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VSEPR Model

VSEPR theory or valence shell electron pair repulsion theory was proposed by Ronald Gillespie and Ronald Sydney Nyholm at 1957 to explain the molecular geometries and properties of molecules based on the geometry. Molecular geometry is the three dimensional arrangement of atoms in a molecule. It determines many of the properties of molecule like melting point, boiling point, dipole moment and reactivity.

According to VSEPR theory the central atom in a molecule is considered to be a sphere with bonded electrons as negative charges. As there is electrostatic repulsion between such negative charges the bonded electrons tend to move away as for as possible. As covalent bond is directional in nature the arrangement of electrons gives a specific shape for the molecule.
Hence, VSEPR theory is applicable for covalent compounds only. But is should not be confused with valence bond theory which deals with physical overlap of orbitals and their mathematical expressions.

 

Applying the VSEPR Model

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To apply the VSEPR model, one has to draw the Lewis dot formula of the molecule first. Then the total number of electrons should be find out which should be kept around the central atom as far as possible. According to the further developments the method to determine the VSEPR model involves the following steps.

  1. The central atom has to be find out which should always have subscript one in a molecule. For example in PCl5, phosphorus is the central atom.
  2. In case if a molecule/ radical is having central atom with subscript other than one it is treated as combination of more than one VSEPR models. For example N2O4 is considered as combination of two NO2 units.
  3. The number of covalent bonds around the central atom is determined. For this multiple bonds like double bond, triple bond are treated as single bond. So in our NO2 case, Nitrogen is central atom which is connected to one oxygen by double bond and with another oxygen by single bond. Hence the number of covalent bonds around nitrogen is 2.
  4. Then the number of lone pair/unpaired electrons in the central atom is find out. The sum of covalent bonds with number of lone pair electrons gives steric factor. In our case of NO2, Nitrogen is having one unpaired electron. Hence the steric factor is 3.
  5. Taking the steric factor as a criteria the shape of the molecule is determined. The following table will help in finding out the structure according to steric factor.

S.No
Steric factor
Geometry
1
2
Linear
2
3
Trigonal
3
4 Tetrahedral
4
5
Trigonal bipyramidal
5
6
Octahedral
6
7
Pentagonal bi pyramidal

VSEPR Model Molecular Geometry

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Lone pair of electrons are the pair of electrons that do not take part in chemical bonding. Lone pair of electrons give more repulsive force than bond pair of electrons. Hence if a molecule contains both bond pair electrons and lone pair electrons, lone pair electrons will give more repulsive force over bond pair electrons.

As a result of this, the angle between the bond pair of electrons will shrink. The geometry of the molecule considering the lone pair of electrons as a bond/steric factor is called as electronic geometry.

On the other hand the geometry of the molecule by considering the atoms involved alone is called as molecular geometry. If lone pair of electrons are present in a molecule the molecular geometry may be different from electronic geometry. For example the electronic geometry of ammonia molecule (NH3) is tetrahedral. But the molecular geometry of ammonia molecule is pyramidal.


Electronic Geometry of Ammonia Molecule

VSEPR Model Chart

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The following chart will give a detailed information about molecular geometries in VSEPR model.

S.No
Steric factor
Bond pair electrons
Lone pair electrons
Electronic structure
Molecular structure
Example
1
2
2
-
linear
linear
BeCl2
2
3
3
-
trigonal
trigonal
BF3
3
4
4
-
tetrahedral
tetrahedral
CH4
4
4
3
1
tetrahedral
pyramidal
NH3
5
4
2
2
tetrahedral
bend shape
H2O
6
5
5
-
trigonal Bi pyramidal
trigonal bipyramidal
PCl5
7
5
4
1
trigonal bipyramidal
seesaw
SF4
8
5
3
2
trigonal bipyramidal
T shape
ClF3
9
6
6
-
Octahedral
Octahedral
SF6
10
6
5
1
octahedral
square pyramidal
XeOF4

VSEPR Model Examples

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1. XeF4 (Xenon tetra fluoride)


There are four bonded electrons and two lone pairs of electrons around xenon. Hence the steric factor is 6. The corresponding electronic geometry is octahedral. The lone pairs of electrons occupy the axial bonds giving the molecular structure of square planar.

Xenon Tetra Fluoride

2. SO2 ( sulfur dioxide)


The covalence of sulfur is 2 (although there is two double bonds between oxygen atoms by the sulfur atom, they are counted as two single bonds). There is one lone pairs of electron. Hence the electronic geometry of the molecule is trigonal and the corresponding molecular geometry is bent shape.

Sulfur Dioxide

3. ClF3 (chlorine trifluoride)


There are three bonded electrons and two lone pairs of electrons around the molecule. Hence the steric factor is 5 with electronic geometry of trigonal bipyramidal. In the trigonal bipyramidal the lone pairs of electrons occupy the plane giving rise to a T shape to the molecule.

Chlorine Trifluoride

4. SO42- (sulphate ion)


The covalent of sulfur is 4 in the molecule (although all the bonds around the sulfur with oxygen are different in nature, they are treated as single mono valent bonds). Hence the electronic geometry is tetrahedral. As there is no lone pairs of electrons, the molecular geometry is also tetrahedral.

Sulphate Ion

Drawbacks of VSEPR Theory

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VSEPR theory cannot be applied to some of the compounds formed by transition elements. This is due to the fact that
  1. There are sp hybrid orbitals which are not included in VSEPR model.
  2. There are 3-center-2-electron bonds (3-c-2-e) which will change the geometry of the molecule.
VSEPR theory predicted the geometry of many metal hydrides like BaCl2 to be linear. But they are not linear and they have a bend shape.
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