# Oxide ions are also formed when aluminium oxide is dissolved. What is the formula of an oxide ion?

Oxygen, $Z = 8$, commonly forms the oxide ion, ${O}^{2 -}$.
We would have got the same formalism had we considered the formula of $A {l}_{2} {O}_{3}$, given that we know that aluminum commonly forms an $A {l}^{3 +}$ cation. Molten alumina is used in the preparation of aluminum metal; see this site for details.