30 OUTLINES OF PHYSICAL CHEMISTRY
one hand and V, Nb, Ta on the other. A satisfactory explanation of the necessity of this subdivision is found by as forming double periods such that the perfect similarity of an element with a previous member of the same vertical column only recurs after an interval of fourteen terms. Between the sections of each of the long periods 3-4, 5-6, and 9-10, we find elements which are very similar and whose atomic weights differ very little from each other. These so-called transition elements are placed in the eighth vertical column.
This classification shows that the differences in the properties of the elements correspond to correlated differ- ences in their atomic weights. The chemical and physical properties of the elements thus appear to be periodic functions of their atomic weights.
But here I only wish to call attention to the manner in which the variations of valency are regulated by this periodic classification.
Deducing the valency of the elements from their hydrogen compounds (or, in the absence of these, from their compounds with chlorine or hydrocarbon radicals), we find that the valency increases progressively from the first to the fourth vertical column, then gradually diminishes to its initial value :
I II III IV V VI VII
Mg(CH 3 ) 2 A1(CH 3 ) 3 CH 4 NH 3 OH 2 FH
PH 3 SH 2 C1H
The elements of the first four groups do not combine with a greater number of atoms of chlorine (bromine or iodine). But it is different with the three succeeding groups. If we consider those compounds of the elements with chlorine, oxygen, or hydroxyl, which contain the maximum number of these atoms or radicals, we find that the valency increases from the first term to the last.
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