An unsubstituted alkane is butane with molecular formula [tex]C_{4}H_{10}[/tex]. In butane, four carbon and ten hydrogen atoms are present which can arranged in only two ways.
The molecules which have same number of atoms of each element but differently arranged atoms are said to be isomers.
The structure of n-butane and iso-butane is shown in the images which exist as isomers (same number of atoms) of butane.
Thus, butane is an unsubstituted alkane that can exist as one of only two isomers.
