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Molecular Geometry and VSEPR Theory

Molecular geometry is the three-dimensional arrangement of atoms in a molecule. The geometry assumed by the species must be such that the repulsion between the electron pairs around the central atom is minimum. This approach is called valence shell electron pair repulsion (VSEPR) theory because it accounts for the geometric arrangements of electron pairs around a central atom in terms of the electrostatic repulsion between electron pairs.

Predicting geometry of species using VSEPR theory

With the help of VSEPR theory, we can predict the geometry of various species in a systematic way. The scheme makes use of the following steps:
  1. Identify the central atom and count the number of valence electrons on the central atom.
  2. Add to this the number of other atoms (which form single bonds only). Here, oxygen atoms are not added as they form two bonds.
  3. If the species is an anion, add negative charges; if it is a cation, subtract positive charges.
  4. This gives us a number, which we refer as N.
  5. Divide N by 2; we get the sum of bonding and non-bonding electron pairs.
     
    Description: 14994.png Number of other atoms + Number of lone pairs
  6. Compare the result (N/2 value) with the value given in Table corresponding to the given number of lone pairs.

N/2 value

No. of lone pairs

Shape of the species

Example

2

0

Linear

HgCl2, BeCl2

3

0

Trigonal planar

BF3, AlCl3, BH3, NO3 , SO3

1

Angular or bent

SnCl2, SO2, NO2

4

0

Tetrahedral

CCl4, BdF4 2–, BF4 , PCl4 +, ClO4

1

Trigonal pyramidal

NH3, PCl3, PF3, ClO3

2

Angular or bent

H2O, H2S, OF2, ClO2

3

Linear

ClO

5

0

Trigonalbipyramidal

PCl5, PF5

1

See-saw or irregular tetrahedron

SF4, IF4 +

2

T-shaped

ClF3, BrF3

3

Linear

I3 , Br3 , XeF2

6

0

Octahedral or square bipyramidal

SF6, PCl6

1

Square pyramidal

BrF5, IF5

2

Square planar

ICl4 , XeF4, IF4

7

0

Pentagonal bipyramidal

IF7

 

Note: XeF6 does not have octahedral structure. Its structure is capped octahedron.




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