Valence Bond Theory, VSEPR Class 11 Notes | EduRev

Chemistry Class 11

Class 11 : Valence Bond Theory, VSEPR Class 11 Notes | EduRev

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Valence Bond (V.B) theory :

According to this theory, a covalent bond is formed by the overlapping of atomic orbitals. Important points of this theory are summarised below.

(I) Orbitals undergoing overlapping should be half-filled.

(II) Half-filled orbitals should contain the electron with opposite spin.

(III) Strength of a covalent bond depends upon the extent of overlapping, for example, axial or lateral overlapping.

(IV) If the atomic orbitals overlap axially, then the bond formed is called a sigma (s) bond.

(V) A sigma bond is always stronger than a pi-bond.

(VI) Covalent bonds formed by the overlap of s-s and s-p orbitals are always sigma.

(VII) By the overlap of p-p orbitals, one sigma and two pi bonds are formed.

(VIII) Increasing strength of s covalent bonds is in the order s - s < s - p < p — p (when internuclear distance is constant)

(IX) A single covalent bond is always a sigma bond. In a double covalent bond, one is sigma and the other is pi-bond. In a triple covalent bond, one is sigma and two are pi-bonds.


Hybridisation :

It is defined as the concept of intermixing of orbitals of same energy or of slightly different energy to produce entirely new orbitals of equivalent energy, identical shapes and which are symmetrically disposed is plane.

Important features of hybridization are given below.

(I) Only the orbitals generated are equal in number to that of pure atomic orbitals which are intermixed.

(II) The hybrid orbitals generated are equal in number to that of pure atomic orbitals which are intermixed.

(III) A hybrid orbital, like an atomic orbital, can have two electrons of opposite spins.

(IV) Hybrid orbitals usually form sigma bonds. If there are pi-bonds, equal number of atomic orbitals must be left unhybridised for pi-bonding.


Valence shell electron pair repulsion (VSEPR) Theory (Gillespie theory) :

(I) The shape of a molecule is determined by repulsion between the electron pairs (lp and bp) present in the valence shell of the central atom.

(II) The order of repulsion is (lp-lp) > (lp-bp) > (bp-bp)

Determination of the Type of Hybridisation :

The number of electron pairs = Bond Pairs + lone pairs

(σ - Bond)


Table Shapes of molecules on the basis of VSEPR theory and hybridisation


Total electron pairs

Bond pairs

Lone pairs

Type of hybridization

Geometry due to repulsion

Bond angle

Example

2

2

0

sp

Linear

180°

BeCl2

3

3

0

sp2

Trigonal Planar

120°

BF2

3

2

1

sp2

Angular

<120°

SO2

4

4

0

sp3 or dsp2

Tetrahedral

109°28'

CH4

4

3

1

spor dsp2

Pyramidal

<109°28'

NH3

4

2

2

spor sp2

Bent

<109°28'

H2O

5

5

0

sp3d

Trigonal bipyramidal

120° & 90°

PCl5

5

4

1

sp3d

See Saw

<120 ° & <90°

SF4

5

3

2

sp2d

Bent T-shaped

<90°

Clf2

5

2

3

sp3d

Linear

180°

I3-

6

6

0

sp3d2

Octahedral

90°

SF6

6

5

1

sp2d2

Square pyramidal

<90°

BrF5

6

4

2

sp2d2

Square planar

90°

XeF4

7

7

0

sp2d2

Pentagonal bipyramidal

90° & 72°

IF7

7

6

1

sp2d2

Pentagonal pyramidal

<90° & <72°

 

7

5

2

sp2d2

Pentagonal planar

72°

XeF5-

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