bonds. In organic chemistry the exceptions to this rule are very
numerous, and, in fact, we do not know that atoms have bonds at
all; but we can best explain the phenomena by supposing them, and
for a general statement we may say that there must be no free
bonds. In binaries the bonds of each element must balance.

56. The Valence, Quantivalence, of an Element is its Combining
Power Measured by Bonds.--H, having the least number of bonds,
one, is taken as the unit. Valence has always to be taken into
account in writing the symbol of a compound. It is often written
above and after the elements [i.e. written like an exponent], as
K^I, Mg^II.

An element having a valence of one is a monad; of two, a dyad;
three, a triad; four, tetrad; five, pentad; six, hexad, etc. It
is also said to be monovalent, di- or bivalent, etc. This theory
of bonds shows why an atom cannot exist alone. It would have free
or unused bonds, and hence must combine with its fellow to form a
molecule, in case of an element as well as in that of a compound.
This is illustrated by these graphic symbols in which there are
no free bonds: H-H, O=O, N[3-bond symbol]N, C[4-bond symbol]C. A
graphic symbol shows apparent molecular structure.

After all, how do we know that there are twice as many Cl atoms
in the chloride of magnesium as in that of sodium? The compounds
have been analyzed over and over again, and have been found to
correspond to the symbols MgCl2 and NaCl. This will be better
understood after studying the chapter on atomic weights. In
writing the symbol for the union of H with O, if we take an atom
of each, the bonds do not balance, H-=O, the former having one;