The Valence shell electron pair repulsion theory (VSEPR)
The basic concept of this theory was suggested by the scientists Sidgwick and Powell (in the year 1940). It gives the general idea for predicting shapes and geometries of the molecules. The concept tells us that, the arrangement of the bonds around the central atom depends upon repulsion's operating between the electron pairs(either bonded or non bonded) around central atom. The Gillespie and Nyholm developed this concept as the VSEPR theory.
The major postulates of VSEPR theory are given as follows
(1) For the polyatomic molecules containing 3 or more atoms, one of the atoms is known as the central atom to which the other atoms are linked.
(2) The geometry of the molecule is dependent upon the total number of valence shell electron pairs (either bonded or not bonded) present around the central atom and their repulsion due to the relative sizes and shapes.
(3) If the central atom is surrounded by the bond pairs only. It gives symmetrical shape to the molecule.
(4) If the central atom is surrounded by the lone pairs (lp) as well as the bond pairs (bp) of then the molecule has the distorted geometry.
(5) The relative order of the repulsion between electron pairs can be given as follows : lp - lp > lp - bp > bp - bp.
A lone pair is concentrated around central atom while a bond pair is pulled out between the two bonded atoms. As such repulsion becomes greater when the lone pair is involved.
|
Type of mole-cule
|
No. of bond pairs of electron
|
No. of lone pairs of electrons
|
Hybridi-zation
|
Bond angle
|
Expected geometry
|
Actual geometry
|
Examples
|
|
|
2
|
1
|
sp2
|
< 120o
|
Trigonal planar
|
V-shape, Bent, Angular
|
SO2, SnCl2, NO2-
|
|
|
2
|
2
|
sp3
|
< 109o 28¢
|
Tetrahedral
|
V-shape, Angular
|
H2O, H2S, SCl2, OF2, NH2-, ClO2-
|
|
|
3
|
1
|
sp3
|
< 109o 28¢
|
Tetrahedral
|
Pyramidal
|
NH3, NF3 , PCl3, PH3, AsH3, ClO3- , H3O+
|
|
|
4
|
1
|
sp3d
|
< 109o 28¢
|
Trigonal bipyramidal
|
Irregular tetrahedron
|
SF4, SCl4, TeCl4
|
|
|
3
|
2
|
sp3d
|
90o
|
Trigonal bipyramidal
|
T-shaped
|
ICl3, IF3, ClF3
|
|
|
2
|
3
|
sp3d
|
180o
|
Trigonal bipyramidal
|
Linear
|
XeF2, I3-, ICl2-
|
|
|
5
|
1
|
sp3d2
|
< 90o
|
Octahedral
|
Square pyramidal
|
ICl5, BrF5, IF5
|
|
|
4
|
2
|
sp3d2
|
-
|
Octahedral
|
Square planar
|
XeF4, ICl4-
|
|
AX7
|
6
|
1
|
sp3d3
|
-
|
Pentagonal pyramidal
|
Distorted octahedral
|
XeF6
|
Table : Geometry of Molecules/Ions having bond pair as well as lone pair of electrons
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