ISOMERISM

Compounds that have the same molecular formula, but are not identical are called isomers.
There are two main classes of isomers, constitutional isomers and stereoisomers. 


Constitutional  isomers will have the same number and types of atoms, but they are connected differently sequentially (they have a different "constitution"). A simple example of constitutional isomers are ethanol and dimethyl ether. 



In stereoisomers, the atoms are connected sequentially in the same way, such that condensed formulas for the two molecules are identical. The isomers differ, however, in the way the atoms are arranged in space. There are two major sub-classes of stereoisomers; conformational isomers, which interconvert through rotations around single bonds, and configurational isomers, which are not readily interconvertable.