Carbon-12 Carbon-12 Carbon-12 (C) is the more abundant of the two stable isotopes of carbon (Carbon-13 being the other), amounting to 98.93% of the element carbon; its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured, thus, its atomic mass is exactly 12 daltons by definition. Its nucleus contains 6 protons and 6 neutrons. Before 1959 both the IUPAP and IUPAC used oxygen to define the mole; the chemists defining the mole as the number of
Atomic mass unit of exactly 16 amu in physics. The divergence of these values could result in errors in computations, and was unwieldy. The chemistry amu, based on the relative atomic mass (atomic weight) of natural oxygen (including the heavy naturally-occurring isotopes O and O), was about as massive as the physics amu, based on pure isotopic O. For these and other reasons, the reference standard for both physics and chemistry was changed to carbon-12 in 1961. The choice of carbon-12 was made to minimise further divergence with prior literature. The new and current unit was referred to as the "unified atomic mass