Question

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

1. $C_2H_2$
2. $C_2H_4$

Answer 1

a. 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 $C_2H_2$ can be represented as:

Hence, there are three sigma and two $pi-bonds$ in $C_2H_2$.

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The structure of $C_2H_4$ can be represented as:

Hence, there are five sigma bonds and one $pi-bond$ in $C_2H_4$.