Valence Shell Electron Pair Repulsion (VSEPR) Theory states

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Theories of Covalent Bonds:

There are three theories of covalent bonds.

These are (i) VSEPRT, Resonance Theory, (ii) VBT and (iii) MOT.

Valence Shell Electron Pair Repulsion (VSEPR) Theory:

VSEPR theory is good enough to explain molecule geometries and bond angles.

A summary of molecular shapes is given in the following table:

shapes of molecules

This theory considers two types of electron pairs around the central atom of a molecule and it has the following postulates:

  • Two types of electron pairs are present around the central atom of a molecule. Electron pair which is shared between two atoms and is responsible for the covalent bond is called a “bond pair” while electron pair which is not shared between two atoms is called a “lone pair”.

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

  • To reduce repulsion these electron pairs move to maximum angles apart from each other and are directed in different directions in space. So they are responsible for molecular geometries.

For two electron pairs, geometry is linear and the bond angle is 180o.

For three electron pairs, the geometry is triangular planar and the bond angle is 120o.

For four electron pairs, the geometry is tetrahedral and the bond angle is 109.5o.

  • If more than one electron pairs are there in the same region they behave like a single pair. e.g.                      

Sulphur in SO2 shows three electron pairs behaviour.

..

           O : : S : O

  • Deviation from the ideal bond angle occurs if lone pair of electrons is there.
  • The effect of a bonding electron pair decreases with the increasing electronegativity of an atom forming a molecule.

Shape of molecules with two electron pairs:

e.g (i)  BeCl2                                                                       

        Cl  Be  Cl          

   or      

   Cl ─ Be ─ Cl  Linear geometry

(  180o )

The central atom Be has two electron pairs so the molecule is linear with a bond angle of 180o.

            e.g (ii) CO2       

          O  : :  C  : :  O          

  or         O = C = O       Linear geometry

Four pairs around the central Carbon atom behave like two pairs so the CO2 molecule is linear with a bond angle of 180o.

            e.g (iii) HCN      

          H  C  : : : N         

   or         H ─  C  ≡  N  Linear geometry

Four pairs around the central Carbon atom behave like two pairs so the HCN molecule is linear with a bond angle of 180o.

Shape of molecules with three electron pairs:

   e.g (i)  BF3

The central atom B has three electron pairs so the molecule is a triangular planner with bond angle 120o.

e.g (ii) SO2

Two electron pairs that Sulphur has shared with one of the oxygen act as single pair so SO2 shows the behaviour of three electron pairs. Hence electron geometry in SO2 is a triangular planner while molecule geometry is an angular or bent structure. In SO2 deviation from the ideal bond, angle occurs due to the presence of a lone pair.     

   e.g (iii) SnCl2

In stannous chloride, Sn has 5s2 5p2 valence electrons. The two unpaired electrons of the 5p orbital take part in the bond formation of a covalent bond with chlorine, while the pair of electrons of 5s remains as lone pair and does not participate in bond formation. Hence electron geometry in SnCl2 is a triangular planner while molecule geometry is an angular or bent structure. In SnCl2 deviation from the ideal bond, angle occurs due to the presence of a lone pair. 

Shape of molecules with four electron pairs:

e.g (i)  CH4          

sp3 hybridization in methane
methane

The central atom C has four electron pairs so the molecule is tetrahedral with a bond angle of 109.5o.

e.g (ii) NH3                             

ammonia pyrimidal structure
ammonia

The central atom N has four electron pairs so electron geometry in NH3 is tetrahedral while molecule geometry is pyramidal. In NH3 deviation from the ideal bond angle occurs due to the presence of a lone pair of electrons and the angle reduces to 107.5 o.

e.g (iii) H2O                   

water

The central atom O has four electron pairs so electron geometry in H2O is tetrahedral while molecule geometry is angular or bent structure. In H2O deviation from the ideal bond angle occurs due to the presence of two lone pairs of electrons and the angle reduces to 104.5 o.

water bent shape

Limitations of VSEPR Theory:

            Although this theory is good enough to explain molecular shapes and bond angles:

  • it fails to explain the mechanism of the formation of covalent bonds.
  • it fails to explain why two electrons of the same pair do not repel each other.
  • it fails to explain the paramagnetic nature of oxygen.

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