State the number of $\sigma$ and $\pi$ bonds in the following molecules:
$(a)$ $C_2H_2$
$(b)$ $C_2H_4$

(N/A) The structure of $C_2H_2$ is $H-C \equiv C-H$.
In the triple bond between the two carbon atoms,there is $1$ $\sigma$ bond and $2$ $\pi$ bonds.
There are also $2$ $C-H$ $\sigma$ bonds.
Thus,$C_2H_2$ has a total of $3$ $\sigma$ bonds and $2$ $\pi$ bonds.
$(b)$ The structure of $C_2H_4$ is $H_2C=CH_2$.
There are $4$ $C-H$ $\sigma$ bonds and in the $C=C$ double bond,there is $1$ $\sigma$ bond and $1$ $\pi$ bond.
Thus,$C_2H_4$ has a total of $5$ $\sigma$ bonds and $1$ $\pi$ bond.