CHAPTER VIIITHE OXY-ACIDS AND SALTS OF THE NON-METALSThe elements which are distinctly and invariably non-metallic
in character are boron in the third group; carbon and silicon in the
fourth group; nitrogen and phosphorus in the fifth group; oxygen,
sulphur, and selenium in the sixth group; and fluorine, chlorine,
bromine, and iodine in the seventh group. Non-metallic elements
have already been studied in Chapter IV, in so far as they enter
binary compounds in which they act as the negative constituents.
When two non-metallic elements unite, the one which is the
less strongly non-metallic is regarded as the positive constituent;
it is, so to speak, compelled to play the positive r61e in the com-
pound. For example, sulphur is regarded as the positive constit-
uent of sulphur trioxide. Although in such a compound the
primary valence of each element seems to be satisfied — sul-
phur, +6, and oxygen, —2, in sulphur trioxide — there must be a
large residual combining power (secondary valence) because the
compound combines so readily with other saturated compounds.
The oxides of non-metals thus unite with metal oxides to form salts,
and with water to form acids, for example, SO3+ Na20 —• Na2SO4j
SO3 + H2O —* H2SO4. When such salts or acids ionize, the non-
metal always appears as a constituent of the complex negative ion.
In the sulphate ion the combination of 1 sulphur, with a valence
of +6, and 4 oxygens, with a valence of —2 each, would leave
the unbalanced valence of —2 for the whole ion; this is the
actual valence of the ion. Hence we conclude that elements
present in molecular compounds and in complex ions still possess
the same primary valence as in the simple compounds.
Although all the non-metals except oxygen and fluorine show
positive primary valences in some of their compounds, nevertheless
it is the most characteristic property of the non-metals that they
do not form positive ions. In ions they are always combined with
enough of an electronegative element to furnish a surplus of nega-
tive charges for the whole ion.
An inspection of the periodic arrangement of the elements shows
that non-metals do not occur at all in the first and second groups;
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