Structural isomers / isomerism is an occurence whereby compounds having different structural formulae, yet have the same molecular formula.
Example : Butane is represented by two different compound with different structural formula :
C4H10 :
• Can be represented by : CH3CH2CH2CH3 ---- n-butane or
• can be represented by : CH3CH-CH3CH3 ....... iso-butane ( 2-methyl propane).