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This entry contributed by David Rovnyak
In 1811, Italian physicist and mathematician Amedeo Avogadro  published a hypothesis (also termed
Avogadro's law or principle) stating that the volume of a gas is directly proportional to the number of molecules
of the gas. This is represented by the formula
where a is a constant, V is the volume of the gas, and N is the number of gas molecules. Therefore, equal volumes
of gases with the same pressure and temperature contain the same number of molecules.
Gay-Lussac and others also believed such a proportionality must exist, but what made Avogadro's hypothesis
complete and correct was his new definition of a molecule as the smallest characteristic particle of a substance which
may be a single elementary atom or a permanent union of several elementary atoms. He proposed that some gases, such as
hydrogen and oxygen, consisted of diatomic molecules and a water molecule could consist of three elementary atoms, two
of hydrogen and one of oxygen. Avogadro's concepts reconciled Dalton's atomic hypothesis with
Gay-Lussac's law of combining volumes.
By comparing the masses of equal volumes of different gases at identical temperatures and pressures, the weight ratios
of the gas molecules could be measured. This was the basis for the first correct measurements of atomic weights.
The combination of Boyle's law, Charles' Law, and Avogadro's hypothesis gives the ideal gas law
where P is the pressure, V the volume, n the number of moles of gas, R the universal gas constant,
and T is the temperature.
For many reasons, Avogadro's hypothesis was ignored for about 50 years. For example, scientists such as
Dalton (who was specifically mentioned in Avogadro's memoir) were threatened by the possibility that many of
their atomic weights were incorrect. Avogadro's hypothesis was not accepted until Cannizzaro convincingly
presented a system of atomic weights around 1858-1860 based on Avogadro's hypothesis.
 Avogadro's Number, Boyle's Law, Charles' Law, Dalton's Atomic Hypothesis, Ideal Gas Law, Law of Combining Volumes
© 1996-2007 Eric W. Weisstein
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