Isomerism. Compounds that have the same molecular formula but different structural formulas are called isomers or isomeric compounds. For example, for the molecular formula C₄H₁₀ you can draw two different structural formulas: butane and 2-methylpropane. These molecules have different physical properties such as density, boiling point etc.

Under the two isomers for C₄H₁₀ above you see two different structures for compounds with the molecular formula C₅H₁₀. There are at least 5 more!

Using a double bond try to find more isomeric compounds for C5H10.

Answer C₅H₁₀ isomers.

A special form of isomerism is more geometrical than constitutional. It is called cis-trans isomerism. It occurs when compounds have double bonds or rings. Here is an example:

You notice that the rigidity of the double bond and the ring, the fact that you cannot rotate the C-C bonds without breaking them, causes the molecule to possess cis-trans isomerism. Build these molecules with a Model Box to verify this phenomenon. In alkenes it is the pi bond that hinders the free rotation. In cycloalkanes it is the ring structure that blocks rotation.