Define the term 'structural isomerism'. Explain why propane cannot exhibit this property. Draw the structures of possible isomers of butane,$C_{4}H_{10}$.

(N/A) $(i)$ Structural isomerism is the phenomenon in which compounds have the same molecular formula but different structural arrangements of atoms.
$(ii)$ Propane $(C_{3}H_{8})$ cannot exhibit structural isomerism because it contains only three carbon atoms. With only three carbon atoms,there is only one possible way to arrange them in a chain,meaning no alternative structural arrangement can be formed.
$(iii)$ Butane $(C_{4}H_{10})$ has two structural isomers:
$(a)$ $n$-butane: $A$ straight-chain structure where four carbon atoms are linked in a single row.
$(b)$ Isobutane ($2$-methylpropane): $A$ branched-chain structure where three carbon atoms form the main chain and one carbon atom is attached as a branch to the central carbon atom.