When non-metals react with non-metals, covalent compounds are formed.

Common practice: write the non-metallic atom that has fewer electrons in the outermost shell first.

e.g. carbon dioxide (CO₂), NOT dioxide carbon (O₂C)
       phosphorus trichloride (PCl₃), NOT trichloride phosphorus (Cl₃P)

Exception: ammonia (NH₃), NOT (H₃N)

The rules for writing the formulae of covalent compounds are discussed below.