Valency

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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, thus a molecule of H2O requires twice as many hydrogen atoms as oxygen atoms

In chemistry, valency (sometimes called valence) is the number of chemical bonds the atoms 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.

IUPAC saw this problem, and proposed oxidation numbers. This means there is one number per chemical element. The problem of this approach is that it leaves aside most chemical properties of the elements in question.

A valence band is the highest occupied molecular orbital normally occupied by valence electrons for a given solid at absolute zero temperature.

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