The VSEPR theory was first presented by Sidgwick and Powell in 1940 and is used to predict the shape of molecules from the electron pairs that surround the central atoms. This theory is based on the assumption that the molecule will take a shape such that the electronic repulsion in the valence shell of the atom is minimized.

Table of Contents

Postulates of VSEPR Theory

Limitations of VSEPR Theory

Predicting the Shapes of Molecules

What is VSEP Number?

Using VSEPR Theory to Predict the Shapes of Molecules

Frequently Asked Questions

VSEPR Theory (Valence Shell Electron Pair Repulsion Theory) is a model used in chemistry to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms.

The Valence Shell Electron Pair Repulsion Theory (VSEPR Theory) is based on the premise that there is a repulsion between the pairs of valence electrons in all atoms. These atoms will always tend to arrange themselves in a way that minimizes this electron pair repulsion, which determines the geometry of the resulting molecule.

The illustration provided below demonstrates the geometries molecules can assume according to the VSEPR theory.

VSEPR Theory – Different Geometries that Molecules can Assume

The two primary founders of the VSEPR theory, Ronald Nyholm and Ronald Gillespie, are also honoured by the alternate name of the theory - the Gillespie-Nyholm theory.

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According to the VSEPR theory, the repulsion between two electrons is caused primarily by the Pauli exclusion principle, which has greater importance than electrostatic repulsion in the determination of molecular geometry.

Postulates of VSEPR Theory:

The Postulates of the VSEPR Theory

In polyatomic molecules (i.e. molecules composed of three or more atoms), one of the atoms is known as the central atom to which all other atoms belonging to the molecule are connected.

The number of valence shell electron pairs determines the shape of the molecule.

The electron pairs have a tendency to orient themselves in a way that minimizes the electron-electron repulsion and maximizes the distance between them.

The valence shell can be thought of as a sphere wherein the electron pairs are localized on the surface in order to maximize the distance between them.

If the central atom of the molecule is surrounded by bond pairs of electrons, then the molecule can be expected to have an asymmetrical shape.

If the central atom were to be surrounded by both lone pairs and bond pairs of electrons, the molecule would likely have an irregular shape.

The VSEPR theory can be applied to each resonance structure of a molecule.

The strength of the repulsion is strongest between two lone pairs and weakest between two bond pairs.

If electron pairs around the central atom are closer to each other, they will repel each other, leading to an increase in the energy of the molecules.

If the electron pairs are spaced far apart, the repulsions between them will be decreased, resulting in a lower energy of the molecule.

Limitations of VSEPR Theory:

Some significant limitations of the VSEPR theory include:

• It does not take into account the effects of charge on molecular shape.
• It does not accurately predict the shape of molecules with more than four atoms.
• It does not account for the effects of lone pairs of electrons on molecular shape.

This theory fails to account for isoelectronic species, which are elements with the same number of electrons but different shapes.

The VSEPR theory is not applicable to compounds of transition metals. This is because the VSEPR theory does not consider the sizes of the substituent groups and the inactive lone pairs, which are essential to correctly describe the structure of such compounds.

Another limitation of VSEPR theory is that it predicts that halides of group 2 elements will have a linear structure, whereas their true structure is actually bent.

Predicting the Shapes of Molecules

In order to decide the shape of a molecule, the following steps must be followed:

The atom with the lowest electronegativity should be chosen as the central atom, as it has the highest capacity to share its electrons with the other atoms in the molecule.

The total number of electrons in the outermost shell of the central atom must be tallied.

The total number of electrons belonging to other atoms that are involved in bonds with the central atom must be taken into account.

The valence shell electron pair number (VSEP number) can be obtained by adding these two values together.

VSEP Number is an acronym for Vehicle Safety Equipment Performance Number. It is a number assigned to each vehicle safety equipment item that is tested according to the National Highway Traffic Safety Administration (NHTSA) standards.

The VSEP number indicates the shape of the molecule, as outlined in the table below.

| 2 | Linear |

| 3 | Trigonal Planar |

| 4 | Tetrahedral |

| 5 | Trigonal Bipyramidal |

| 6 | Octahedral |

| 7 | Pentagonal Bipyramidal |

The illustration provided earlier also contains each of the corresponding shapes. However, the VSEPR theory cannot be used to accurately determine the bond angles between the atoms in a molecule.

Now, Let’s Discuss Each Shape in Detail:

Linearity of Molecule:

In this type of molecule, the valence shell of the central atom has two places filled.

They should be arranged in such a manner that repulsion is minimized (pointing in opposite directions).

Answer: **BeF2**

Trigonal Planar Shape of Molecule:

A trigonal planar molecule is a type of molecule that has three atoms connected by single bonds in a flat, triangular shape.

In this type of molecule, the central atom is attached to three molecules.

They are arranged in such a manner that the repulsion between the electrons is minimized, with the electrons located at the corners of an equilateral triangle.

Example: **BF3**

The Tetrahedral Structure of Molecules

In two-dimensional molecules, atoms lie in the same plane and if we apply this condition to methane, we will get a square planar geometry with a bond angle of 900° between the H-C-H atoms.

Now, if we consider all these conditions for a three-dimensional molecule, we will get a tetrahedral molecule in which the bond angle between H-C-H is 109.028° (toward the corners of an equilateral triangle) CH₄

Trigonal Bipyramid Shape of Molecule:

The trigonal bipyramid shape is a molecular geometry consisting of five atoms or groups of atoms that are arranged with one atom at the center and the other four at the corners of a triangular base.

In a trigonal bipyramid, repulsion can be minimized by evenly distributing electrons towards the corners and along the equator of the molecule. Additionally, two positions lie along an axis perpendicular to the equatorial plane. An example of this is PF5.

What is the VSEPR Theory and How Can it be Used to Predict the Shapes of Molecules?

The order of repulsion between electron pairs is:

1. Lone Pair - Bond Pair
2. Lone Pair - Lone Pair
3. Bond Pair - Bond Pair

Lone Pair - a pair of electrons not shared with another atom
Bond Pair - a pair of electrons shared between two atoms

1. Total number of electron pairs around the central atom = ½ (number of valence electrons of central atom + number of atoms linked to central atom by single bonds)

Add the number of electrons equal to the units of negative charge on the negative ions to the valence electrons of the central atom.

Subtract the valence electrons of the central atom by the number of electrons equal to the units of positive charge on the positive ion.

2. The number of Bond pairs = Total number of atoms linked to the central atom by single bonds.

3. Number of lone pairs = Total number of electrons - Number of shared pairs

The electron pairs around the central atom repel each other and move so far apart that there are no greater repulsions between them. This results in the molecule having minimum energy and maximum stability.

The shape of a molecule with only two atoms is always a straight line.

For molecules with three or more atoms, the atom at the center is referred to as the central atom, with the other atoms attached to it.

If the central atom is linked to similar atoms and is surrounded by bond pairs of electrons only, the repulsions between them are similar, resulting in a symmetrical shape for the molecule and thus the molecule is said to have regular geometry.

If the central atom is bonded to different atoms and has a lone pair of electrons, the repulsion between them is similar. This leads to an irregular or distorted geometry for the molecule.

The shape of the molecule is determined by the number of electron pairs surrounding the central atom.

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#Frequently Asked Questions About VSEPR Theory

The premise of the VSEPR Theory is that the electron pairs surrounding an atom will repel each other and adopt an arrangement that minimizes this repulsion.

The repulsion between electron pairs in the valence shell leads to atoms arranging themselves in a way that reduces this repulsion. This has a direct effect on the geometry of the molecule formed by the atom.

What is the molecular shape of a molecule with a VSEPR number of 5?

The molecule would have a Trigonal Bipyramidal Structure.

What are the Benefits of the VSEPR Theory?

Once the geometry of the molecule is understood, the theory can be used to accurately predict the shapes of many compounds, making it easier to understand their reactions.