Write down the formulae of aluminium chloride.

Explanation:

Aluminium Chloride is an ionic compound of Al⁺³ and Cl^-1. The valency of Al is +3 which means that it loses 3e^- to gain it's stable octet configuration of Al⁺³.These 3e^- are then taken up by 3 Cl atoms, which gain an e^- each to attain the stable octet configuration of Cl^-.

Final Answer:

Hence, the formulae of aluminum chloride is AlCl_3.