chemical bonding

Valence Shell Electron Pair Repulsion (VSEPR) Theory states

Do you know !!

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.

Summary of molecular shapes is given in following table:

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

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

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

  • To reduce repulsion these electron pairs move to maximum angle apart 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 bond angle is 180o.

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

For four electron pairs geometry is tetrahedral and 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 ideal bond angle occurs if lone pair of electrons is there.
  • The effect of a bonding electron pair decreases with 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 molecule is linear with bond angle 180o.

            e.g (ii) CO2                 O  : :  C  : :  O            or         O = C = O       Linear geometry

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

            e.g (iii) HCN                H  C  : : : N             or         H ─  C  ≡  N  Linear geometry

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

Shape of molecules with three electron pairs:

   e.g (i)  BF3

The central atom B has three electron pairs so molecule is 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 behaviour of three electron pairs. Hence electron geometry in SO2 is triangular planner while molecule geometry is angular or bent structure. In SO2 deviation from ideal bond angle occurs due to presence of a lone pair.     

   e.g (iii) SnCl2

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

Shape of molecules with four electron pairs:

e.g (i)  CH4          

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

e.g (ii) NH3                             

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

e.g (iii) H2O                   

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 ideal bond angle occurs due to presence of two lone pairs of electrons and angle reduces to 104.5 o.

Limitations of VSEPR Theory:

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

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

                    

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