How to Find Valency

How to find valency?

Definition: The valency of an element is a measure of it’s combining power with other elements. The number of electrons that an atom can share, lose or gain during the chemical reaction is termed as valency. It denotes the combining capacity of the atom of an element.

The valency of a radical or of an element is the number of hydrogen atoms that will combine with or displace one atom of that element or radical.

How to find valency? 

The valency of an element can be found by formula;

Valency = 8 – (number of electrons in valence shell)

For example; If the number of electrons present in the valence outermost shell are 4,3,2,1 then their valency will be 4,3,2,1 respectively. If the number of electrons present in the valence outermost shell, then their valency is 8-5=3, 8-6=2, 8-7=1 respectively.

On moving down a given group, in the valence shell (outermost shell) of the element, the number of electrons (valence electrons) remains the same. Therefore, we can say that in a group valency will remain the equal (same).

In a given period, in the outermost shell of the element, the number of electrons will increase from the position left to the position right. But the valency of an element increases upto Group 14, where it becomes 4 and then it decreases, i.e., in Group 17 it becomes 1.

Elements with 1,2 or 3 electrons in their valence sheel are usually metals. To attain stable electronic configuration, these atoms lose their valence electrons and form positive ions (captions). Examples;

Na – e^– = Na⁺

Mg – 2e^– = Mg²⁺

Elements with 5,6 or 7 electrons in their valence shell are usually non-metals. To attain stability, these atoms have to gain the electrons in order to follow the octet rule (have 8 electrons in the outermost shell) and form negative ions (anions). Examples;

Cl + e^– = Cl^–

O + 2e^– = O^2-

Examples of sharing of atoms;

• One atom of chlorine combines with one hydrogen atom to form a molecule of hydrogen chloride. So, the valency of chlorine is one.

H-Cl

• One atom of oxygen combines with 2 atoms of hydrogen to form a molecule of water. So, the valency of oxygen is 2.

O-H-O