Oxygen has seventeen known isotopes with atomic masses ranging from 12.03 u to 28.06 u. Three are stable, 16O, 17O, and 18O, of which 16O is the most abundant (over 99.7%). The radioisotopes all have half-lives of less than three minutes.
An atomic weight of 16 was assigned to oxygen prior to the definition of the unified atomic mass uni based upon 12C. Since physicists referred to 16O only, while chemists meant the naturally abundant mixture of isotopes, this led to slightly different atomic weight scales.Oxygen has three stable isotopes. These are 16O, 17O, and 18O. 16O is the most common in earths atmosphere, with a 99.762% majority abundance. It also has fourteen Radioactive Isotopes, all with half-lives under three minutes.