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Meaning of ionium

i•o•ni•um

Pronunciation: (ī-ō'nē-um), [key]
— n. Chem.
  1. a naturally occurring radioactive isotope of thorium. Symbol: Io; at. no.: 90; at. wt.: 230.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.