The conception of valency may be applied also to radicals, groups of atoms which take part as a whole in chemical reactions. An inspection of the table:
leads to the recognition of the following radicals in the compounds:
-NO3, =SO4, ≡PO4, and -NH4.
If we know the valencies of the elements and of common radicals, we can at once write down the formulae of all the compounds which can theoretically be formed from them.
It is usually most convenient to remember the formulae of a few typical compounds, from these to deduce the valencies of the elements or radicals, and thence to write down the formula of the compound required. Thus, if we wish to write down the formula'of aluminium sulphate, we remember the formulae AlCl3 and H2SO4. Hence we find that Al is tervalent and SO4 bivalent: -Al<, >SO4. In order to satisfy the valencies of Al by those of SO4, we shall have to take 2Al, i.e., 6 valencies, and 3SO4, also 6 valencies. No free valencies must be left over. Hence aluminium sulphate is Al2(SO4)3.