Valence electrons:
The electrons in the outermost shell of an atom are called its valence electrons.
Example : The number of valence electron in sodium is one. The number of valence electrons in chlorine is seven.
Explanation of Valency with respect to Configuration
As per the modern defnition, The number of electrons gained, lost or contributed for sharing by an atom of the element to acquire the stable configuration of the nearest noble gas element gives us directly the combining capacity or valency of the element.
Valency of an element is determined by the number of valence electrons in an atom of the element.
Valency, a whole number : Valency of an element varies from 1 to 8. It may be zero for an element which does not combine with any other element. (inert gases like He, Ne, Ar, Xe)
The valency of an element = number of valence electrons
(when number of valence electrons are from 1 to 4)
The valency of an element = 8 - number of valence electrons.
(when number of valence electrons are more than 4)