O16

Oxygen-16, often written as 16O or oxygen-16, is the most abundant stable isotope of the element oxygen. It accounts for about 99.76% of natural oxygen. Its nucleus contains eight protons and eight neutrons, giving a mass number of 16. The atomic mass is 15.99491461956 atomic mass units. 16O is stable and has no known radioactive decay.

In nuclear physics, 16O is noteworthy as a doubly magic nucleus, with both proton number Z = 8 and neutron number N = 8 occupying closed shells in the nuclear model. This configuration contributes to its high binding energy and exceptional stability relative to many other light nuclei. Its overall binding energy per nucleon is substantial, reflecting strong nuclear cohesion.

16O is produced in stars during helium burning, primarily through the fusion reaction 12C(α,γ)16O. It becomes one of the dominant oxygen isotopes in stellar and galactic chemistry. On Earth, it is the predominant isotope of oxygen in atmospheric O2, water, and rocks, forming the basis of the planet’s oxygen inventory.

Analytically and geochemically, the ratios involving 16O, such as 16O/18O and 17O/16O, are used to study past temperatures, hydrological cycles, and planetary processes. As a stable reference isotope, 16O serves as a standard in various mass spectrometric and isotope-dratio measurements.