Oxygen isotope

Main isotopes of oxygen

iso­tope NA half-life DM DE (MeV) DP 16O 99.76% is stable with 8 neutrons 17O 0.04% is stable with 9 neutrons 18O 0.20% is stable with 10 neutrons
Standard atomic weight (Ar)	[7001159990300000000♠15.99903, 7001159997700000000♠15.99977] Conventional: 15.999

There are three stable isotopes of oxygen (₈O): ¹⁶O, ¹⁷O, and ¹⁸O. Radioactive isotopes with mass numbers from ¹²O to ²⁴O have also been characterized, all short-lived, with the longest-lived being ¹⁵O with a half-life of 122.24 seconds. The shortest-lived is ¹²O with a half-life of 580(30)×10⁻²⁴ second.

Naturally occurring oxygen is composed of three stable isotopes, ¹⁶O, ¹⁷O, and ¹⁸O, with ¹⁶O 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.

The relative and absolute abundance of ¹⁶O 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 ¹⁶O is synthesized at the end of the helium fusion process in stars; the triple-alpha reaction creates ¹²C, which captures an additional ⁴He to make ¹⁶O. The neon burning process creates additional ¹⁶O.

Both ¹⁷O and ¹⁸O are secondary isotopes, meaning that their nucleosynthesis requires seed nuclei. ¹⁷O is primarily made by the burning of hydrogen into helium during the CNO cycle, making it a common isotope in the hydrogen burning zones of stars. Most ¹⁸O is produced when ¹⁴N (made abundant from CNO burning) captures a ⁴He nucleus, making ¹⁸O common in the helium-rich zones of stars. Approximately a billion degrees Celsius is required for two oxygen nuclei to undergo nuclear fusion to form the heavier nucleus of sulfur.

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