t is in water. It will thus be seen that the
structure of a compound must be known before the valences of
the atoms making up the compound can be definitely decided
upon.

Such formulas as H-O-H and H-O-O-H are known as _structural
formulas_, because they are intended to show what is known in
regard to the arrangement of the atoms in the molecules.

~Valence and the replacing power of atoms.~ Just as elements having the
same valence combine with each other atom for atom, so if they replace
each other in a chemical reaction they will do so in the same ratio.
This is seen in the following equations, in which a univalent hydrogen
atom is replaced by a univalent sodium atom:

NaOH + HCl = NaCl + H_{2}O.

2NaOH + H_{2}SO_{4} = Na_{2}SO_{4} + 2H_{2}O.

Na + H_{2}O = NaOH + H.

Similarly, one atom of divalent calcium will replace two atoms of
univalent hydrogen or one of divalent zinc:

Ca(OH)_{2} + 2 HCl = CaCl_{2} + 2H_{2}O.

CaCl_{2} + ZnSO_{4} = CaSO_{4} + ZnCl_{2}.

In like manner, one atom of a trivalent element will replace three of a
univalent element, or two atoms will replace three atoms of a divalent
element.

~Valence and its applications to formulas of salts.~ While the true nature
of valence is not understood and many questions connected with the
subject remain unanswered, yet many of the main facts are of much help
to the student. Thus the formula of a salt, differs from that of the
acid from which it is derived in that the hydrogen of the acid has been
replaced by a metal. If, then, it is known that a given metal forms a
normal salt with a certain acid, the formula of the salt can at once be
determined if the valence of the metal is known. Since sodium is
univalent, the sodium salts of the acids HCl and H_{2}SO_{4} will be
respectively NaCl and Na_{2}SO_{4}. One atom of divalent zinc will
replace 2 hydrogen atoms, so that the corresponding zinc salts will be
ZnCl_{2} and ZnSO_{4}.

The formula for aluminium sulphate is somewhat more difficult to
determine. Aluminium is trivalent, and the simplest ratio in which the
aluminium atom can replace the hydrogen in sulphuric acid is 2 atoms of
aluminium (6 valences) to 3 molecules of sulphuric acid (6 hydrogen
atoms). The formula of the sulphate will then be Al_{2}(SO_{4})_{3}.

~Valence and its application to equation writing.~ It will be readily seen
that a knowledge of valence is also of very great ass