/ Dictionary / Index I isotopic number: Meaning and Definition of Find definitions for: i'sotop'ic num'ber Pronunciation: [key] — Physics. Physics. the number of neutrons minus the number of protons in an atomic nucleus. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. isotope isotopic spin Related Content Daily Word Quiz: sequester Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language