/ Dictionary / Index I ionium: Meaning and Definition of Find definitions for: i•o•ni•um Pronunciation: (ī-ō'nē-um), [key] — n. Chem. 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. ion implantation ionization chamber Related Content Daily Word Quiz: tactful Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language