Valence Shell Electron Pair Repulsion (VSEPR) Theory – Shape of Molecules

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ChemistryReference Notes | Science
Valence Shell Electron Pair Repulsion (VSEPR) Theory
Unit: Chemical Bonding and Shape of Molecules
Chemistry Class 12

This theory was developed by Gillespie in 1957 to predict and explain molecular shape and bond angles more exactly on the basis of electrostatic repulsion between electron pairs. It states that “the electron pairs (both lone pairs and shared pairs) surrounding the central atom will be arranged in space as far apart as possible to minimize the electrostatic repulsion between them

The postulates of VSEPR theory are as follows:

  1. The central atom of covalent molecule is attached with other surrounding atoms.
  2. Long pairs of bond pairs or both electrons pair are present in central atom.
  3. Order of repulsion between electron pairs are:
    Lone pair – lone pair > lone pair – bond pair > bond pair – bond pair.
  4. When a molecule has lone pair of electrons, the bonding electron pairs are pushed closer as result the bond angle is decreased.
  5. Molecules or ions containing 2, 3, 4, 5 and 6 electron pairs in central atom have linear trigonal planer, tetrahedral, trigonal bipyramidal and octahedral geometry respectively.
  6. The shape of molecule is determined by repulsion between all of the electron pairs present in the valence shell.

On the basis of VSEPR theory the geometry of some molecules are as follows:

1. Using VSEPR theory, explain the shape of Beryllium fluoride (BeF2). [HSEB 2056]

The Lewis structure of BeF2 is:

shape of BeF2 - 1

The central atom Be contain two bond pairs to have minimum repulsion, bond pair electron stretch and angle 180° and the molecule assume linear structure.

Linear structure of BeF2

2. Using VSEPR theory, explain the shape of boron trifluroide (BF3). [HSEB 2056]

The Lewis structure of BF3 is:

Lewis Strucute of BF3

The central atom “B” contains three bond pairs. To have minimum repulsion, these three bond pairs are stretched to assume a plane triangular or trigonal geometry with each bond angle equal to 120°.

trigonal structure of BF3

Limitation of VESPR Theory:

  1. It can’t explain the shape of molecules having very polar bonds, eg. Li2O should have the same structure as H2O but actually it is linear.
  2. This fails to explain the shape of molecules having extensive delocalized pi electron system.
  3. This theory is not able to predict the shape of certain transition metal complexes.
  4. It does not help in determining the exact bond angle.

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One Comment

  1. yogesh pratap
    Posted August 29, 2016 at 9:54 am

    hi give me all notes related to chemistry class 11 cbse board

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