The given definition is best illustrated by an example: consider the isomers possible for #"butane"#, whose formula is #C_4H_10#.
Two structual isomers are possible: #"n-butane"#, #H_3C-CH_2CH_2CH_3#, and so-called #"isobutane"#, #H_3C-CH(CH_3)CH_3#. Clearly, these have the the same chemical formula, #C_4H_10#, but they have different connectivity, which results in different physical properties, viz. boiling point, #-0.5# #""^@C#, #"n-butane"#, versus #-11.7# #""^@C#, #"isobutane"#.
As expected the longer hydrocarbyl chain of #"n-butane"# allows for greater #"van der Waals"# interaction between the chains, and an increased boiling point with respect to #"isobutane"#.
