Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 77: 2.82b

Butane and pentane.

Ethane and propane cannot form isomers because their hydrocarbon chain is very small. Hence, they cannot form branched isomers. Butane and pentane can form isomers such as 2-methylpropane, 2-methylbutane, 2,2-dimethylpropane.