Oxygen-16

Oxygen-16 (¹⁶O) is a stable isotope of oxygen, having 8 neutrons and 8 protons in its nucleus. It has a mass of 15.99491461956 u. Oxygen-16 is the most abundant isotope of oxygen, and accounts for 99.762% of oxygen's natural abundance. The relative and absolute abundance of ¹⁶O 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 ¹⁶O is synthesized at the end of the helium fusion process in stars; the triple-alpha process creates ¹²C, which captures an additional ⁴He to make ¹⁶O. The neon-burning process creates additional ¹⁶O.

Isotopes are species which have the same number of protons, but different atomic masses due to a different number of neutrons. Oxygen 16 and 16/8 O both have 8 protons, 8 neutrons, and an atomic weight of 16 amu.