46 QUANTITATIVE ASPECTS OF CHEMISTRY
Avogadro's Principle. In 1811 Avogadro suggested the hy-
pothesis that at the same temperature and pressure all gases con-
tain in equal volumes an equal number of molecules. He defined
the molecule as the smallest particle of a substance and drew a
clear distinction between molecules and atoms; a molecule of a
compound would of necessity contain two or more atoms, at least
one atom of each constituent element. But the startling feature
of Avogadro's hypothesis was that it demanded as a necessary
deduction that the molecules of the elementary gases, oxygen,
nitrogen, hydrogen, and chlorine, should consist of two atoms each.
Such an idea was regarded by his contemporaries as preposterous
and they would have nothing to do with his hypothesis.
It was not until 1858 that Cannizzaro showed how Avogadro's
principle accounted for the reactions of gases and a great variety
of chemical combinations. Since then it has come to be recog-
nized as one of the most fundamental laws of chemistry. It has
been said that modern chemistry dates from 1858.
In order to show that the molecules of oxygen, nitrogen, hydro-
gen and chlorine must be diatomic if Avogadro's principle is
tenable, let us consider the following data which have been estab-
lished by measuring combining volumes of gases and of course
reducing all volumes to standard conditions:
2 volumes of hydrogen + 1 volume of oxygen give 2 volumes
of water vapor.
1 volume of hydrogen + 1 volume of chlorine give 2 vol-
umes of hydrogen chloride.
2 volumes of ammonia give 3 volumes of hydrogen + 1
volume of nitrogen.In the first set of data let us assume that the 1 volume of oxygen
contains 1 million molecules. Then the 2 volumes of water vapor
according to Avogadro's principle will contain 2 million molecules
of water. But the water vapor is a homogeneous substance, and,
since it contains oxygen, every molecule of it must contain at
least 1 atom of oxygen. Therefore, there must be at least 2 million
atoms of oxygen which were derived from the 1 million molecules.
Therefore, every molecule of oxygen must contain at least 2 atoms.
Following the same line of reasoning 1 million molecules of
hydrogen and 1 million molecules of chlorine produce 2 million
molecules of hydrogen chloride which must contain at least 2