hydrogen or two atoms of oxygen.

Evidently, then, one atom is not always equivalent to another
atom of a different kind in combining powers. A recognition of
this fact by Frankland about 1852, and its further investigation
by others (notably A. Kekule and A. S. Couper), led to the
introduction of the word equivalent into chemical terminology in
a new sense, and in particular to an understanding of the
affinities or "valency" of different elements, which proved of
the most fundamental importance. Thus it was shown that, of the
four elements that enter most prominently into organic compounds,
hydrogen can link itself with only a single bond to any other
element--it has, so to speak, but a single hand with which to
grasp--while oxygen has capacity for two bonds, nitrogen for
three (possibly for five), and carbon for four. The words
monovalent, divalent, trivalent, tretrava-lent, etc., were coined
to express this most important fact, and the various elements
came to be known as monads, diads, triads, etc. Just why
different elements should differ thus in valency no one as yet
knows; it is an empirical fact that they do. And once the nature
of any element has been determined as regards its valency, a most
important insight into the possible behavior of that element has
been secured. Thus a consideration of the fact that hydrogen is
monovalent, while oxygen is divalent, makes it plain that we must
expect to find no more than three compounds of these two
elements--namely, H--O--(written HO by the chemist, and called
hydroxyl); H--O--H (H2O, or water), and H--O--O--H (H2O2, or
hydrogen peroxide). It will be observed that in the first of
these compounds the atom of oxygen stands, so to speak, with one
of its hands free, eagerly reaching out, therefore, for another