In nuclear physics, a magic number is the number of protons or neutrons in a very stable atomic nucleus. For both particles, the magic numbers are 2, 8, 20, 28, 50, and 82 (corresponding, for protons, to the elements helium, oxygen, calcium, nickel, tin, and lead). Proton number 114 is also a magic number, as are 126 and 184 for neutrons. A shell model of the nucleus (analogous to electron shells in the atom) was proposed to account for the extra stability conferred by the magic numbers.