Answer:

OKAY, well I am answering this because many people have answered to this query either in a wrong way or they are just partially right.

As many said that it's a necessity to have functional group for POSITION isomers, well, they are wrong. I will be giving examples to state my point.

CHAIN ISOMERS

NON CYCLIC CHAINS

Parent chain must be of different NUMBERS of carbon. For eg. Iso pentane and n pentane and neo pentane, all among themselves are chain isomers.

Functional group if present must be same in both/all isomers. For eg. Pentane-2-ol and 2-methylbutan-1-ol.

CYCLIC RINGS

Parent carbon chain can be same, but then the side chain or BRANCH must form DIFFRENT types of radicals( should have diffrent word roots.). For eg. 1)Ethyl cyclohexane and 1,3 methyl cyclohexane and 2) Ethyl cyclohexane and 1,2- methyl cyclo (NOTE- 1,3-methylcyclohexane and 1,2- methyl cyclo hexane are positional isomers among themselves but are chain isomers when compared to Ethyl cyclohexane.)

POSITION ISOMERS

CYCLIC/ non CYCLIC

Parent carbon chain has to be same and it's a mandate.

Only the position of functional group or side branch must be different. Eg 2-methylpentane and 3-methylpentane.

I hope it helps.