B) three electron pairs

 

If there are three electron pairs on the central atom, the angle between the bonds is 120^o.

 

 

Molecules which adopt this shape are said to be TRIGONAL PLANAR.

E.g. BF₃, AlCl_3, CO₃^2-, NO₃^-

 

 

If one of these electron pairs is a lone pair, the bond angle is slightly less than 120^o due to the stronger repulsion from lone pairs, forcing them closer together.

 

bond angle = 118^o

 

Molecules which adopt this shape are said to be BENT.

 

E.g. SO₂, NO₂^-

 

C) Four electron pairs

 

If there are four bonded pairs on the central atom, the angle between the bonds is approx 109^o.

Molecules which adopt this shape are said to be TETRAHEDRAL.

 

E.g. CH₄, SiCl_4, NH₄⁺, SO₄^2-

 

If one of the electron pairs is a lone pair, the bond angle is slightly less than 109^o, due to the extra lone pair repulsion which pushes the bonds closer together (approx 107^o).

 

 

Molecules which adopt this shape are said to be TRIGONAL PYRAMIDAL.

 

E.g. NH₃, PCl₃

 

If two of the electron pairs are lone pairs, the bond angle is also slightly less than 109^o, due to the extra lone pair repulsion (approx 104^o).

 

Molecules which adopt this shape are said to be BENT.

 

E.g. H₂O, OF₂

D) Five electron pairs

If there are five bonded pairs on the central atom, the three bonds are in a plane at 120^o to each other, the other 2 are at 90 ^o to the plane.

Molecules which adopt this shape are said to be TRIGONAL BYPRYMIDAL.

E.g. PF_5, PCl₅