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Homologous series

What is homologous series?
can we put alkanes , alkenes , alkynes  in to same group or this is homologous series or not if yes then why if no then why not! 
 
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1 Answer

No, Prasenjeet, we cannot lump alkanes, alkynes and alkenes into one "homologous" series.
 
The general formula for an alkane is CnH2n + 2 (n ≥ 1) in which the repeating unit is -CH2-
 
For 1-alkenes the general formula is CnH2n (n ≥ 2) for which the repeating unit is also -CH2-, but notice how the general formula differs from that of alkanes.
 
As a final example consider 1-alkynes for which the general formula is CnH2n − 2 (n ≥ 2).
 
Compounds form a homologous series as long as they have the same general formula.  Thus methane, ethane, propane, butane etc. form one homologous series (the alkanes).  Alkenes and alkynes form their own series.