Isomers are compounds which have the same molecular formula but different structures. $\underset{\text{n- butane}}{ \mathrm{H}-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\mathrm{H} }$  $\underset{\text{Iso butane}}{ \mathrm{H}-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}-\underset{ \vert_{\mathrm{H}} }{ \mathrm{C} }-\mathrm{H}} }{ \mathrm{C} } }-\overset{ \vert^{\mathrm{H}} }{ \underset{ \vert_{\mathrm{H}} }{ \mathrm{C} } }-\mathrm{H} }$ Since branching is not possible in first 3 members thus isomers of these first members of alkane series is not possible.