Open In App

VSEPR Theory

Last Updated : 23 Jul, 2025
Comments
Improve
Suggest changes
Like Article
Like
Report

VSEPR Theory tells us about the basic structure of the molecules based on the force of repulsion between lone pair and bond pair of electrons. It states that any molecule arranged in such a structure minimizes the repulsion between the lone pair and bond pair of the molecule.

Let's learn more about VSEPR theory in detail, including its postulates, limitations, and examples.

What is VSEPR Theory?

VSEPR stands for Valence Shell Electron Pair Repulsion. As the name suggests this theory is used to find the shape of the molecule by taking into account the force of the electron pairs and lone pairs of the molecule. According to this theory, any molecule formed by sharing of electron pair between two or more atoms tends to take such shapes that minimize forces between the electron pairs or lone pairs of electrons.

The various geometries of the molecules according to the VSEPR theory are discussed in the table below:

Number of Electron Or Lone Pair

Electron Pair Geometry

No Lone Pair

1 Lone Pair

2 Lone Pairs

3 Lone Pairs

4 Lone Pairs

2

LinearLinear

-

-

-

-

3

Trigonal PlanarTrigonal PlanarBent

-

-

-

4

TetrahedralTetrahedralTrigonal PyramidalBent

-

-

5

Trigonal BipyramidalTrigonal BipyramidalSawhorseT-shapedLinear

-

6

OctahedralOctahedralSquare PyramidalSquare PlanarT-ShapedLinear

VESPR Theory states that the repulsion between two electrons is because of the Pauli Exclusion Principle.

Postulates of VSEPR Theory

Various postulates of the VSEPR Theory are,

  • All the electron pairs in the molecule are arranged in such a way that they minimize the repulsion between the electron pairs of the atom.
  • The central atom of the polyatomic atom is the atom to which all the other atoms of the molecule are linked.
  • Valance Shell electrons of the molecule are responsible for the shape of the molecule.
  • The valence shell of the molecules is arranged in such a way that the distance between them is maximum and the repulsion between them is minimum.
  • If the central atom of the molecule is surrounded by bond pairs of electrons, then the asymmetrically shaped molecule is formed.
  • If the central atom of the molecule is surrounded by lone pairs and bond pairs of electrons, then the shape of the molecule so formed is distorted.
  • In each resonance state of the molecule, their structure is explained using the VSEPR theory.
  • The force of repulsion between the lone pairs, lone pair, and bond pair, and bond pairs follows the order.

Lone Pair - Lone Pair > Lone Pair - Bond Pair > Bond Pair - Bond Pair

Predicting Shapes of Molecules

We can easily predict the shape of the molecules using the VESPR theory using the following rules :

  • The least electronegative element of the molecule must be chosen as the central element. This atom can easily share its electron with other atoms of the molecule.
  • The lone pairs of the central atom are counted by taking the central atom's outermost shell into account.
  • The electrons shared by other atoms with the central atom are accounted as the bond pair of electrons.
  • The lone pair and the pair are sums to find the VESP number of the molecule which is used to explain the shape of the molecule.

VSEP Number

VSEP number of a molecule is the number that describes the shape of the molecule. We easily find this number by taking the sum of the Lone Pairs and Bond Pairs of the molecule. Various Shapes of molecules according to the VSEP number is tabulated below:

VSEP Number

Shape of Molecule

2Linear Structure
3Trigonal Planar Structure
4Tetrahedral Structure
5Trigonal Bipyramidal Structure
6Octahedral Structure
7Pentagonal Bipyramidal Structure

The various shape of the molecules are discussed in the article below,

Linear Shape of Molecule

The molecule in a linear shape is arranged in such a way that it has two valance shells and the bond pair in this arrangement are arranged in such a manner that their repulsion is minimum, this is achieved by taking the bond pair in opposite directions.

BeF2 is an example of a Linear Shape Molecule.

Linear VSEPR Shape

Trigonal Planar Shape of Molecule

There are three molecules attached to the central atom in the Trigonal Planer molecule. The molecules in the trigonal planar structure are arranged in the form of an equilateral triangle which helps them to reduce the repulsion between the electron pairs of the atoms.

BF3 is an example of a Trigonal Planar Shape Molecule.

Trigonal Planar VSEPR Shape

Tetrahedral Shape of Molecule

There are four molecules attached to the central atom in the Tetrahedral molecule. The molecules in the tetrahedral structure are arranged in the form of a tetrahedron, which helps them to reduce the repulsion between the electron pairs of the atoms.

CH4 is an example of a Tetrahedral Shape Molecule.

Tetrahedral VSEPR Shape

Trigonal Bipyramid Shape of Molecule

There are five molecules attached to the central atom in the Trigonal Bipyramid molecule. The molecules in the trigonal bipyramid structure are arranged in the form of a triangular pyramid, which helps them to reduce the repulsion between the electron pairs of the atoms.

PF5 is an example of a Trigonal Bipyramid Shape Molecule.

Trigonal Bipyramidal VSEPR Shape

Octahedral Shape of Molecule

In Octahedral shape, the central atom attaches 6 different atoms i.e. in total there are 6 bond pairs. These 6 bond pairs and the central atom arrange Octahedral to minimize electron repulsion.

An example of an Octahedral Shape is SF6.

Octahedral VSEPR Shape

Pentagonal Bipyramidal Shape of Molecule

In Pentagonal Bipyramidal Shape, the central atom is attached to seven atoms at the corner to minimize the repulsion between the electron pairs. 

An example of a Pentagonal Bipyramidal Shape is IF7.

Pentagonal Bipyramidal VSEPR Shape

VSEPR Shapes of Molecules

We know that VSPER theory minimizes the force between the bond pair and lone pair electrons of atoms and the structure so formed also follows this rule. So using this theory we can easily predict the shape of the molecule if all the bond pairs and the lone pairs of the molecule are known to us.

The force of repulsion between Lone Pair- Lone Pair, Lone Pair- Bond Pair, Bond Pair- Bond Pair (BP - BP) is in the order mentioned below:

Lone Pair- Lone Pair (LP - LP) > Lone Pair- Bond Pair (LP - BP) > Bond Pair- Bond Pair (BP - BP)

Now we can predict the shape of the molecule by first finding the total electron pairs around the central atom using the formula,

Total Electron Pairs =  ½ (Number of Valence Electrons of Central Atom + Number of atoms attached to Central Atom by single bonds)

For Negative and Positive ions electron pairs are calculated as,

Negative Ions: In negative ions, we add the number of electrons equal to the units of negative charge on the ion to the valance electrons of the central atom.

Positive Ions: In positive ions, we subtract the number of electrons equal to the units of positive charge on the ion from the valance electrons of the central atom.

  • If we have only two atoms then the shape of the molecule so formed is linear.
  • If the central atom is only surrounded by the bond pair of similar atom then the structure of the atom is symmetrical and have regular geometry.
  • If the central atom is only surrounded by the bond pairs and lone pair of electrons then the structure of the atom is unsymmetrical because of the lone pair-lone pair and bond pair-lone pair repulsions, and thus the structure so formed have a distorted geometry.

Limitations of VSEPR Theory

The limitations of VESPR Theory are :

  • It fails to account for the different structures of isoelectronic species (i.e. elements having the same number of electrons). Why do these species having the same number of electrons, differ in their shapes?
  • It is unable to explain the formation of metal complexes.
  • The bent structure of halides is also not explained using the VESPR theory.

Related :


Next Article

Similar Reads