Write the chemical formulae for the following compounds. (i) Aluminium nitrate (ii) Calcium oxide (iii) Ferric oxide
Al(NO₃)₃. Aluminium has a charge of 3+ and nitrate has a charge of 1−. By criss-crossing the valencies, we get the formula Al(NO₃)₃, using brackets around the polyatomic ion.
CaO. Calcium has a charge of 2+ and oxide has a charge of 2−. Since the valencies of the two ions are the same, they cancel out, and the formula is simply written as CaO.
Fe₂O₃. Ferric ion has a charge of 3+ and oxide has a charge of 2−. Criss-crossing the valencies gives the formula Fe₂O₃.
Explanation
The chemical formulae are derived using the criss-cross method. For aluminium nitrate, Al³⁺ and NO₃⁻ give Al(NO₃)₃, requiring brackets around the polyatomic anion. For calcium oxide, Ca²⁺ and O²⁻ give CaO as equal valencies cancel out. For ferric oxide, Fe³⁺ and O²⁻ give Fe₂O₃.