Allotropes are unlike isotopes, while isotopes are the same element with the same number of protons and different number of neutrons, allotropes are different forms of the same element for example, carbon has diamond and graphite as two of its allotropes. There are allotropes because elements bond themselves differently, it only happens in elements though not compounds.

external image 300px-Eight_Allotropes_of_Carbon.png external image 300px-Diamond_and_graphite.jpg