A nucleus typically has roughly the same number of neutrons as protons, or a few more. Unlike the proton count, which is fixed for any given element, the neutron count varies. Normal carbon has six protons and six neutrons, giving a total ‘mass number’ of 12 (since the mass of electrons is negligible and a neutron weighs approximately the same as a proton). It is therefore called carbon 12. Carbon 13 has one extra neutron, and carbon 14 two extra neutrons, but they all have six protons. Such different
