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Valence Shell Electron Pair Repulsion (VSPER) Theory 

  • Regular Geometry of Molecules

Geometry of the molecules in which the central atom has no lone pairs are regular and can be predicted simply.

Bond ange of any molecule with regular geometry = 360o  /Number of bond pairs

Number of electron pairs

Arrangement of electrons

Molecular geometry 

Examples

2

 

1338_Chemical compound 100.JPG

 

B – A – B

Linear

BeCl2, HgCl2

3

 

1545_Trigonal Planar 1.JPG

θ = 120°

 

1072_Trigonal Planar 2.JPG

θ = 120°

 

BF3, AlCl3

4

 

2171_Regular Geometry 1.JPG

 

237_Regular Geometry 2.JPG

 

CH4, NH4+, SiF4

5

 

1947_Trigonal bipyramidal 1.JPG

 

2119_Trigonal bipyramidal 2.JPG

 

PCl5, PF5

6

 

1063_Octahedral 1.JPG

2340_Octahedral 2.JPG

SF6
  • Irregular Geometry of Molecules and VSPER Theory 

Geometry of the molecules having  lone pair of electrons can not be predicted simpley using above mentioned method. The geometric arrangement of atoms in molecules and ions may be predicted by means of the valence-shell electron-pair repulsion (VSEPR) theory. This type includes  molecules which may or may not obey the octet rule but have only single bonds.

Postulates of VSEPR theory: 

  1. The shape of the molecule is determined by repulsions between all of the electron pairs present in the valence shell.

  2. A lone pair of electrons takes up more space around the central atom than a bond-pair, since the lone pair is attracted to one nucleus whilst the bond pair is shared by two nuclei. It follows that repulsion between two lone pairs is greater than repulsion between a lone pair and a bond pair, which in turn is greater than the repulsion between two bond pairs. Thus the presence of lone pairs on the central atom causes slight distortion of the bond angles from the ideal shape. If the angle between a lone pair, the central atom and a bond pair is increased, it follows that the actual bond angle between the atoms must be decreased.The descending order of repulsion is 
    (lp – lp) > (lp – bp) > (bp – bp)
    where lp-Lone pair; bp-bond pair

  3. The magnitude of repulsions between bonding pairs of electrons depends on the electronegativity difference between the central atom and the other atoms.

  4. Double bonds cause more repulsion than single bonds and triple bonds cause more repulsion than double bonds.

  5. A brief summary of molecular shapes resulting from different configurations of electrons pairs is presented below:

  6. With very few exceptions, the predictions based on the VSEPR theory have been shown to be correct.

Refer to the following video  for geometry of the different molecules

Molecule Type

No. of Bonding pairs

No. of lone pair

Arrangement of electrons pairs

Shape (Geometry)

Examples

AB2E

2

1

 

25_Trigonal Planar 3.JPG

Bent

SO2, O3
AB3E

3

1

1476_Tetrahedral 1.JPG

Trigonal pyramidal NH3
AB2E2

2

2

1020_Tetrahedral 2.JPG

Bent H2O
AB4E

4

1

1807_Trigonal bipyramidal 3.JPG

See saw SF4
AB3E2

3

2

1690_Trigonal bi-pyramidal 4.JPG

T – shaped CIF3
AB5E

5

1

600_Octahedral 3.JPG

Square pyramidal BrF5
AB4E2

4

2

2442_Octahedral 4.JPG

Square planar XeF4
 

To find the shape of a molecule follow the steps given below:

  1. Identify the central atom and count the number of valence electrons.

  2. Add to this, number of other atoms.

  3. If it is an ion, add negative charges and subtract positive charges. Call it total N

  4. Divide N by 2 and compare the result with chart I and obtain the shape.

Total N/2

Shape of molecule or ion

Example

2

Linear

HgCl2/BeCl2

3

Triangular planar

BF3

3

Angular

SnCl2, NO2

4

Tetrahedral

CH4, BF4-

4

Trigonal Pyramidal

NH3, PCl3

4

Angular

H2O

5

Trigonal bipyramidal

PCl5, PF5

5

Irregular tetrahedral

SF4, IF4+

5

T-shaped

CIF3, BrF3

5

Linear

XeF2, I3-

6

Octahedral

SF6, PF6-

6

Square Pyramidal

IF5

6

Square planar

XeF4, ICI4

You can also refer to  

To read more, Buy study materials of Chemical Bonding comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here

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