Question

Oxide ions are also formed when Aluminum oxide is dissolved. What is the formula for an oxide ion?

Answer 1

An oxide is a chemical compound that has at least one oxygen atom along with some other element in its chemical formula. Metal oxides thus contain an oxide anion and a metal cation. Most of the earth’s crust consists of oxides due to the oxidation of elements by the oxygen in air and water.

Complete answer:
Aluminum Oxide when dissolved in water, doesn’t dissolve completely. It is insoluble in water, unlike other oxides like sodium oxide or potassium oxide. This is because the oxide ions are held strongly in the lattice of the Aluminum Oxide and don't react with water.
The oxide anion is a Dianion of Oxygen. It has a chemical formula of ${O^{2 - }}$ . Metal oxides thus contain one oxygen anion and have the oxidation state of -2. Aluminium oxide is often formed as a coat on the aluminium foil that protects the foil from corrosion.
The formula for Aluminium Oxide is $A{l_2}{O_3}$ . The common oxidation state of Aluminium is +3. The overall charge on the compound is zero. From this we can find out the oxidation state of Oxygen.
$A{l_2}{O_3} = 2 \times ( + 3) + 3(x) = 0$
$3x = - 6 \to x = - 2$
Hence the oxidation state of Oxide ion is -2 and its formula will be ${O^{2 - }}$ .

Note:
Aluminium oxide is found to be amphoteric in nature, meaning it can act as both acid and base depending on the reaction. For example, on reaction with Hydrochloric acid it gives Aluminium Chloride, which means aluminium oxide acts as a Base. On reaction with NaOH, it will act as an acid to give $NaAl{(OH)_4}$