The 11 possible isomers of C5H10 are pentene, trans-2-pentene, cis-2-pentene, 2-methylbutene, 9-methylbutene, 2-methyl-2-butene, cyclopentane, methylcyclobutane, ethylcyclopropane, 1,1-dimethylcyclopropane and 1,2-dimethylcyclopropane. All of these are configurational isomers, except for the cis and trans isomers of 2-pentene, which are geometrical isomers of each other.
Configurational isomers are molecules with the same atoms but different connectivity between then. Geometric isomers have the same connectivity but different orientations around a double bond. The largest hydrocarbon with five carbon atoms is C5H12. Therefore, C5H10 has an index of hydrogen deficiency of two. These two missing hydrogens must be replaced by either one double bond or one ring. The first six isomers listed above are alkenes, which are hydrocarbons with double bonds. Each isomer has the double bond and its substituent alkanes in different positions. The last five are cycloalkanes containing one ring. The ring can be made up of five carbon atoms and no alkane substituents, four carbons and one substituent, or three carbons and two substituents. Like all cyclopropanes, the last three isomers are unstable, since they have three carbon-to-carbon bonds at a 60-degree angle. This kind of instability is known as ring strain, and it occurs because the most thermodynamically stable carbon-to-carbon bond angle is 109.5 degrees.