Number of electrons pairs left after bonding are called lone pair of electrons
You can easily Calculate number of lone pairs using following steps
Example – Find number of lone pair of electrons on Oxygen in H2O
Step 1: Find number of valence electrons on Oxygen
Number of Valence Electrons on Oxygen is 6 because it is from group 16 ( V1A) and Valence Electrons are equal to group number.
Step 2 : Find total number of electrons used in bonding from all valence electrons of Oxygen
Here two hydrogens are bonded to oxygen and one Hydrogen requires one electron to be stable by completing it’s duet.
So two electrons from oxygen will be taken by hydrogen to form two Sigma bonds
Number of electrons on Oxygen left after bonding will be 6-2= 4
Pair of electrons left after bonding is 4/2 = 2
Number of lone pair of electrons = 2
Let’s take another example
Find Number of lone pair of electrons on Nitrogen in NH3
Valence Electrons on Nitrogen = 5
Electrons of Nitrogen used in bonding with hydrogen 1 X 3 = 3
Number of electrons left on Nitrogen after bonding is 5-3 = 2
Number of lone pair of electrons is 1
Calculate number of lone pairs of electrons in following compounds
1. CO2 Valence Electrons on Carbon 4, Number of electrons required by Oxygen to complete it’s octet is 2 ( oxygen having 6 valence electrons so 2 more electrons are required to complete it’s octet )
Total number of electrons of Carbon used by both oxygen is 2 X 2 = 4
Number of electrons left after bonding is 4-4 = 0
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