molecule: Avogadro's Hypothesis

A different theory from Dalton's that could explain the combining volumes of gases was proposed by the Italian physicist Amadeo Avogadro in 1811. According to his theory, under given conditions of temperature and pressure, a given volume of any gas contains a definite number of particles. From the earlier observation that one volume of hydrogen gas and one volume of chlorine gas react to form two volumes of hydrogen chloride gas he deduced that the particles in gaseous hydrogen or chlorine could not be single atoms, but must be some combination of atoms. He called this combination a molecule. He reasoned that the two volumes of hydrogen chloride that are formed must contain twice as many particles as either single volume of hydrogen or chlorine. Thus, if there were 100 particles each of hydrogen and chlorine, there would be 200 particles of hydrogen chloride produced; but there could be only 100 particles produced if the original particles of hydrogen and chlorine were indivisible atoms, since each particle of hydrogen chloride contains both hydrogen and chlorine. An assumption that there are two atoms in a molecule of gaseous hydrogen or chlorine and one atom each of hydrogen and chlorine in a molecule of hydrogen chloride preserves both the hypothesis of indivisible atoms and the hypothesis of equal numbers of particles in equal volumes of gases. Similar reasoning would allow a larger even number of atoms in the molecules of hydrogen or chlorine, but Avogadro favored a rule of simplicity, using the smallest possible number. In the model of gases proposed by Avogadro, the particles were not in contact and much of the volume of the gas was empty space.

Sections in this article:

• Introduction
• Cannizaro's Compromise
• Avogadro's Hypothesis
• Dalton's Theory
• Evolution of Molecular Theory
• Nature of Molecules

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