VSEPR theory-definition-postulates-shape and limitations

VSEPR  theory and shape of covalent molecules 

The geometry of covalent molecules depends upon the number and arrangement of electron pairs [which present in the valence shell of the central atom], due to electrostatic repulsion between bond pair and lone pair of electrons.

To explain the geometry or shape of covalent molecules, Gillespie and Nyholn put forward a new theory which is known as valence shell electron pair repulsion theory or in a nutshell VSEPR  theory.

VSEPR theory postulates for covalent molecules

There are five main postulates of this theory, which may be summarized as follows .

Postulate: I  The shape of a covalent molecule is determined by the repulsion among all the electron pairs [ bond pair(b.p ) of electrons and lone pair( l.p ) of electrons ] present in the valence shell of the central atom .

If only bonding electron pairs are present on the central atom, the covalent molecule will have a regular geometry .

Thus for AB ₂ type of molecule with two bond pair [such as BeCl2, CO2 etc ], the geometry is linear.

Similarly, molecules with three bond pair, four bond pair, five bond pair and six bond pair, the geometry of the concern molecules are trigonal , tetrahedral , trigonal  bipyramidal  and octahedral respectively .

For examples, BCl₃ ( trigonal ) , CH₄ ( tetrahedral ), PCl₅ ( trigonal bipyramid ) and SF₆ ( octahedral ) etc.

Postulate: II  When both the bond pair( b.p ) and lone pair ( l.p ) are present on the central metal atom, the molecules will have a distorted geometry.

Because, the lone pair takes up more space on the central atom than a bond pair and the lone pair is attracted by one nucleus while the bond pair is attracted by two nuclei .

Hence, the l.p–l.p repulsion is greater than l.p – b.p repulsion and l.p – b.p repulsion is greater than b.p –b.p repulsion .

That is , l.p–l.p repulsion > l.p – b.p repulsion > b.p –b.p repulsion.

For examples, ammonia molecule contain one lone pair and three bond pair of electrons . The geometry of ammonia molecule is distorted tetrahedral with sp3 hybridization .

That is, it has pyramidal structure .The H –N –H bond angle is 107^o28’ instead of 109^o28’ .

Similarly, H₂O molecule contain two bond pair and two lone pair of electrons .Hence ,the geometry of water molecule is distorted tetrahedral or V–shaped geometry with sp3 hybridization.

The H –O –H bond angle in water molecule is 104^o27’ instead of 109^o28’ .

Postulate: III  The magnitude of repulsion between bond pairs depend on the electronegativity difference between central atom (A ) and other bonded atom ( B ).

That is , the B – A – B  bond angle decreases with increasing in electronegativity of ‘B’ as the electronegativity of ‘B’ increases.

The bond pair move away from the central atom and repulsion between bond pair decreases. So the bond angle decrease . 

For examples, the order of bond angle of PI₃ , PBr₃ and PCl₃ is PI₃ > PBr₃ > PCl₃.

Postulate: IV  Triple bond take more space than double bond . Similarly , double bond take more space than single bond.

So double bond causes more repulsion than single bond and triple bond causes more repulsion than double bond .

That is, t.b –t.b repulsion > d.b–d.b repulsion > d.b–s.b repulsion > s.b –s.b repulsion . [ t.b = triple bond, d.b = double bond and s.b = single bond ].

For example , F – C – F and F – C – O bond angle of carbonic difluoride ( COF₂ ) .

Postulate: V  The bond angle contraction of incomplete valence shell is greater than complete valence shell due to the l.p–l.p or l.p – b.p repulsion.

For examples, in case of ammonia the H –N –H bond angle is 107^o28’ instead of 109^o28’ . But in case of phosphine , the H – P –H bond angle becomes 94^o instead of 109^o28’.

Limitations of VSEPR Theory.

Like other theory, VSEPR Theory also some draw back. The main two limitation of VSEPR Theory are discussed below .

( I ) VSEPR Theory fails to explain isoelectronic species. Isoelectronic species are elements, ions and molecules that share the same number of electrons.

According to VSEPR theory, shape of the molecule depends on number of bond pair and lone pair of electrons of the central atom.

But, isoelectronic species can differ in geometry , despite the fact that they have the same numbers of valence electrons. 

( II ) VSEPR Theory does not explain the geometry of transition metal compounds. This theory  is also unable to guess the structure of certain compounds.

This is because of that it does not take associated sizes of the substituent’s and inactive lone pairs into account.

 Summary 

•  VSEPR theory and shape of covalent molecules
•  VSEPR theory postulates for covalent molecules
•  Limitations of VSEPR Theory.

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