Q.Write the formula of the compound Iron (III) sulphate.

Writing Chemical Formulae from Valency

The chemical formula of a compound tells us the ratio in which atoms or ions combine. For ionic and simple molecular compounds this ratio is fixed by the valency (combining capacity) of each element or the charge on each ion. The compound as a whole must be electrically neutral, so the total positive charge must exactly balance the total negative charge.

The criss-cross method: write the two ions side by side with their charges (valencies) on top, then swap each number to become the subscript of the other ion. Finally reduce the subscripts to the simplest whole-number ratio.

Worked example — iron(III) sulphate:

Iron(III) is $Fe^{3+}$ and sulphate is $SO_4^{2-}$. Crossing over gives the 3 to sulphate and the 2 to iron:

Check: $2 \times (+3) = +6$ and $3 \times (-2) = -6$; the charges cancel. A polyatomic ion taken more than once is enclosed in brackets, as with $(SO_4)_3$. …