Compounds having the same molecular formula but different structures are called structural isomers. The given compounds have the same molecular formula but they differ in the positions of the functional group(ketone group).

In structure 1, the ketone group is at the C-3 of the parent chain(hexane chain) and in structure 2, the ketone group is at the C-2 of the parent chain(hexane chain). Hence, the given pairs represents structural isomers.