VSEPR Theory: Definition, Table, Chart, Formula, Examples, Questions

VSEPR refers to a chemistry concept that highlights a computational model for the prediction of the geometry of individual molecules. The theory was developed by Ronald Gillespie together with Ronald Nyholm. According to this theory, electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion. VSEPR considers both bonding electron pairs—pairs of electrons that are involved in the formation of bonds—and lone pairs of electrons that do not participate in bonding when predicting the geometry of a molecule. The common molecular geometries that result from VSEPR include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. For example, in a molecule like methane, CH₄, which has four bonding pairs and no lone pairs, the geometry is tetrahedral. In the case of water (H₂O), owing to the presence of two lone pairs of oxygen, it is bent. Chemists use the VSEPR theory to determine and predict the three-dimensional arrangement of atoms in a molecule. This helps in understanding chemical reactivity and properties.

VSEPR Theory

Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate bond angles around a central atom of a molecule from the estimation of the number of bonds and lone pairs of electrons in its Lewis structure.

The main postulates of VSEPR theory are:

• The actual shape of a molecule depends upon the number of electron pairs (bonded or non–bonded) around the central atom.

• The electron pairs tend to repel each other due to their negative charge.

• Electron pairs arrange themselves in such a way that there exists a minimum repulsion between them.

• The valence shell is considered as a sphere with the electron pairs placed at a distance.

• A multiple bond is treated as if it is a single electron pair & the electron pairs that constitute the bond as a single pair.

• The repulsive interaction of electron pairs decreases in the order as mentioned below:

Lone pair (lp) – Lone pair (lp) > Lone pair (lp) – Bond pair (bp) > Bond pair (bp) – Bond pair (bp).

• Double bonds cause more repulsion than single bonds, and triple bonds cause more repulsion than double bonds. This repulsion decreases sharply with increasing bond angle between the electron pairs.

Let us understand VSEPR theory using a gaseous BeF₂ molecule. In the Lewis structure of BeF₂ as shown in the figure, there are only two electron pairs around the central beryllium atom. With two bonds and no lone pairs of electrons on the central atom, the bonds are as far apart as possible, and the electrostatic repulsion between these regions of high electron density is reduced to a minimum when they are on opposite sides of the central atom, thus the bond angle is 180°.

The BeF₂ molecule adopts a linear structure in which the two bonds are at maximum distance from each other and maintain an angle of 180°.

As given in the table below, two regions of electron density around a central atom in a molecule form a linear geometry, three regions form a trigonal planar geometry, four regions form a tetrahedral geometry, five regions form a trigonal bipyramidal geometry, and six regions form an octahedral geometry.

Recommended topic video on ( VSEPR Theory)

Some Solved Examples

Example 1: The molecule having smallest bond angle is :

1)NCl₃

2)AsCl₃

3) SbCl₃

4)PCl₃

Solution

As we discussed in the concept

VSEPR Theory -

1. The shape of the molecule is determined by repulsions between all of the electron pairs present in the valence shell.

2. Order of repulsion

lone pair - Lone pair $>$ Lone pair - Bond pair $>$ Bond pair - bond pair

3 Repulsion among the bond pair is directly proportional to the bond order and electronegativity difference between the central atom and the other atom.

As we move down the group the size of the atom increases and as the size of the central atom increases lone pair-bond pair repulsion also increases. Thus bond angle decreases.

Increasing order of atomic radius:
$
N<P<A s<S b
$

Decreasing order of bond angle :
$
\mathrm{NCl}_3>\mathrm{PCl}_3>\mathrm{AsCl}_3>\mathrm{SbCl}_3
$

Hence, the answer is the option (3).

Example 2: The correct order of bond angles (smallest first ) in $\mathrm{H}_2 \mathrm{~S}, \mathrm{NH}_3, \mathrm{BF}_3$, and $\mathrm{SiH}_4$ is
1) $\mathrm{H}_2 \mathrm{~S}<\mathrm{SiH}_4<\mathrm{NH}_3<\mathrm{BF} F_3$
2) $\mathrm{NH}_3<\mathrm{H}_2 \mathrm{~S}<\mathrm{SiH}_4<B F_3$
3) $\mathrm{H}_2 \mathrm{~S}<\mathrm{NH}_3<\mathrm{SiH}_4<\mathrm{BF}_3$
4) $\mathrm{H}_2 \mathrm{~S}<\mathrm{NH}_3<\mathrm{BF}_3<\mathrm{SiH}_4$

Solution

The correct order of bond angle ( smallest first )is
$
\begin{aligned}
& H_2 S<N H_3<S i H_4<B F_3 \\
& 92.6^{\circ}<107^{\circ}<109^{\circ} 28^{\prime}<120^{\circ}
\end{aligned}
$

Bond angle order = H₂S < NH₃ < SiH₄ < BF₃

Example 3: The type of hybridization and number of lone pair (s) of electrons of Xe in XeOF₄ respectively, are :

1) $s p^3 d^2$ and 1
2) $s p^3 d$ and 2
3) $s p^3 d^2$ and 2
4) $s p^3 d$ and 1

Solution

As we have learnt in Hybridisation:

Hybridization is sp³d²

It contains 5 sigma bonds and 1 pi bond.

It has 1 lone pair of electrons.

Hence, the correct answer is Option (1)

Example 4: The shape of a molecule is determined by

1)Valence bond theory

2)Molecular orbital theory

3) Valence shell electron pair repulsion theory

4)None

Solution

VSEPR (Valence Shell Electron Pair Repulsion) Theory - Valence shell electron-pair repulsion theory (VSEPR theory) enables us to predict the molecular structure, including approximate bond angles around a central atom of a molecule from the estimation of the number of bonds and lone pairs of electrons in its Lewis structure. The shape of a molecule depends upon the number of valence shell electron pairs (bonded or non-bonded) around the central atom. These pairs of electrons in the valence shell repel one another since their electron clouds are negatively charged.
Hence, the answer is the option (3).

Example 5: The reason for the change in bond angle in the different molecules having the same hybridization is given by:

1)Molecular orbital theory

2) Valence shell electron pair repulsion theory.

3)Valence bond theory

4)None

Solution

1. Electron pairs are always repelling each other and try to remain far apart

2. Different electron pairs have different orders of repulsion: L.P-L.P > L.P-B.P > B.P-B.P

3. If lone pairs are not present then geometry is equal to shape else geometry is not equal to the shape.

This all is explained by V.S.E.P.R. theory.
Hence, the answer is the option (2).

Summary

The Valence Shell Electron Pair Repulsion is one of the primary models used in computing the geometry of molecules, given electron pair repulsion. It was developed by Gillespie and Nyholm. The theory postulates that electron pairs around a central atom arrange themselves as far away from each other as possible, thereby minimizing the repulsion and thus determining the shape of the molecule. The theory describes all geometries of the molecules by considering both bonding and lone pairs of electrons. Common geometries include linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. For example, methane, CH4, is tetrahedral because it has four bonding pairs of electrons, whereas water, H2O, is bent because of two bonding pairs and two lone pairs on the oxygen atom. Knowledge of VSEPR theory is essential in predicting molecular geometries that turn out useful in explaining the chemical reactivity and physical properties of substances. This theory can easily enable the illustration of the three-dimensional arrangement of atoms in a molecule to help in the study and application of chemical interactions and reactions.