A form of an element that has isotopic abundances that are different from the naturally occuring form. Compare with isotope and polymorph .
Some elements occur in several distinct forms called allotropes.
Allotropes have different chemical and physical properties. For example, graphite and diamond are allotropes of carbon. A particle that is commonly ejected from radioactive nuclei, consisting of two protons and two neutrons. Alpha particles are helium nuclei. A stream of alpha particles .