noun.
The occurrence of more than one form of a chemical element with difference in physical properties is explained, in the light of the atomic theory, as depending on a difference in the number, and possibly in the arrangement, of the atoms which go to make up the molecule. Thus it is believed that in the more common form of oxygen there are two, but in the allotropic ozone three, atoms to the molecule.
noun.
Allotropical variation; allotropy.