n–Butane and isobutane are a pair of

The chemical formulas of n-butane and iso-butane are identical, but the number of carbon in  chain is differ from the main chain . As a result, they are chain isomers.

n-butane - ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_3$

iso-butane - ${\mathrm{CH}}_3\mathrm{CH}\left({\mathrm{CH}}_3\right){\mathrm{CH}}_3$