VSEPR Theory





Valence Shell Electron Pair Repulsion (VSEPR) Theory

This theory was developed by Gillespie and Nyholm. It is based on the effect of electron repulsion on the bond angles. The shape of the molecule or ion depends upon the number of bonding electron pairs (bp’s) and nonbonding electron pairs or lone pairs (lp’s) in the central atom.

The central atoms are oriented in such a way that there is minimum repulsion (maximum stability) between them. The molecule has a definite shape because; there is only one orientation of orbitals corresponding to minimum energy. Gillespie postulated the following rules of VSEPR theory:

(i)  If the central atoms in a molecule is surrounded by only bonding electron pairs (bp’s) and not by non-bonding electron pairs or lone pairs (lp’s) it will have regular geometry or shape.

(ii) If the central atom in a molecule is surrounded by both bp’s, and lp’s it will have distorted or irregular geometry or shape, because, the lp’s repel adjacent bonding electron pairs more strongly than do bonding electron pairs which increases as:

(bp – bp) < (bp – ip) < (ip – ip)

Due to this reason the bond angels decrease in the order of

CH_4 (109.5^0) > NH_3 (107.8^0) > H_2O (105.5^0)

 

And the number of lp’s increases as:

CH_4 (0) < NH_3 (1) < H_2O (2)

In other words we can say that as the number of lp’s increases, repulsion increases and bond angle decrease or distortion increases. The other example is as follows:

NO^+_2 (180^0) > NO_2 (134^0) > NO^-_2 (115^0)

 

(iii) Repulsion between electron pairs in filled shell is greater than that in incomplete shell. Here we take the hydrides of 16th group elements of the periodic table with their bond angels:

OH_2(105.5^0) > > SH_2(92.2^0) > SeH_2 (91.0^0) > the_2(89.5^0)

 

(iv) If the electronegativity of the central atom decreases, bond angle decreases, at the same time if the electronegativity of the surrounding atoms decreases (central atom remains the same) the bond angel increases.

E.g.

PI_3 (102^0) > PBr_3 (101.5^0) > PCI_3(100^0) \\ AsI_3 (101^0) > AsBr_3 (100.5^0) > AsCL_3(98.4^0)

 

(v)  Although the multiple bonds do not effect the geometric of the molecules but the bond angles involving single bonds are generally smaller than those of multiple bonds.

  • sp^2 \to when there are 2bp and one lpe^-. Shape will be angular.

VSEPR THEORY

  • sp^2 \to when there are 3bp, 1lpe^-. Shape is pyramidal.

Ex.

VSEPR THEORY

  • When there are tow bp, two lp lpe^-. Shape is angular or bent

VSEPR THEORY

  • When there are 3bp, 1lpe^-. Shape is linear.

Ex. I – Cl.

  • sp^2d \to when there are 4bp, 1lpe^-. Shape is Sea Saw. Ex

Ex.

VSEPR THEORY

  • When there are two lp and 3bpe^-. It has T-shape

Ex. XeOF_2, CIF_3

 

VSEPR THEORY

  • When there are 3lp 2bpe^-. Shape is linear.

Ex. I^-_3, XeF_2, ICL^-_2

 

VSEPR THEORY

sp^3p^2 \to (A) when there are one lp and5bpe^-. Shape is square pyramidal e.g. XeOF_4, IF_5, CIF_5

 

(B). When there are two l_p \text{and} 4b_pe^-. Shape is square planar.

E.g.

XeF_4, ICl^-_4

 

VSEPR THEORY

sp^3d^3 \to when there are one lp and six bpe^-. Shape is distorted octahedral.

E.g. XeF_6.

Limitations of VSEPR Theory

(i) It does not explain the shapes of the molecules having very polar e.g. Li_2O \text{and} H_2O should have same structure but Li_2O is linear while H_2O is angular.

(ii) It does not explain the shapes of the molecules or ions which are extensive by delocalized \pi-electron system.

(iii) It does not explain the shapes of some molecules which have an inert pair of electrons.

(iv) It does not explain the shapes of certain compounds of transitional metals e.g. the shape of the compound p^8 electronic configuration of the central atom, is square planar and not tetrahedral as predicted b this theory.



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