What is "isomerism"?

An isomerism (isomer) is a phenomenon whereby two
compounds share the same chemical formula whilst behaving differently.  The German
chemist Johannes Wislicneus suggested in 1873 that this is possible because of
differences in the molecular structure or arrangement of the atoms between the two
bonds.  The former is called a structural isomer and the latter is called a
stereoisomer.  In order for a stereoisomer to exist, there must be carbon atoms bonded
with other carbon atoms in the molecular
structure.

Exmaples:

The
carbon atoms in butane (C4H10) can connect in a straight-line (n-butane) or the fourth
carbon atom can branch off of the middle carbon atom in a chain of three
(iso-butane).

i.e.
H H H H
|  |  | 
|
n-butane:   H-C-C-C-C-H
|  |  |  |
H H H
H

H
|
H-C-H
H | H
|  | 
|
iso-butane:    H-C-C-C-H
|  |  |
H H
H