<P> There are three known stable isotopes of oxygen (O): O, O, and O . </P> <P> Radioactive isotopes with mass numbers from O to O have also been characterized, all short - lived, with the longest - lived being O with a half - life of 122.24 seconds, while the shortest - lived isotope is O with a half - life of 580 (30) × 10 second . </P> <P> Naturally occurring oxygen is composed of three stable isotopes, O, O, and O, with O being the most abundant (99.762% natural abundance). Depending on the terrestrial source, the standard atomic weight varies within the range of (15.99903, 15.99977) (the conventional value is 15.999). Known oxygen isotopes range in mass number from 12 to 24 . </P> <P> The relative and absolute abundance of O is high because it is a principal product of stellar evolution and because it is a primary isotope, meaning it can be made by stars that were initially made exclusively of hydrogen . Most O is synthesized at the end of the helium fusion process in stars; the triple - alpha reaction creates C, which captures an additional He to make O . The neon burning process creates additional O . </P>

How many isotopes of oxygen occur naturally on earth