How is a molecular formula of a compound written?

In order to write formula of a compound comprised of cations and anions, the valencies of the ion must be known. Let us consider a compound composed of cation A with valency x⁺ and anion B with valency y^– . Then the following steps may be followed to write a molecular formula.

(i) Write down the symbols of the cation and anion side by side.

A B

(ii) Write their valencies at top corners as

A^x+ B^y–

(iii) Interchange between the ions their valencies and these are placed on the lower side of each radical or used as subscripts.

(iv) If a radical is multi-atomic, use a small bracket around it.

(v) Eliminate the common factor, if any, from the numbers used in subscripts.

vi) The valencies or charges on the ion must balance.

vii) When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO) etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

viii) In compounds formed with polyatomic
ions, the ion is enclosed in a bracket
before writing the number to indicate the
ratio. Thus calcium phosphate, compound of calcium ion (valency 2⁺) and phosphate ion (valency 3^–) is written as Ca₃(PO₄)₂.

The same method is used to write formula even for non-ionic substances.