A History of Chemistry: Avogadro's Law

While working with compounds, Amedeo Avogadro developed his theory that all gases with the same volume, at the same temperature and pressure, have the same number of particles. In his experiment decomposing water, Avogadro found that the hydrogen gas and the water vapor had the same volume.

How was this possible? Adding the hydrogen and oxygen together to make water vapor should result in a larger volume, but it didn’t.

Avogadro believed that when gaseous atoms combined into compounds like water, they took up the same space Amedeo Avogadro as the individual atoms. This is possible because atoms are mostly empty space. Therefore, as gases, two atoms chemically combined have the same volume as one atom.

Avogadro needed a word for these combinations of atoms, so he called them molecules. A molecule is any combination of two or more atoms connected by chemical bonds. For example, hydrogen gas is a molecule because it is made up of two hydrogen atoms. Hydrochloric acid is also a molecule because it is made up of one hydrogen atom and one chlorine atom.

So, as gases, molecules and atoms take up the same amount of space when they are at the same temperature and pressure. We call this idea Avogadro’s Law.

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