Define valency. How is it calculated for various elements?

• Valency of an element is the number of electrons it needs to gain or lose in order to achieve the stable octet configuration of the nearest noble gas.
• It is the combining capacity of an element to form bonds in a compound.

Method of calculating valency:-

Octet rule:

• According to this rule, an element is said to be stable if it has octet complete, or its outermost shell has 8 electrons.
• The electrons in the outermost shell of an atom are called valence electrons.
• For elements having one to four valence electrons, valency = valence electrons.
• For elements having five to seven valence electrons, valency = (8-valence electrons).

Example: Lithium has atomic number 3. So, the number of valence electrons is 1.

Hence, valency of $\mathrm{Li}$ = 1.

Nitrogen has atomic number 7. So, the number of valence electrons is 5.

Hence, valency of $\mathrm{N}$ = 8-5 = 3.