VSEPR Theory: Predicting Shapes, Postulates and Limitations

The VSEPR theory or the Valence Shell Electron Pair Repulsion theory is used in chemistry to predict the geometry of molecules from the number of electron pairs surrounding the central atom. This theory is based on the repulsive interactions of electron pairs in the valence shell of an atom.

Read More: Tetravalency

Key Terms: VSEPR, postulates, shape, geometry, limitations

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What is VSEPR Theory?

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VSEPR or the Valence Shell Electron Pair Repulsion theory is based on the basic idea of minimizing the repulsion between valence shell electrons by increasing the distance between them so that the stability of the molecule is increased. This helps in determining the molecular geometry. Electron-electron repulsion is a greater deciding factor than electrostatic repulsion in the determination of molecular geometry.

3D Arrangement of molecules

Lewis electron dot diagrams gave the 2-D model of the molecule. The VSEPR theory gives the 3-D arrangement of the molecules. Electron-electron repulsion in molecules having central atoms with zero lone pairs results in 5 basic geometries or shapes. The molecules having central atoms with lone pairs have compressed bond angles due to lone pair-bond pair repulsion.

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Postulates of The VSEPR Theory

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The following list of points forms the basis of the VSEPR theory:

1. The shape of molecules is decided by the total number of bond pairs and lone pair valence shell electron pairs that are present around a central atom.
2. Both the lone pair and bond pair valence shell electrons repel each other. Lone pair- Lone pair repulsion is stronger than bond pair-bond pair repulsion.

Bond length and angle

3. Minimum energy → Maximum stability. The electron pairs arrange themselves in a fashion that tends to reduce the repulsion forces between them i.e they tend to align far apart from each other.
4. The entire arrangement of the lone pair and bond pair electrons, in a manner that reduces repulsion to achieve stability of the molecule, is considered to be the valence shell.

Molecular Bonding

5. The single super pair: In the case of multiple bonds i.e double or triple bonds where more than one pair of electrons are involved, all the bond pairs are taken as one single pair or super pair. 
6. A lone pair-lone pair repulsion is strongest followed by lone pair-bond pair. Bond pair-bond pair repulsion is the weakest.
7. VSEPR theory is applicable to any existing resonating structure of a molecule, in case it has 2 or three resonating structures.

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Steps to Use The VSEPR theory To Predict the Shape of Molecules

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Step 1: First, draw the lewis electron dot arrangement for a given ion or molecule.

Step 2: The least electronegative atom is taken as the central atom in the structure.

Step 3: Count the total number of bonded pairs and valence shell electrons by analyzing the atoms bonded with the central atom.

Shapes of Molecules

Step 4: Determine a stable arrangement of electron pairs around the central atom so that they experience the least repulsion.

Step 5: Observe and find the lone pair-lone pair interaction, lone pair-bond pair interaction and bond pair-bond pair interaction. This is useful in determining the bond angles.

VSEPR Number

The VSEPR number helps in determining the shape of the molecule. VSEPR numbers 2,3,4,5,6 represent Linear, Trigonal Planar, Tetrahedral, Trigonal Bipyramidal and Octahedral shapes respectively.

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Geometry of the Molecules Using The VSEPR theory

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The VSEPR theory gives the 3D arrangement of the molecules. Electron-electron repulsion in molecules having central atoms with zero lone pairs results in 5 basic geometries or shapes. The molecules having central atoms with lone pairs have compressed bond angles due to lone pair-bond pair repulsion.

The molecules where the central atom has no lone pair of electrons are represented by 5 types of geometric shapes as given below:

In the above table, A is the central atom and B is the outer atom.

Now let us have a look at the geometrical shape of molecules in which the central atoms have lone pairs. Here A is the central atom, B is the outer atom and E represents the number of lone pair electrons.

On the basis of lone pair and bond pair arrangement in the valence shell the above tables classified the different shapes of molecules.

Linear Shape

• The central atom is connected to two outer atoms 
• Valence shell has two places
• The best possible arrangement to have minimum repulsion outer atoms have an angle of 1800 between them.
• Example: BeF₂ , BeH₂ , ZnCl₂ , BeCl₂

Linear

Trigonal-Planar Shape

• The central atom is linked with three outer atoms.
• To reduce the repulsive interaction the outer atoms are arranged at corners of an equilateral triangle having 1200 angles with each other.
• Examples: BF₃, AlCl₃ 

Trigonal Planar

Tetrahedral Shape

• The central atom is joined to four outer atoms.
• Each outer atom is separated by 109.50.
• Example: CH₄ ,SiH₄ , SiF₄

Tetrahedral

Trigonal Bipyramidal Shape

• The central atom has a connection with 5 outer atoms.
• Three outer atoms are at equatorial corners of a planar triangle, while the rest two are axially located above and below the central atom forming 900 with equatorial corner atoms.
• Example: PF₅, SbCl₅ 

Trigonal Bipyramidal

Octahedral Shape

• The central atom is connected with 6 outer atoms.
• All the outer atoms have 900 separations in between.
• Example : SF₆ , TeF₆ 

Octahedral

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Limitations of The VSEPR Theory

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1. The VSEPR theory is unable to predict the geometrical shapes of the transition metals.
2. The VSEPR theory fails in the case of isoelectronic elements i.e. elements with an identical number of electrons.
3. The VSEPR theory does not account for the inactive lone pairs.
4. The VSEPR theory also ignores the orbital movement of electrons that is bound to affect the overall molecular shape.
5. The VSEPR theory fails in providing exact bond angles, hence the geometrical shapes according to the theory are not the actual shapes of the molecule.

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Things To Remember

1. The VSEPR theory predicts the geometrical shapes of molecules based on the lone pair-lone pair, bond pair-bond pair and lone pair-bond pair interactions.
2. The main basis of the theory is that electrons present in the valence shell of an atom arrange themselves in a fashion which ultimately leads to the maximum separation between them so that the overall energy of the molecule gets minimized and the compound attains a stable state. This is used to predict geometrical shapes.
3. When central atoms do not possess lone pairs of electrons in their valence shells, they show different geometry as compared to central atoms with lone pairs in their valence shells. Further, if bond pair-lone pair interaction gives rise to changing bond angles.
4. The VSEPR theory does not take into consideration the orbiting movement and interactions of electrons in the valence shell of the central atoms. Thus it ignores a big factor in deciding the actual shape of the molecule.
5. The VSEPR number helps in determining the shape of the molecule. VSEPR numbers 2,3,4,5,6 represent Linear, Trigonal Planar, Tetrahedral, Trigonal Bipyramidal and Octahedral shapes respectively.

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