158 NON-METALLIC ELEMENTS IN BINARY COMPOUNDSceive of the action as consisting merely of a transfer of electricity
so that the metallic constituent becomes positively charged and the
non-metallic constituent negatively charged. The electrostatic
attraction between the charges holds the constituents rigidly in
place in a solid compound, and so the solid substance is a non-
conductor. We have seen in Chapter III that many compounds,
viz., acids, bases, and salts, when they are dissolved in water give
a solution which conducts an electric current. The compounds
are ionized in the solution. Our idea of ionization by no means
signifies that the opposite charges have been separated from each
other; it simply regards the single positive constituent as no
longer bound exclusively to a particular negative constituent, but
as free to wander about through the solution holding under its
attractive force first one negative constituent, then another, and
so on.
Were it not for the discovery of ionization, we perhaps would
never have suspected that the constituents of compounds are
electrically charged. This conclusion, however, seems to be well
warranted, and it is for this reason that one speaks of metallic
elements as positive and non-metallic elements as negative when
they are combined, even in compounds that are not ionized.
The number of charges associated with a simple ion as A1+++ or
Cl" determines its valence, the valence of aluminum thus being
+3 and that of chlorine —1 in the compound aluminum chlo-
ride. Such a compound of two elements is called a binary com-
pound, and the constituents are held by the forces of the primary
valence. It is the purpose of this chapter to study the behavior
of the non-metallic elements in binary compounds in which they
show their primary negative valence.
OXIDES
Nearly all the elements combine with oxygen forming simple
binary compounds.
No divalent negative ion of oxygen, 0 , has ever been found;
nevertheless the valence of oxygen is believed to be —2 because
of the composition of these compounds.
- Treat a little cupric oxide with hydrochloric acid. The
liquid acquires a blue color and the black solid finally disap-
pears entirely. Evaporate the solution and a residue is left
which becomes brown on further heating.