The chemical formula of a compound is a symbolic representation of its composition. The combining power or combining capacity of an element is known as its valency. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound. Some elements show more than one valency. Then we must crossover the valencies of the combining atoms. The positive and negative charges must balance each other and the overall structure must be neutral.