For metals, it is more convenient to define valency in terms of the number of chlorine atoms with which one metal atom combines to form their normal chlorides. Thus, the valencies of sodium, magnesium and aluminium are 1, 2 and 3 as they form NaCl, MgCl2 and AlCl3 in the atomic ratios 1:1, 1:2 and 1:3 respectively.