Who is the first member of the alkene series? Provide information about its bond and structure.

(N/A) The general formula for alkenes is $C_{n}H_{2n}$.
For $n=1$,the formula would be $CH_2$ (methene),but it is highly unstable and has an extremely short lifetime,so it is not considered a stable member.
Therefore,the first stable member of the alkene series is ethene $(C_{2}H_{4})$.
In ethene,the two carbon atoms are connected by a double bond $(C=C)$,which consists of one sigma $(\sigma)$ bond and one pi $(\pi)$ bond.
The structure of ethene is planar,with a bond angle of approximately $120^{\circ}$ around each carbon atom,exhibiting $sp^2$ hybridization.