On Atomic Properties which make an Element a Metal

A criterion for determining when an element will show metallic conductivity.—From the dielectric constant of a gas or from its refractive index, extrapolated to long wave-lengths, the molar refractivity $R$ can be calculated. In the solid or liquid state we have $\frac{(n^2-1)}{(n^2+2)}=\frac{\mathrm{Rd}}{M}$. The left side cannot be larger than 1. If the right side becomes larger than 1, the dispersion electron is set free and the body then has metallic conductivity. The necessary and sufficient condition for metallic conductivity is therefore $R>\mathrm{}\frac{M}{d}$. The prediction is verified in the case of metals for which the refractive index is known or for which it can be calculated. It is shown that the condition is probably satisfied in the case of metals for which the refractive index cannot be calculated with certainty. The conductivity of sodium dissolved in liquid ammonia is discussed.