Carbon-13 and Carbon-14 are isotopes of the element carbon. All carbon atoms have 6 protons in their nucleus and most have 6 neutrons. Since the mass number consists of # of protons + # of neutrons, Carbon 12 has an atomic mass of 12 ( = 6 protons + 6 neutrons). Carbon-13 atoms have one extra neutron, giving it a total of 7 neutrons. Carbon-14 atoms have two extra neutrons, giving them a total of 8 neutrons. Therefore the mass number of C-12= 12, C-13= 13 and C-14= 14.
