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This article is about the chemical concept of valence, or valency. Other meanings are at valence.

Oxygen has twice the valence of hydrogen, so a molecule of H₂O requires twice as many hydrogen atoms as oxygen atoms.

In chemistry, valency or valence is the number of chemical bonds that an atom of a certain element can form.

For a long time, people thought that this number was a fixed property of the element in question. They thought that carbon always has four bonds, oxygen always has two, and hydrogen always has one. The problem was seen only later. For example, phosphorous sometimes behaves as if it had three bonds, a valence of three. At other times, though, it seems to have five bonds.