Allotropism as a noun is the ability (of an element) to exist in more than one physical form without change of state. while Allotropy as a noun is a property, exhibited by some elements of existing in multiple forms with different atomic structures.
Part of speech: noun
Definition: The ability (of an element) to exist in more than one physical form without change of state.
Part of speech: noun
Definition: A property, exhibited by some elements of existing in multiple forms with different atomic structures.
We hope you now know whether to use Allotropism or Allotropy in your sentence.