electrons, it is capable of forming bonds with four other atoms may be
    carbon or atoms of some other monovalent element. When carbon reacts
    with hydrogen,four hydrogen atoms allow their‘s’orbitals containing one
    electron each to overlap the four sp3orbitals of carbon atom which are
   &nbs;poriented at an angle of 109o28/.(Four orbitals of an atom in the outer
   &nbs;pshell orient along the four corners of a tetrahedron to have minimum
    repulsion between their electrons).The nucleus of the atom is at the centre
    of the tetrahedron. See figures below:
    
          This leads to form four sp3 – s sigma bonds between carbon atom and
   four hydrogen atoms. All these bonds are of equal energy.

   Sp2 hybridisation 
   
           How do you explain the ability of C – atom to form two single covalent
   bonds and one double bond?
   Consider ethene / ethylene (CH2 = CH2) molecule as the example.
   
    In the formation of CH2=CH2 each carbon atom in its excited state
    undergoes sp2 hybridisation by intermixing one s-orbital (2s) and two
    p-orbitals (say 2px, 2py) and reshuffling to form three sp2 orbitals. Now
    each carbon atom is left with one ‘p’ orbital (say pz) unhybridised. The
    three sp2 orbitals having one electron each get separated around the nucleus
    of carbon atoms at an angle of 120⁰. When carbon is ready to form bonds
    one sp2 orbital of one carbon atom overlaps the sp2  orbital of the other
    carbon atom to form sp2 – sp2 sigma (σ) bond. The remaining two sp2
    orbitals of each carbon atom get overlapped by‘s’ orbitals of two hydrogen
    atoms containing unpaired electrons. The unhybridised pz orbitals on the 
    two carbon atoms overlap laterally as shown in figure to form a π bond.