Hybridisation and Shapes of Molecules:
Hybridisation is the process of intermixing of the pure atomic orbitals of slightly different energies so as to distribute their energies resulting in the formation of new set of orbitals of equivalent energies and shape. The new orbitals formed as a result of hybridization are called hybrid or hybridized orbitals.
1) Sp3 Hybridisation:
To form four equivalent bonds, one 2s and three 2p-orbitals of carbon hybridise and form four new orbitals called sp3-hybrid orbitals.
The carbon atoms in alkanes involve sp3 hybridisation. As a result, the four bonds formed by each carbon atom are directed towards the corners of a regular tetrahedron. For example, in case of methane (CH4), the carbon atom involves sp3 hybridisation and forms four sp3 hybrid orbitals. Each of these forms sigma bond by overlapping with 1s orbitals of hydrogen. The four bonds are directed towards the corners of a regular tetrahedron as shown below. The H—C—H bond angle in this molecule is 109.5°.
Fig: Shape of methane
In ethane (CH3—CH3) molecule, each carbon atom undergoes sp3 hybridisation. One of the four sp3 hybrid orbitals of one carbon atom overlaps axially with similar orbital of the other carbon atom to form C—C sigma bond. The remaining three hybrid orbitals belonging to both the carbon atoms overlap axially with the half- filled orbitals of hydrogen atoms to form C—H sigma bonds as shown below.
2) Sp2 Hybridisation:
Alkenes are planar molecules and the carbon atoms of the C=C bond involve sp2hybridisation. Carbon atom has four unpaired electrons in the excited state. The three orbitals (one 2s and two 2p) get hybridised to form three sp2-hybrid orbitals leaving one 2pz-unhybridised orbital.
For example in ethylene one sp2 hybrid orbital of one carbon atom overlaps with sp2 hybrid orbital of the other carbon atom to form C—C sigma bond. The remaining two sp2-hybrid orbitals of both the carbon atoms overlap with 1s-orbitals of two hydrogen atoms to form C—H sigma bonds. The unhybridised orbital participates in the formation of pi (π) bond.
Fig: Structure of ethylene
2) Sp-Hybridisation:-
The two carbon atoms constituting the triple bond are sp-hybridised. In this, carbon undergoes sp-hybridisation forming two sp-hybrid orbitals. The two 2p-orbitals (2py and 2pz) remain unhybridised.
Fig: Structure of ethyne
For example, in the case of acetylene, one sp-hybrid orbital of one carbon atom overlaps with sp-hybrid orbital of the second carbon atom and forms C—C sigma bond. The remaining sp-hybrid orbital of each C-atom forms sigma bond with H-atom. Each of the unhybridised orbitals of one carbon atom forms π-bond with the second carbon atom so that there are two π-bonds in acetylene molecule. The structure of acetylene (ethyne) is shown above.
Note:
i) Sigma (σ) bond is formed by end to end overlapping of bonding orbitals along the internuclear axis.
ii) Pi (π) bond is formed by the sidewise overlapping of the half filled atomic orbitals of bonding atoms.
