Question

How many isomers of propene and ethene?
Actually,in my view propene has two isomers - n-propene, and cyclopropane. But some of the websites has mentioned that their is no isomers of propene. and some other websites has mentioned that the isomer of propene really exist as cylopropene and n-propene.

Answer 1

there is no isomer of propene and ethene 
c=c-c or c=c just form same number of carbon   and ethene c=c too and we know 
                 c
isomer are the compound having same formula but different structure and here both the structure are forming same compound.

Answer 2

There is no isomers of both propene and ethene.