When two or more elements combine to form compounds, the combining capacity of an atom of an element with other atoms is determined from its ‘valency’. # Old definition of Valency: - *It is the number of H atoms with which one atom of an element can combine.* Hydrogen atom was taken as standard because it is monovalent/univalent (valency = 1). For example, in methane (CH~4~), carbon has a valency of 4 (tetravalent); in ammonia (NH~3~), nitrogen has a valency 3 (trivalent) and in H~2~O, oxygen has a valency 2 (divalent/bivalent) and in HF/HCl/HBr/HI, the valency of each halogen atom (F/Cl/Br/I) is 1. So halogens (F/Cl/Br/I) are also monovalent (valency = 1) like H. - *Valency of an element can also be defined as the number chlorine or other monovalent atoms with which one atom of the element can combine.* In phopshorous pentachloride (PCl~5~), P has a