The numbers of nucleons for which shells are filled are called magic numbers. Magic numbers of 2, 8, 20, 28, 50, 82 and 126 have been observed for neutrons, and the next number is predicted to be 184. Protons share the first six of these magic numbers, and 126 has been predicted as a magic proton number since the 1940s. Nuclides with a magic number of each—such as O (Z = 8, N = 8), Sn (Z = 50, N = 82), and Pb (Z = 82, N = 126)—are referred to as "doubly magic" and are more stable than nearby nuclides as a result of greater binding energies.