why hydrocarbons having less than 4 carbon atoms does not show isomerism

because methane , ethane and propane cant be arranged in the form of branched chain. They can be arranged only in the form of straight chain. Thus they dont show isomerism.
Atleast 4 carbon atoms are required for arranging in the form of branched chain. thus they show isomerism.

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And also branching cannot take place on first & last carbon atoms...
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we cannot have isomers of the first 3 members of the alkane series bcoz of the following laws of isomers:
1) the parent chain should have the most number of carbon atoms
2) The branching cannot be done from the first and the last carbon atom of the structure
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