# Finding structural isomers of an alkene

I am doing problems on finding structural isomers. I am asked to find the structural isomers of $$\ce{C5H10}$$. The answer to problem states there are 10 structural isomers possible. I was able to find only 9. Could any one please tell what is the 10th isomer which I missed. Further could any one please tell is there any other method to find structural isomers other than to list them out, like a standard formula?

• Ethylcyclopropane is missing. – Ivan Neretin Aug 7 at 5:24