The Electronic Theory of Valency

Abstract

WHEN the electronic structure of matter had been demonstrated, and the electrons in the atom had been not only counted but also classified by means of spectroscopic data, it was inevitable that attempts should be made to correlate the new data in reference to atomic structure with the commonplace facts of chemistry. In the case of metallic salts, which have been shown by X-ray analysis to be ionised completely even in the solid state, the application of physical data has been comparatively easy, since the attractive forces in an aggregate of ions can be calculated, and the principal unknown quantity is the compressibility or deformability of the ion. This can be expressed as a repulsion varying inversely as the wth power of the distance, the value of n being about 9 for crystals of the sodium chloride type, in which each ion is surrounded by six atoms of opposite sign; in crystals of the caesium chloride type, however, where the envelope includes eight ions of opposite sign, the value of n is greater, whilst the four ion envelopes of the zinc sulphide type require a smaller index. These simple considerations are complicated by the mutual polarisation of the ions, which introduces another independent constant in the calculations; but, by assigning an arbitrary value to this constant, it has been possible in a considerable number of cases to calculate, by means of data derived from independent sources, the physico chemical properties (e.g. the heat of sublimation, and the molecular volume) of crystalline compounds of this type.

The Electronic Theory of Valency

By Dr. Nevil Vincent Sidgwick. Pp. xii + 310. (Oxford: Clarendon Press; London: Oxford University Press, 1927.) 15s. net.