Valency
The combining capacity of an element, equal to the number of electrons an atom gains, loses or shares when forming compounds.
Unit
Dimensionless
In depth
Valency is set by the number of electrons needed to reach a stable octet or duplet. Many elements, especially transition metals, show variable valency because d-electrons are close in energy to s-electrons. The modern quantitative successor to valency is the oxidation number, which carries a sign as well as a magnitude.
Examples in the real world
Oxygen has valency 2, aluminium 3, and iron shows both 2 (in FeO) and 3 (in Fe₂O₃).