What are the isomers of C4H8?

I count six isomers of ##"C"_4"H"_8##.

An alkane with 4 carbon atoms would have the formula ##"C"_4"H"_10##.

This hydrocarbon has two hydrogens less, so it must contain either a double bond or a ring.

The six possibilities are:

But-1-ene

cis-But-2-ene

(from commons.wikimedia.org)

trans-But-2-ene

2-Methylpropene

Cyclobutane

Methylcyclopropane

And there you are!