Hybridisation is explained some structural properties like shape of molecules, bonds that cannot be explained by simple theories of valency.The bonds formed between central atoms and other atoms in a covalent molecule can have different orientation with respect to the central atom. Due to the different orientation, the molecules possess different molecular geometry and different bond angles.
Some important concept about the Hybridisation is that;The orbitals must have little energy difference which take part in hybridisation. That are also have equal amount of energy and identical in shapes.The formed hybrid orbitals are equal to the number of orbitals that take part in hybridisation. Half and completely filled valance shell orbitals can take part in hybridisation. The electron density is concentrated on the one side of nucleus.
They are directed in space and in some preferred directions with maximum stability so they control the geometry of molecule.The various types of hybridisation depending upon the type of orbitals that takes part like sp, sp2, sp3, dsp2, and dsp3 etc. The molecular geometry chart of various molecules is basically utilized for better differentiation of structures.For example the molecular geometry of NH3 is pyramidal shape molecular geometry of NH3In ammonia molecule the nitrogen adopts sp3 hybrid state. Three sp3 hybrid orbitals of N atom are used to form three sigma bonds with other three s orbitals of three H atoms respectively.
Thus this forms three sp3-sσ bonds. While the fourth sp3 hybrid orbital contains one lone pair, as the lp-lp interaction is larger than the lp-bp and bp-bp. So the bond angle of tetrahedral shape 109 º28 decreases to 107º. So the shape is changed to pyramidal.Similar if we take example of CO2, then the molecular geometry of CO2 is linear shape .
The carbon atom in CO2 is in sp hybrid state. The two sp hybrid orbitals are linear ans oriented in opposite direction at an angle 180 º. Two sp-hybrid orbital overlaps axially with half filled orbitals of oxygen atom and form two C-O sigma bond while the other two un-hybridized orbitals of carbon atom overlap sidewise to form two pi-bonds. Similar H2S molecular geometry is bent like water. The S atom is in sp3 hybridized state. Two half filled hybrid orbital overlap with s orbital of H atoms form two sigma bond while the other two filled oribtals contain lone pairs. Due to the large size and low electro negativity of S atom, the lp-lp repulsion is higher than lp-bp repulsion. This reduces the bond angle from 109 º28 to 92º and give the bent molecular geometry.