Dear Student
Ethane and propane are not isomers.
Isomers are two molecules with the same molecular formula but they differ structurally.
So, isomers contain the same number of atoms of each element, only atomic arrangement differs.
Ethane has molecular formula C2H6.
Propane has molecular formula C3H8.
Hence, they do not contain same number of atoms each element, so they are not isomers. Their molecular formula is different.
Regards