MAGIC NUMBER

in physics, in the shell models of both atomic and nuclear structure, any of a series of numbers that connote stable structure. They designate the sum of electrons in atoms or the sum of either protons or neutrons in nuclei that occupy completely filled, or closed, shells. The magic numbers for atoms are 2, 10, 18, 36, 54, and 86, corresponding to the total number of electrons in filled electron shells. In the chemical elements of atomic number 17 to 19, for example, the chloride ion (Cl-), the argon atom (Ar), and the potassium ion (K+) have 18 electrons in closed-shell configurations and are chemically quite stable. The number of electrons present in the neutral atoms constituting the relatively unreactive noble gases exactly correspond to the atomic magic numbers. The magic numbers for nuclei are 2, 8, 20, 28, 50, 82, and 126, corresponding to the total number of protons or neutrons in filled nuclear shells. Thus, tin (atomic number 50), with 50 protons in its nucleus, has 10 stable isotopes, whereas indium (atomic number 49) and antimony (atomic number 51) have only 2 stable isotopes apiece. The doubly magic alpha particle, or helium-4 nucleus, composed of two protons and two neutrons, is very stable.