Oxygen-16 (symbol: ¹⁶O or ¹⁶
₈O
) is a nuclide. It is a stable isotope of oxygen, with 8 neutrons and 8 protons in its nucleus, and when not ionized, 8 electrons orbiting the nucleus. Oxygen-16 has a mass of 15.99491461956 u. It is the most abundant isotope of oxygen and accounts for 99.762% of oxygen's natural abundance.^[2]

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The relative and absolute abundances of oxygen-16 are high because it is a principal product of stellar evolution and because it is a primordial isotope, meaning it can be made by stars that were initially made exclusively of hydrogen.

Most oxygen-16 is synthesized at the end of the helium fusion process in stars; the triple-alpha process creates carbon-12, which captures an additional helium-4 to make oxygen-16. The neon-burning process also makes it.

Oxygen-16 is doubly magic.

Solid samples (organic and inorganic) for oxygen-16 studies are usually stored in silver cups and measured with pyrolysis and mass spectrometry.^[3] Researchers need to avoid improper or prolonged storage of the samples for accurate measurements.^[3]

Originally, one atomic mass unit was defined as one sixteenth of the mass of oxygen-16, but the atomic mass unit has since been redefined as one twelfth of the mass of carbon-12.