allotropism vs allotropy

allotropism and allotropy both are nouns.

Word	Noun	Adjective	Verb	Adverb
allotropism	Yes	No	No	No
allotropy	Yes	No	No	No

As nouns, allotropism and allotropy are synonyms defined as:

allotropism and allotropy: the phenomenon of an element existing in two or more physical forms

allotropism (noun)	allotropy (noun)
the phenomenon of an element existing in two or more physical forms	the phenomenon of an element existing in two or more physical forms