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Oxygen-16
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<p>Oxygen-16 (symbol: 16O or 168O) is a <a href="/facts/Nuclide/97Ykvzr3">nuclide</a>. It is a <a href="/facts/Stable_isotope/vgcdxgjA">stable</a> <a href="/facts/Isotopes_of_oxygen/ektDXb8B">isotope of oxygen</a>, with 8 <a href="/facts/Neutron/oRCM1geb">neutrons</a> and 8 <a href="/facts/Proton/clCjepXP">protons</a> in its <a href="/facts/Atomic_nucleus/orpIm5Xl">nucleus</a>, and when not ionized, 8 electrons orbiting the nucleus. The atomic mass of oxygen-16 is 15.99491461956 <a href="/facts/Dalton_(unit)/IMATU3vf">Da</a>. It is the most abundant isotope of oxygen and accounts for 99.757% of oxygen's <a href="/facts/Natural_abundance/dGjaAwc9">natural abundance</a>. </p><p>The relative and absolute abundances of oxygen-16 are high because it is a principal product of <a href="/facts/Stellar_evolution/Qa4vAGmn">stellar evolution</a> and because it is a <a href="/facts/Primordial_isotope/lFAUqHDv">primordial isotope</a>, meaning it can be made by stars that were <a href="/facts/Population_III_stars/8ur1qDRK">initially made exclusively of hydrogen</a>. </p><p>Most oxygen-16 is synthesized at the end of the <a href="/facts/Helium_fusion/wV6lacSF">helium fusion</a> process in stars; the <a href="/facts/Triple-alpha_process/wV6lacSF">triple-alpha process</a> creates carbon-12, which captures an additional helium-4 to make oxygen-16. It is also created by the <a href="/facts/Neon-burning_process/gFaHhLDK">neon-burning process</a>. </p><p>Oxygen-16 is <a href="/facts/Magic_number_(physics)/v9PwXFzQ">doubly magic</a>. </p><p>Solid samples (organic and inorganic) for oxygen-16 studies are usually stored in silver cups and measured with <a href="/facts/Pyrolysis/h1cEegI7">pyrolysis</a> and <a href="/facts/Mass_spectrometry/RH5WBHJ2">mass spectrometry</a>. Researchers need to avoid improper or prolonged storage of the samples for accurate measurements. </p><p>Historically, one atomic mass unit was defined as one sixteenth of the mass of an oxygen-16 atom, but has been redefined as one twelfth of the mass of a <a href="/facts/Carbon-12/nJYECzEr">carbon-12</a> atom, with the name <i>unified atomic mass unit</i> or <i><a href="/facts/Dalton_(unit)/IMATU3vf">dalton</a></i>. </p>