Question

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

Answer 1

Hint: Alkanes, alkenes and alkynes are hydrocarbon chains without any functional groups in them. This classification is based on the number of bonds between carbon and hydrogen atoms.

Complete answer:
Let us first understand about the homologous series;
The homologous series is a sequence of compounds with identical chemical properties, though certain functional groups vary from the successive $\,C{H_2}\,$ components.
Now, in this question ethene mentioned belongs to alkenes;
Alkenes are unsaturated hydrocarbons in which two carbon atoms have a double bond. In these compounds, the presence of the double bond is due to unsaturation. General formula of alkenes is $\,{C_n}{H_{2n}}\,$, where $\,n\,$ represents the number of carbon atoms.
Which means, if $\,n\,$is $\,2\,$, the alkene is ethene and the formula is $\,{C_2}{H_4}\,$. So, in this question the first member of the homologous series is ethene. So, in order to find out the second and third member, we can use this formula by putting n as $\,3\,$ and $\,4\,$;
If $\,n = 3\,$, $\,{C_3}{H_6}\,$ which is propene
If $n = 4\,$, $\,{C_4}{H_8}\,$ which is butene

Additional Information: Since alkenes contain less than the maximum possible number of atoms of hydrogen per atom of carbon, it is said that they are unsaturated. Alkenes will have isomers based on certain factors, in addition to the various isomers that can be formed based on chain branching. The position of the double bond is one such factor.

Note: We have to note that, there is no alkene with only one carbon atom, hence the first member is ethene. Olefins are another concept used to classify Alkenes. Due to the presence of the double bond, alkenes are more reactive than alkanes.