The energy difference between the 2s
and 2p orbitals isvery small. When carbon
atom is ready to form bonds it gets a small
amount of energy from bond energies and
gets excited to promote an electron from
the 2s to the empty 2pz to give four unpaired
electrons.

We have got four unpaired electrons
ready for bonding, but these electrons are
in two different kinds of orbitals and their
energies are different. We are not going to
get four identical bonds unless these
unpaired electrons are in four identical
                          orbitals.

How to explain the four orbitals of carbon containing unpaired
electrons as energetically equal?

We can explain it by the phenomenon called hybridisation.

 
     The concept of hybridisation was introduced by Linus Pauling (1931).
The redistribution of orbitals of almost equal energy in individual atoms
to give equal number of new orbitals with identical properties like energy
and shape is called “hybridisation”. The newly formed orbitals are called
as ‘hybrid orbitals.’
 In the excited carbon atom its one s-orbital (2s) and three p-orbitals
(2px,2py,2pz) intermix and reshuffle into four identical orbitals known as
SP3 orbitals. Thus, carbon atom undergoes SP3 hybridisation.
     The four electrons enter into the new four identical hybrid orbitals
known as sp3 hybrid orbitals one each as per Hund’s rule. (because they
are made from one ‘s-orbital’ and three ‘p-orbitals’ they are called sp3
orbitals).
NOTE:Read“sp3 “as“ s p three”. or“yes pee three”




     
   The hybridisation enables the carbon to have four identical sp3 hybrid
orbitals and these have one electron each. Since carbon has four unpaired.