��I
�in
�IV
�V VI VII
�LiCl
�PCI5 SC1 4 IBr
�BeO
The hydroxyl compounds enumerated for the elements of the last columns have no real existence ; they lose one or more molecules of water in forming the acids of maxi-
Thus Si(OH) 4 , P(OH) 5 , S(OH) 6 , 1(OH) 7 ,
become Si0 3 H 2 , P0 4 H 3 , S0 4 H 2 , I0 4 H.
Collecting the valencies of the groups with respect to hydrogen and with respect to electronegative radicals in a single table, we get the following :
��Valency with respect
to hydrogen Valency -with respect to
chlorine or oxygen .
��I.
�II.
�m.
�rv.
�v.
��Mendeteeffs system conveniently expresses the periodic variations of the valency. Elements which belong to the same group have, as a rule, the same valency, and this is constant with respect to hydrogen, and more or less variable with respect to oxygen and the electronegative elements.
��1 The reader may object that we have made use of formulse for certain substances whose true molecular weight (by the vapour density) we do not know. To this we reply that we can deduce the size of the molecule of the substances in question : firstly, from chemical considerations. For example, sulphuric acid has the formula H 2 S0 4 because we may replace half of the hydrogen (NaHS0 4 , KHS0 4 ) or all of it (Na^O,, I^SOJ. If the formula were double or three times H 2 S0 4 , then a fourth or a sixth of the hydrogen would be replaceable, but this is not the case ; and secondly, from physico-chemical considerations which we shall go into later (see the chapters on osmotic pressure, cryoscopy, <fec).
�� �
