetic effect, is usually negligible. The first term, the paramagnetic effect, leads to an orientation of the molecule parallel to the magnetic field, an orientation which remains, however, incomplete on account of the energy of thermal agitation and the collisions of the atoms. In the case of the alkali vapours for which the molecules are free to orientate themselves in any direction, the molecular susceptibility may be shown to be X m = 1/3 N m 0 2 / k T. In solid bodies the number of positions which the molecule is free to assume is limited by the action of the neighbouring atoms with which it composes the crystal lattice, and the magnetic field may not be able to overcome the forces in the solid body and not succeed in orientating the magnetons. The paramagnetic properties are then hidden, and the diamagnetic factor may become predominant. Furthermore, lower electron shells become important, as is known in the case of the rare earths.