valency

Valency most often refers to the number of connections something can make. In chemistry, it means the combining power of an atom or element, especially the number of bonds it can form with other atoms in a compound. This idea helps explain why atoms join together in regular patterns to make molecules.

For example, hydrogen usually has a valency of 1, oxygen usually has a valency of 2, nitrogen often has a valency of 3, and carbon commonly has a valency of 4. That is why water is written as $\mathrm{H}_{2}\mathrm{O}$ and methane as $\mathrm{C}\mathrm{H}_{4}$: the numbers match the usual bonding capacity of the atoms involved. The concept developed from early observations of how elements combine, and modern chemistry connects it to the behavior of electrons in the outer part of the atom.

Some elements can show more than one valency. Iron, for instance, can act with a valency of 2 or 3 in different compounds. This is why older chemistry writing sometimes speaks of "variable valency." The term is close in meaning to related ideas such as bonding capacity, but it is not always identical to oxidation state, which is a different way of describing an atom in a compound.

In linguistics, valency has a parallel meaning. It describes how many participants a verb takes or requires in a sentence. A verb like rain has very low valency, sleep usually takes one participant, eat often takes two, and give commonly takes three. In this sense, the word still keeps its core idea: it is about how many links or partners are needed to complete the pattern.