Question

What is the total number of sigma and pi bonds in the following molecules?

(a) $C_2{H}_2$ (b) $C_2{H}_4$

Answer 1

A single bond is a result of the axial overlap of bonding orbitals. Hence, it contributes a sigma bond. A multiple bond (double or triple bond) is always formed as a result of the sidewise overlap of orbitals. A pi-bond is always present in it. A triple bond is a combination of two pi-bonds and one sigma bond.

Structure of C2H2 can be represented as:

Hence, there are three sigma and two pi-bonds in $C_2{H}_2$.
The structure of $C_2{H}_4$ can be represented as:

Hence, there are five sigma bonds and one pi-bond in $C_2{H}_4$.