posted by pranu .
An allotrope is a variant of a substance consisting of only one type of atom. It is a new molecular configuration, with new physical properties. Substances that have allotropes include carbon, oxygen, sulfur, and phosphorous. Allotropes of a given substance will often have substantial differences between each other.
got this froma website called wisegeek
Example;-Oxygen exists as two gaseous allotropes,'normal oxygen(O2)and (Ozone(O3),which differ in their molecular configurations.