Valency is the measure of the ability of to combine with other elements. It is expressed as the number of atoms of hydrogen capable of uniting with (or replacing) its atoms. The number of electrons in the outermost shell of the atom dictates the combining ability of an element.

Oxidation state is the number of electrons to be added or subtracted (removed) from an atom in a combined state to convert it to elemental form. It is also known as oxidation number.