A measure of the reactivity of an element.

(chemistry) a property of atoms or radicals; their combining power given in terms of the number of hydrogen atoms (or the equivalent)

The combining power which determines the number of other atoms with which an atom of the element can combine, eg Sodium (Na) has a valency of one, oxygen (O) has a valency of two which means that two sodiums will combine with one oxygen, Na2O.

The number of electrons which an atom of an element must either gain or lose to attain a noble gas structure.