Molecular Geometry and VSEPR Theory
Molecular geometry is the threedimensional 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:
 Identify the central atom and count the number of valence electrons on the central atom.
 Add to this the number of other atoms (which form single bonds only). Here, oxygen atoms are not added as they form two bonds.
 If the species is an anion, add negative charges; if it is a cation, subtract positive charges.
 This gives us a number, which we refer as N.
 Divide N by 2; we get the sum of bonding and nonbonding electron pairs.
 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 
HgCl_{2}, BeCl_{2} 
3 
0 
Trigonal planar 
BF_{3}, AlCl_{3}, BH_{3}, NO_{3 }^{â€“}, SO_{3} 
1 
Angular or bent 
SnCl_{2}, SO_{2}, NO_{2 }^{â€“} 

4 
0 
Tetrahedral 
CCl_{4}, BdF_{4 }^{2â€“}, BF_{4 }^{â€“}, PCl_{4 }^{+}, ClO_{4 }^{â€“} 
1 
Trigonal pyramidal 
NH_{3}, PCl_{3}, PF_{3}, ClO_{3 }^{â€“} 

2 
Angular or bent 
H_{2}O, H_{2}S, OF_{2}, ClO_{2 }^{â€“} 

3 
Linear 
ClO^{â€“} 

5 
0 
Trigonalbipyramidal 
PCl_{5}, PF_{5} 
1 
Seesaw or irregular tetrahedron 
SF_{4}, IF_{4 }^{+} 

2 
Tshaped 
ClF_{3}, BrF_{3} 

3 
Linear 
I_{3 }^{â€“}, Br_{3 }^{â€“}, XeF_{2} 

6 
0 
Octahedral or square bipyramidal 
SF_{6}, PCl_{6 }^{â€“} 
1 
Square pyramidal 
BrF_{5}, IF_{5} 

2 
Square planar 
ICl_{4 }^{â€“}, XeF_{4}, IF_{4} 

7 
0 
Pentagonal bipyramidal 
IF_{7} 
Note: XeF_{6} does not have octahedral structure. Its structure is capped octahedron.