In acetylene molecule there exists a triple bond between two carbon
    atoms and the fourth valency of each carbon atom is satisfied by hydrogen
    atoms (H–C C–H ).
     
          In C2H2 molecule there are two carbon atoms and two hydrogen atoms.
   In excited state each carbon atom undergoes sp- hybridisation by mixing
   its one ‘s’ orbital (2s)and one ‘p’ orbital (2px) and reshuffling to form two
   identical orbitals known as sp-orbitals. Each carbon atom has two
   unhybridised p-orbitals (say 2py, 2pz). 
   
   
         One sp-orbital of a carbon overlaps the sp-orbital of other carbon to
   give sp-sp sigma bond. The other sp-orbital of each carbon atom overlaps
   ‘s’orbital of a hydrogen atom to form a s-sp sigma bond. The unhybridised
   ‘p’orbitals of one carbon atom laterally overlap the unhybridised‘p’orbitals
   of other carbon atom to give two π bonds between two carbon atoms
   (say πpy-py, πpz-pz, see figure). Thus ethyne molecule H–C C–H and
   there exists three σ-bonds and two π-bonds in the molecule.
   
   
   
    Allotropes of Carbon
 
         The property of an element to exist in two or more physical forms
   having more or less similar chemical properties but different physical
   properties is called allotropy.The different forms of the element are called
   Allotropes form due to the difference in the arrangement of
   atoms in the molecules.
   
         The allotropes of carbon are classified into two types. They are

          • Amorphous forms                • Crystalline forms