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Explain the effect of pi bond and lone pairs on hybridization according to bent rule

Explain the effect of pi bond and lone pairs on hybridization according to bent rule

Grade:12th pass

1 Answers

Arun
25750 Points
6 years ago
Dear Deepika
 
 The effect lone pairs exert is the same in VBT as it was 
in VSEPR. In this post we will study a few examples in which central atom possess 
lone pairs.
First, we will take the example of NH3 molecule. When you draw its Lewis dot 
structure, you will find three bonding pairs of electrons and one lone pair on N. Now 
write the ground state configuration of N.
7N: 1s2
, 2s2
, 2p3
N already has 3 unpaired electrons in ground state to make bond with 3 H atoms. You 
may think why does N need hybridization in this case when it has 3 unpaired electrons 
of the same orbital p? Because hybridized orbitals are more competent in overlapping 
which results in a stronger bond.
In NH3 molecule, N chooses sp
3
hybridization. This may get you puzzled again, if N 
has 3 unpaired electrons in p why does it include s orbital which has paired electrons? 
You know that the atoms follow Hund’s rule for filling electrons in orbitals, similarly 
they have to consider energy order of orbitals in hybridization also. When an atom 
undergoes hybridization, it has to pick orbitals in the increasing order of their energies. 
In NH3, N has to first pick 2s then 2px
, 2py and then 2pz
. N can’t ignore 2s even if it is 
filled because 2s has the lowest energy and N has to include all three 2p orbitals because it needs them for bonding with three H atoms.
 
Now four sp
3
hybridized orbitals get arranged in tetrahedral shape. One sp
3
orbital is 
occupied by lone pair and other three are used for making sigma bond with 1s orbitals 
of three H atoms.
We have already studied that lone pairs are like aloof ghosts; they repel bonding pairs 
and decrease the bond angles. In NH3molecule, the lone pair repels bonding pairs, 
compresses them and distorts the tetrahedral shape to pyramidal shape.
 
 
Regards
Arun (askIITians forum expert)

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