Each element has a definite electronic configuration. The electrons in the outermost shell of an atom are termed as its valence electrons. The valence electrons take part in the chemical reactions. The combining capacity of an element is given by the number of valence electrons which take part in chemical reactions.
The number of valence electrons of an element which actually tart in the chemical reactions is called as the valence of that element.
Valence of an element may also be defined as follows:
“The number of Hydrogen atoms, or chlorine atoms, or double the number of oxygen atoms which combine with one atom of element is called its valence.”
For example, the valence of nitrogen (N) in ammonia (NH3) is three: one atom of nitrogen combines with three atoms of hydrogen.
The valence of a metal Make in its oxide M2O3 can be obtained as follows:
No. of oxygen atoms combining with two atoms of M = 3
No. of oxygen atoms combining with one atom of M = 3/2
So, Valence of metal Make =2× 3/2 = 3.
Generally, the valence of an element is equal to the number of valence electrons, or equal to eight minus the number of valence reaction in which that element is involved.
The modern approach to the concept of valence is based only the electronic concept of atom.