oms by Roman numerals placed above
their symbols, we have the formulas

II II II III III IV IV
HCl, ZnO, BN, CSi.

A divalent element, on the other hand, will combine with two atoms of a
univalent element. Thus we have

II II II II
ZnCl_{2} and H_{2}O

(the numerals above each symbol representing the sum of the valences of
the atoms of the element present). A trivalent atom will combine with
three atoms of a univalent element, as in the compound

III III
H_{3}N.

If a trivalent element combines with a divalent element, the union will
be between two atoms of the trivalent element and three of the divalent
element, since these numbers are the smallest which have equal valences.
Thus the oxide of the trivalent metal aluminium has the formula
Al_{2}O_{3}. Finally one atom of a tetravalent element such as carbon
will combine with four atoms of a univalent element, as in the compound
CH_{4}, or with two atoms of a divalent element, as in the compound
CO_{2}.

We have no knowledge as to why elements differ in their combining power,
and there is no way to determine their valences save by experiment.

~Valence and the structure of compounds.~ Compounds will be met
from time to time which are apparent exceptions to the general
statements just made in regard to valence. Thus, from the
formula for hydrogen dioxide (H_{2}O_{2}), it might be
supposed that the oxygen is univalent; yet it is certainly
divalent in water (H_{2}O). That it may also be divalent in
H_{2}O_{2} may be made clear as follows: The unit valence of
each element may be represented graphically by a line attached
to its symbol. Univalent hydrogen and divalent oxygen will then
have the symbols H- and -O-. When atoms combine, each unit
valence of one atom combines with a unit valence of another
atom. Thus the composition of water may be expressed by the
formula H-O-H, which is meant to show that each of the unit
valences of oxygen is satisfied with the unit valence of a
single hydrogen atom.

The chemical conduct of hydrogen dioxide leads to the
conclusion that the two oxygen atoms of its molecule are in
direct combination with each other, and in addition each is in
combination with a hydrogen atom. This may be expressed by the
formula H-O-O-H. The oxygen in the compound is therefore
divalent, just as i