Chemical symbols. Each element is given its own chemical symbol, like O for oxygen and Cl for chlorine. Chemical symbols are usually one or two letters long, but sometimes three letters are used. Every chemical symbol starts with a capital letter, with the second or third letters written in lower case.For example, Mg is the correct symbol for magnesium, but mg, mG and MG are wrong.hemical symbols are usually one or two letters long, but sometimes three letters are used. Every chemical symbol starts with a capital letter, with the second or third letters written in lower case.