n–Butane and isobutane are a pair of

$\underset{\mathrm{n}-\mathrm{butane}}{{\mathrm{CH}}_3-\stackrel{{\mathrm{H}}_2}{\mathrm{C}}-\stackrel{{\mathrm{H}}_2}{\mathrm{C}}-{\mathrm{CH}}_3}$

$\underset{\mathrm{Iso} \mathrm{butane}}{{\mathrm{CH}}_3-\underset{\underset{\mathrm{H}}{|}}{\stackrel{\stackrel{{\mathrm{CH}}_3}{|}}{\mathrm{C}}}-{\mathrm{CH}}_3}$

Both are having same M.F −${\mathrm{C}}_4{\mathrm{H}}_{10}$ but different structural chains.