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VSEPR THEORY: Definition, Postulates of the VSEPR theory

VSEPR THEORY: DEFINITION, POSTULATES OF THE VSEPR MODEL OR VSEPR THEORY

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What is VSEPR theory?

Valance Shell Electron Pair Repulsion theory also called VSEPR theory was the theory given by Sidgwick and Powell. This theory describes the influence of the lone pair of electrons on the bonding electron and also predicts the shape of the molecules. This theory is based on the number of valance electrons present in the central atom of a molecule.

Statement: "A molecule takes a shape that minimizes the repulsion between the bonding and non-bonding pair of an electron."

Postulates of VSEPR theory:

Following are the postulates of the VSEPR theory:

1. In any covalent molecule, there is a central atom around which other atoms are arranged. 

2. The shape of the molecule is determined by the number of electron pairs present in the valence shell of the central atom.

3. If the central atom of a molecule is surrounded by electron pair of dissimilar atoms then the molecule will have irregular geometry due to different repulsive interactions.

4. If the central atom in a molecule is surrounded by both lone pair and bond pair of electrons then the molecule will have irregular geometry due to different repulsive interactions. 

5. The order of repulsion between electrons pair are:

Lone pair - lone pair > Lone pair - Bond pair > Bond pair

6. The magnitude of repulsion between bonding pairs of electrons depends upon the electro-negativity difference between the central atom and other atoms.

7. Triple bonds have more repulsive force double bond and double bonds have more repulsive than a single bond.

Explain the shape of BeF2 and BF2 using the VSEPR theory.

Ans: In BeF2 molecules, the central atom Behave two valence electrons. These two electrons are shared by two F atoms and two bond pairs of electrons are formed around the central atom Be. According to VSEPR theory, these two bond pairs of electrons are arranged in such a way that there must be minimum repulsion. These two bonds are 180 degrees apart from each other. The geometry of BeF2 is linear.

IN BeF2 molecules, the central atom B has three valence electrons. These three electrons are shared by three F atoms from three bond pairs of electrons. They have an equal force of repulsion among other each and are oriented in triangular geometry with a bond angle of 120 degrees.

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