Question

Write the chemical formula of aluminium oxide.

Answer 1

Hint: Valency of aluminium is 3 and valency of oxygen is 2 in normal oxides. Cross multiplication of valencies of atoms will give the chemical formula of the compound.

 Complete step by step answer:
Aluminium oxide molecules are electrically neutral and have no charge. Atomic number of Al is 13, so total electrons in an aluminium atom are 13. Electronic configuration of aluminium atom is: $1s^2\text{ 2}{s}^2\text{ 2}{\text{p}}^6\text{ 3}{s}^2{\text{ 3p}}_x^1{\text{ 3p}}_y^0{\text{ 3p}}_z^0$. Valence electrons in an aluminum atom = 3 and valency of aluminium atom is 3. Atomic number of O (oxygen atom) is 8, so total electrons in an oxygen atom are 8. Electronic configuration of oxygen atom is: $$1s^2\text{ 2}{s}^2{\text{ 2p}}_x^2{\text{ 2p}}_y^1{\text{ 2p}}_z^1$$. Valence electrons in an oxygen atom = 6 and valency of oxygen atom is 2. Aluminium atoms form trivalent cation by losing 3 electrons form completion of octet, which is $A{l}^{3+}$. Oxygen atom forms bivalent anion by accepting 2 electrons for completion of octet, which is $O^{2-}$. For electrical neutrality 2 $A{l}^{3+}$ ions are combined with 3 $O^{2-}$ ions to form aluminium oxide molecules and it has a total charge zero. So, the chemical formula of aluminium oxide is $A{l}_2{O}_3$.

Additional information: Aluminium has +3 oxidation state in aluminium oxide molecules. Oxygen has -2 oxidation state in aluminium oxide molecules. Total charge on the molecule of aluminium oxide is zero as it is electrically neutral. Aluminium is electron donor and oxygen is electron acceptor. Aluminium oxide molecules have a total 5 atoms in which 2 atoms of aluminium and 3 atoms of oxygen.


Note:
Aluminium oxide molecules must be electrically neutral. Al forms cation by losing electrons and O forms anion by accepting electrons.