CBSE Class 11-science Answered

In ethane there is only single bond between C-C which is sigma bond whereas in ethylene there is a double bond between C=C which consists of a pi-bond formed due to sideways overlap of 2p orbitals of carbon atoms in addition to sigma bond.

pi and sigma bonds together form a double bond which is stronger and shorter than a single sigma bond, thus carbon atoms C=C are closer to each other in ethylene than in ethane C-C

Also, the bond energy of C=C is 610 kJ mol^-1 while C-C bond energy of ethane is 370 kJ mol^-1. Therefore, carbon-carbon bond lengths are in ethane (154 pm), in ethylene (134 pm) and in acetylene (120 pm).

More bonds make the total bond shorter and stronger.