Valence Shell Electron Pair Repulsion (VSPER) Theory

Regular Geometry of Molecules

Geometry of the molecules in which the central atom has no lone pairs are regular and can be predicted simply.

Bond ange of any molecule with regular geometry = 360^o  /Number of bond pairs

Number of electron pairs	Arrangement of electrons	Molecular geometry	Examples
2		B – A – B Linear	BeCl2, HgCl2
3	θ = 120°	θ = 120°	BF3, AlCl3
4			CH4, NH4+, SiF4
5			PCl5, PF5
6			SF6

Irregular Geometry of Molecules and VSPER Theory 

Geometry of the molecules having  lone pair of electrons can not be predicted simpley using above mentioned method. The geometric arrangement of atoms in molecules and ions may be predicted by means of the valence-shell electron-pair repulsion (VSEPR) theory. This type includes  molecules which may or may not obey the octet rule but have only single bonds.

Postulates of VSEPR theory: 

• The shape of the molecule is determined by repulsions between all of the electron pairs present in the valence shell.

• A lone pair of electrons takes up more space around the central atom than a bond-pair, since the lone pair is attracted to one nucleus whilst the bond pair is shared by two nuclei. It follows that repulsion between two lone pairs is greater than repulsion between a lone pair and a bond pair, which in turn is greater than the repulsion between two bond pairs. Thus the presence of lone pairs on the central atom causes slight distortion of the bond angles from the ideal shape. If the angle between a lone pair, the central atom and a bond pair is increased, it follows that the actual bond angle between the atoms must be decreased.The descending order of repulsion is 

• (lp – lp) > (lp – bp) > (bp – bp)

• where lp-Lone pair; bp-bond pair

• The magnitude of repulsions between bonding pairs of electrons depends on the electronegativity difference between the central atom and the other atoms.

• Double bonds cause more repulsion than single bonds and triple bonds cause more repulsion than double bonds.

• A brief summary of molecular shapes resulting from different configurations of electrons pairs is presented below:

• With very few exceptions, the predictions based on the VSEPR theory have been shown to be correct.

Refer to the following video  for geometry of the different molecules

Molecule Type	No. of Bonding pairs	No. of lone pair	Arrangement of electrons pairs	Shape (Geometry)	Examples
AB2E	2	1		Bent	SO2, O3
AB3E	3	1		Trigonal pyramidal	NH3
AB2E2	2	2		Bent	H2O
AB4E	4	1		See saw	SF4
AB3E2	3	2		T – shaped	CIF3
AB5E	5	1		Square pyramidal	BrF5
AB4E2	4	2		Square planar	XeF4

To find the shape of a molecule follow the steps given below:

• Identify the central atom and count the number of valence electrons.

• Add to this, number of other atoms.

• If it is an ion, add negative charges and subtract positive charges. Call it total N

• Divide N by 2 and compare the result with chart I and obtain the shape.

You can also refer to  

• JEE Physical Chemistry Syllabus 

•  Reference books of Physical Chemistry

To read more, Buy study materials of Chemical Bonding comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here