Shell

Name:
Shell
Detailed information:
(1) In physical chemistry, this term is applied to any of the several sets of, or orbits of, the electrons in an atom as they revolve around its nucleus. They constitute a number of principal quantum paths representing successively higher energy levels. There may be from one to seven shells, depending on the atomic number of the element and corresponding to the seven periods of the periodic table. The shells are usually designated by number, though letter symbols have been used, i.e., K, L, M, N, O, P, Q. The laws of physics limit the number of electrons in the various shells as follows: two in the first (K), eight in the second (L), 18 in the third (M), and 32 in the fourth (N). With the exception of hydrogen and helium, each shell contains two or more orbitals, each of which is capable of holding a maximum of two electrons.See Quantum Number; Orbital Theory; Pauli Exclusion Principle.(2) The hard integument of mollusks and crustaceans, consisting mostly of calcium carbonate, chitin, etc.See Nacre; Oyster Shells.(3) The brittle covering of avian eggs, chiefly calcium carbonate, lime, etc. The formation of proper shell structures in certain species of birds is said to be adversely affected by DDT and similar insecticidal contaminants of their food.(4) The shells of nuts are cellulosic in character. Some contain industrially useful oils.