The rules which are to be kept in mind while writing a chemical formula is:

i. The valencies or charges on the ion must balance.

ii. When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first.

iii. For ex: calcium oxide (CaO), sodium chloride (NaCl) where oxygen and chlorine are written on the right whereas calcium and sodium are written on the left.

iv. If the valencies of two elements are same, we will simplify the formula.

v. In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.