<P> The shell terminology comes from Arnold Sommerfeld's modification of the Bohr model . Sommerfeld retained Bohr's planetary model, but added mildly elliptical orbits (characterized by additional quantum numbers l and m) to explain the fine spectroscopic structure of some elements . The multiple electrons with the same principal quantum number (n) had close orbits that formed a "shell" of positive thickness instead of the infinitely thin circular orbit of Bohr's model . </P> <P> The existence of electron shells was first observed experimentally in Charles Barkla's and Henry Moseley's X-ray absorption studies . Barkla labeled them with the letters K, L, M, N, O, P, and Q. The origin of this terminology was alphabetic . A "J" series was also suspected, though later experiments indicated that the K absorption lines are produced by the innermost electrons . These letters were later found to correspond to the n values 1, 2, 3, etc . They are used in the spectroscopic Siegbahn notation . </P> <P> The physical chemist Gilbert Lewis was responsible for much of the early development of the theory of the participation of valence shell electrons in chemical bonding . Linus Pauling later generalized and extended the theory while applying insights from quantum mechanics . </P> <P> The electron shells are labeled K, L, M, N, O, P, and Q; or 1, 2, 3, 4, 5, 6, and 7; going from innermost shell outwards . Electrons in outer shells have higher average energy and travel farther from the nucleus than those in inner shells . This makes them more important in determining how the atom reacts chemically and behaves as a conductor, because the pull of the atom's nucleus upon them is weaker and more easily broken . In this way, a given element's reactivity is highly dependent upon its electronic configuration . </P>

What is the maximum number of electrons that can occupy the innermost electron orbital