Chemistry: Chemical Bonding: Valence Shell Electron Pair Repulsion Theory (For CBSE, ICSE, IAS, NET, NRA 2022)

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Valence Shell Electron Pair Repulsion (VSEPR) Theory

  • The molecules have a definite shape and atoms that are present in them have definite positions relative to one another.
  • Sidgwick and Powell in 1940 put forward a simple theory called VSEPR theory to explain the shapes of molecules.
  • It was later extended by NY Holm and Gillespie in 1957 which focuses on the electron pairs present in the valence hell of the central atom of the molecule.
  • It can be stated in terms of two postulates:

Postulate 1: The electron pairs (both bonding and non-bonding) around the central atom in a molecule arrange themselves in space in such a way that they minimize their mutual repulsion. The chemical bonds will be stable when they are far from each other.

Examples:

BeCl2

  • It is a triatomic molecule.
  • The central atom, beryllium has two electrons in its valence shell and its electronic configuration is 1 s2 2 s2.
  • In the process of covalent bond formation, two chlorine atoms contribute two more electrons are contributed to the valence shell.
  • 4 valence electrons or two pairs of valence electrons are formed by this process.
  • According to this postulate, the electron pairs make the two electron pairs to be at an angle of 180o which gives the molecule a linear shape.
Linear Molecule

BF3

  • In boron trifluoride, boron has an electronic configuration of 1 s2 2 s2 2 p1.
  • It has three electrons in its valence shell.
  • In the process of covalent bond formation, three fluorine atoms contribute three more electrons to the valence shell.
  • There are a total of 6 valence electrons or three pairs of valence electrons.
  • According to the VSEPR postulate, it makes the three electron pairs to be located at an angle of 120o which gives the molecule a planar trigonal shape.
Planar Trigonal Shape
  • The different molecules have different shapes which depend on the number of valence shell electrons that are involved in it.
  • The geometric shapes associated with various numbers of electron pairs surrounding the central atom are given below:
Geometric Shapes

Postulate 2: The repulsion of a lone pair of electrons for another lone pair is greater between a bond pair and a lone pair which in turn is greater than between two bond pairs.

  • The order of repulsive force follows lone pair - lone pair > lone pair - bond pair > bond pair - bond pair.
  • Example: Methane, Ammonia and Water.
  • All the above three substances contain 4 electron pairs around their central atom.
  • In methane molecule the central carbon atom has 4 valence electrons and it shares 4 electrons with four hydrogen atoms, so it is in tetrahedral shape.
  • In case of ammonia there are four pairs of electrons but three of these electrons are bond pairs, while one is a lone pair.
  • Similarly, in case of water there are four pairs of electrons where the two are bond pairs, while two are lone pairs.
  • Due to the differences in the mutual repulsion between bond pair - bond pair and lone pair - bond pair the molecular shape would be slightly distorted from the expected tetrahedral shape.
  • The processes of bond formation and the bonding in simple molecules can be conveniently represented in terms of electron dot structures.
  • The VSEPR theory provides a good idea of the shapes of the molecules.

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