Valency implies the existence of something equivalent to the possession by an atom of points of attachment, and is inseparable from the idea of chemical combination; but it must not be confused, as has often been done, with the force acting at those points. The properties of a mass of matter are the sum of those of its constituents, and, since molecules combine together, the affinities and valencies effecting their union must be those of the constituent atoms. The satisfaction of such additional valencies will represent a comparatively small expenditure of energy, for, even when principal valencies are concerned, as when an element forms pairs of oxides or halides, the heat evolution on combining with the additional oxygen atom is, on the average, only 38 per cent. of what it is on combining with the first atom or atoms (17 cases), and 52 per cent. in the case of the halides (10 cases). At this rate residual affinity must soon become reduced to a quantity insufficient for the attachment of another free atom, but might still be sufficient to unite with it if it were already in combination, and had only its own residual affinity available. Platinum is hexavalent in Cl
4
Pt
ClK
ClK
, though Cl
6
Pt cannot exist. It is, therefore, only in so-called molecular compounds that the highest valencies of an atom can be expected, though no fundamental differences can be imagined between the higher and the principal valencies; this is admitted even by those who have coined a distinctive name for the higher valencies (
e. g.
, Werner’s “co-ordination values”). The general rule that additional valencies come into existence in pairs is not invalidated by certain apparent exceptions. Thus, the most stable arrangement of five atoms round a central one is two in one plane, and three in the plane at right angles; according as the positions occupied are those in the one, the other, or both planes, the atom will show valencies of 2, 3 or 5, and by the suppression or development of pairs of valencies, other values of 1, 4, 7, 9, etc., might be shown.
Problems bearing on residual affinity