Write the molecular formula of the $2^{nd}$ and $3^{rd}$ member of the homologous series whose first member is ethene.

The homologous series starting with ethene ($C_2H_4$) is the alkene series, which follows the general formula $C_nH_{2n}$.

1. The first member is ethene ($n=2$): $C_2H_4$.
2. The second member is propene ($n=3$): $C_3H_6$.
3. The third member is butene ($n=4$): $C_4H_8$.

As per the NCERT text (Section 1.1.1), chemical formulae are used to represent substances concisely. In a homologous series, each successive member differs by a $-CH_2-$ unit.