Examining the kinds of isotopes that exist gives clues about the nature of the strong force. The most stable nucleus, helium-4, has two neutrons having spins in opposite directions and two protons also having spins in opposite directions. A majority of non-radioactive isotopes have even numbers of neutrons and even number of protons, allowing them to form pairs with opposite spins. Most of the other isotopes have either an even number of protons and an odd number of neutrons or the opposite. Isotopes with both an odd number of protons and neutrons are extremely rare. So, the strength of the strong force clearly depends on the spins of the nucleons.

The mass of a nucleus is less than the sum of the masses of the protons plus the sum of masses of the neutrons. The larger the mass difference, the stronger the forces holding the nucleus together and the more energy needed to pull the nucleus apart. Studies of the mass of the nuclei can thus be used to gain further insight into the strong force. Maria Goeppert-Mayer (1906-1972) was a German-born American physicist who explained why nuclei with certain numbers of protons and/or neutrons, called magic numbers, were extremely stable. Her theory showed that the nuclear force depended on both the spin of the nucleon and its orbital angular momentum. The magic numbers are 2, 8, 20, 28, 50, 82, and 126. Thus helium-4, the most stable nucleus has a magic number of protons and a magic number of neutrons, and so is called doubly magic. Oxygen-16, calcium-40 and calcium-48, and lead-208, which is the heaviest stable nuclide, are also doubly magic. But, are all nuclei stable?