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6 CHEMICAL BONDING eLearn.Punjab

1 Molecules Containing Two Electron Pairs (AB2 type)

In such, molecules two electrons, pairs around the central
atom are arranged at farther distance apart at an angle of 180°, in
order to minimize repulsions between them. Thus, they form a linear
geometry.
Beryllium chloride is a typical linear molecule, which contains
two electrons pairs. MgCl2, CaCl2, SrCl2. CdCL2 and HgCl2 are also
linear molecules. The central atoms have two electrons in outer most
orbitals.

2. Molecules Containing Three Electron Pairs — (AB3 type:)

(a) AB3 Type with no Lone Pairs

In such molecules, central atom contains three bonding electron pairs, which are arranged at
maximum distance apart at a mutual angle of 120°, giving a triangular planar geometry. The boron
atom in BH3 is surrounded by three charge clouds, which remain farthest apart in one plane, each
pointing towards the corners of an equilateral triangle. Thus, BH3, molecules has a trigonal planar
geometry, with each H- B-H bond angles of 120°.
We expect similar geometries in hydrides of group III-A (AlH3, GaH3, InH3 and TlH3)and their
halides (BF3, AlCl3 ,etc.)

(b) AB3-Type with One Lone Pair and Two Bond Pairs

In SnCl2, one of the corner of the triangle is occupied by a lone pair, giving rise to a distorted
triangular structure in vapour phase.

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