Isomers are two or more chemical compounds that have the same number of each kind of atom, but differ in the way the atoms are arranged. Because the atom arrangement varies, isomers are generally differ from each other in both their physical properties and their chemical behavior.
For example, n-butane and isobutane (the two types of the gaseous butane) are isomers. The two have the same formula, C4H10. But, because the arrangement of atoms in their molecules differs, they have different boiling points.
The four carbon atoms in a molecule of n-butane form of a zigzag chain. N-butane boils about at -0.5 C.
In a molecule of isobutene, three carbon atoms are attached to the fourth in a pyramid pattern. Isobutane boils at -11.7 C.