Valence shell electron pair repulsion theory
Later it was developed by Nyholm and Gillespie.
POSTULATES 1)
The shape of the molecule depends upon number of electron pairs in valency shell of central atom
Ans:
Vale
1)1
L
A lone pair of electrons occupies more space around the central atom atom and bonding atom.
than bond pair of electrons.
Bond
C-C
C=C
Bond length
1.54 A
6
1.34 A
1.20 A
The bond angle of the molecule depends upon repulsions between electron pairs and electronegativity difference between central
The repulsions among electron pairs decrease in the order
(p-p>(p-bp>b.p-b.p.
Repulsion among triple, double, single bonds decrease in the order triple bond > double bond single bond.
Repulsion forces decreases sharply with increase in angle between electron pairs. 8) Shape and bond angle of the molecule basing on bond pairs and lone pairs around centra atom as are follows:
Formula Example
Bond Pairs
Lone Pairs
Shape
Bond angle
AB
BeC
2
0
Linear
180°
AB,
BC
3
0
Trigonal Planar
120°
AB,
CH
4
0
Tetrahedral
109°28′
AB
PC
5
0
Trigonal Bipyramidal
90°,120°
AB
SF
6
0
Octahedral
90°,180°
ABE
H₂O
2
2
Angular
104.5°
ABE
NH,
3
1
Pyramidal
107°
Note: E lone pair of electrons. B bond pair of electrons. A central atom.
Eg: Structure of NH, molecule:
The central atom in NH, molecule is N. It contains one lone pair and three bond pairs of electrons.
ii) Nitrogen in NH, undergoes sp¹ hybridisation, so shape is tetrahedral and bond angle is 109°28'.
iii) According of VSEPR theory, the repulsion between lone pair and bond pair of electrons causes the decrease in bond angle from 109°28' to 107° and shape changes from tetrahedral to pyramidal.
H
H
ii) Oxygen in H₂O undergoes sp¹ hybridisation, so shape is tetrahedral and bond angle is
109°28'.
iii) According to VSEPR theory, the repulsion between lone pair and bond pair of electrons bond angle decreases from 109°28' to 104.5° and shape is changed H from tetrahedral to angular.
The Valence Shell Electron Pair Repulsion (VSEPR) theory initially developed by Nyholm and Gillespie, is based on several postulates:
. The shape of a molecule depends on the number of electron pairs in the valence shell of the central atom.
Molecule shape is also influenced by repulsions between electron pairs.
Lone pairs of electrons occupy more space around the central atom compared to bond.
The bond length varies with different types of bonds: single (1.54 Å), double (1. Å), and triple (1.20 Å).
Bond angles in molecules are affected byulsions between electron pairs and electronegativity differences between central atoms.
The repulsion order among electron pairs is: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.
Repulsion forces decrease as angle between electron pairs increases.
Molecule shape and bond angles depend on the arrangement of bond pairs and lone pairs around the central atom, as exemplified by various molecular formulas.
Furthermore, VSEPR theory explains the structure of specific molecules. For instance, in the case of NH3, nitrogen's sp3 hybridization determines its tetrahedral with a bond angle of 109°28', which is altered to a pyramidal shape with a bond angle of107° due to repulsion between the lone pair and bond pair. Similarly, the H2O molecule with oxygen's sp3 hybridization, initially exhibits a tetrahedral shape a bond angle of 109°28', but this changes to angular shape with a bond angle of 104.5° due to repulsion between the lone pair and bond pair.