Valence, (from the Latin valentia — strength), the ability of the atoms of the elements to form chemical bonds; quantitatively it is characterised by a number. Valence may be regarded as the ability of an atom to give up or take on a definite number of electrons from its outer electron shells (valence electrons). For an ionic bond the valence is the number of electrons given up or taken on by the atom in question; for a covalent bond the valence equals the number of shared electron pairs. Many elements can have different valences, depending on the compounds they enter into. In this case the terms “oxidation state” or “oxidation number” are often used. Valence is sometimes a conventional notion and cannot be characterised quantitatively.