allotrope n.

in chemistry, "property of existing in two or more distinct forms, variation of physical properties without change of substance," 1850, from French or German allotropie (1840), from Greek allotropia "variety," from allos "different, other" (from PIE root *al- (1) "beyond") + tropos "manner, way" (from PIE root *trep- "to turn").

word-forming element meaning "other," from Greek allos "other, different," cognate with Latin alius "other," from PIE root *al- (1) "beyond."

word-forming element meaning "that which turns," from Greek tropos "a turn, direction, course, way," from PIE root *trep- "to turn."