* Electronegativity Difference: Aluminium (Al) is a metal with an electronegativity of 1.61, while sulfur (S) is a nonmetal with an electronegativity of 2.58. The difference in electronegativity between them is large enough (0.97) to indicate an ionic bond.

* Metal and Nonmetal: Ionic compounds are typically formed between metals and nonmetals. Aluminium is a metal, and sulfur is a nonmetal.

* Transfer of Electrons: In the formation of Al₂S₃, aluminium atoms lose three electrons each to become Al³⁺ ions, while sulfur atoms gain two electrons each to become S²⁻ ions. This transfer of electrons creates electrostatic attraction between the oppositely charged ions, forming the ionic bond.

Therefore, due to the significant electronegativity difference, the presence of a metal and a nonmetal, and the transfer of electrons, aluminium sulphide is classified as an ionic compound.