We gots #"methane"#, a formula of #CH_4#...given a tetrahedral structure, there is only one possible structural arrangement, one isomer. And this is the case for #"ethane"# #H_3C-CH_3#, AND ALSO for #"propane"#, #H_3C-CH_2-CH_3#; the three carbons must lie in a single chain. And go to #"butane"#, and now we have the possibility of the straight chain isomer, #"n-butane"#, and a branched chain isomer, i.e. #"methylpropane"#: TWO ISOMERS in total....
And go to #"pentane"#, there is the possibility of THREE structural isomers, and #"hexane"# offers the possibility of FIVE structural isomers. As the formula gets bigger, the possibility for isomerism greatly expands....
