An isomer possible due to different permutations of element positions in a molecule. Position isomers of pentane are possible since there are 3 ways to build a "tree" of 5 carbon atoms: n-pentane, isopentane, and neopentane. Geometric structural isomers are related to lack of rotatability in cyclic and multiply bonded species. "Cis" isomers have identical species are located on same side of the multiple bond or ring; "trans isomers" have identical species are located on opposite sides of the multiple bond or ring.