The atomic arrangement in compounds and groups ${\left(A_m{X}_n\right)}^{-p}$ for which $v=n\times 8+m\times 2$ is discussed, $v$ being the total number of valence electrons per stoechiometric molecule or group. Available experimental data show that in such groups or compounds the atom $A$ is usually displaced away from the center of the polyhedron formed by the surrounding atoms $X$. Simple rules for the amount of displacement are given. The interpretation of the observations in terms of the ionic theory and the theory of electron pair bonds is discussed.

• Received 20 April 1932

DOI:https://doi.org/10.1103/PhysRev.40.914