There is a rule in naming polyatomic anions containing oxygen.
Those names ending in <em>–ate</em> are given to the most common oxyanion of an element.
While those names ending in <em>–ite</em> are for those that contains fewer O atom for
the same charge. Since P has fewer O atoms, its name ends in <em>–ite</em>.
Thus, P
is phosphite while P
is phosphate.