When atoms of two or more elements combine, a molecule of a compound is formed. The capacities of these elements to combine with each other determine the formula of the compound formed.
The capacity of an element to combine with other elements is known as its valency.
Explanation of Valency
General Defnition of Valency :
Number of hydorgen atoms or chlorine atoms or double the number of oxygen atoms with which one atom of an element combines is called its Valency.
It will be evident from the following that the combining capacities of all elements are not the same.
1. One atom of Cl combines with one atom of H to form a molecule of hydrogen chloride (HCl).
2. One atom of O combines with two atoms of H to form a molecule of water (H2O).
3. One atom of N combines with three atoms of H to form a molecule of ammonia (NH3).
Thus, the combining capacity of O is twice that of Cl, and that of N is thrice that of Cl.
H is assigned a valency of 1. So the valencies of Cl, O and N are 1, 2 and 3 respectively. However, many atoms do not combine with H. Their valencies are calculated by the number of Cl atoms they combine with, since Cl and H have the same valency, i.e., 1.
² The valency of an element is given by the number of H or Cl atoms mainly that an atom of the element combines with.