The above two compounds exhibit chain isomerism. Isomers are compounds that have same molecular formula but different structures. In the above structures, both have the same functional group (thioether / sulphide), but there is difference in the arrangement of atoms in the main chain. The first one is linear( propylmethylsulphide) , while the second one(isopropylmethylsulphide) is branched, i.e., there is difference in the chains attached to –S-. hence they are exhibiting chain isomerism.