Carbon-12 is the more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons. Carbon-12 is of particular importance as it's used as the standard from which atomic masses of all nuclides are measured: its mass number is by definition 12. History Prior to 1959 both the IUPAP and IUPAC tended to use oxygen to define the mole, the chemists defining the mole as the number of atoms of oxygen which had mass 16 g, t… (More on Carbon-12)