Write the formulae for the compounds formed by the reaction of:

(A) Magnesium and nitrogen

(B) Lithium and nitrogen

(C) Sodium and sulfur

(D) Aluminium and oxygen

(A) The formula for magnesium nitride:

Hence, criss-crossing the charges 2 and 3 gives Mg₃N₂

(B) The formula for lithium nitride:

Since the valency of lithium is 1, the formula is written as Li₃N.

(C) The formula for sodium sulphide:

Hence, criss-crossing the charges gives Na₂S.

(D) The formula for aluminium oxide:

Hence, criss-crossing the charges 3 and 2 gives Al₂O₃.