Isomers are compounds with the same molecular formula but different structures.
  1. Structural isomers such as pentanes and 2-methyl butane (isopentane) differ in the covalent arrangements of their atoms.

  2. Geometric isomers differ in their spatial arrangements around a double bond.
    Cis isomers have two groups on the same side relative to the double bond.
    Trans isomers have two groups on opposite sides relative to the double bond.

    For example, the biochemistry of vision involves a light–induced change of rhodopsin, a chemical compound in the eye, from the cis isomer to the trans isomer.

  3. Enantiomers (or optical or stereo isomers) contain an asymmetric carbon and are mirror images of each other.