Why can't we have the isomers of first 3 members of alkane series ?

The first three members of the alkane series are methane, ethane and propane. Methane has the chemical formula CH4. Ethane has the chemical formula C2H6 and is also represented as CH3-CH3. Propane has the chemical formula C3H8 and is also represented as CH3-CH2-CH3. In alkanes, isomerism arises when a particular compound can be represented in the form of both straight chain and branched isomer (by placing alkyl groups as substituents on different carbon atoms, example in butane, pentane etc). The structural formulae of methane, ethane and propane shows that they do not have sufficient number of carbon atoms to exist in the form of branched isomer, that is they can be represented in only one way. In other words, branching is not possible in these compounds. Hence the first three members of alkanes do not have isomers. 

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