Shell Of Atom (Energy Level)

           An electron shell, or a principal energy level, may be thought of as an orbit followed by electrons around an atom’s nucleus.

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          The closest shell to the nucleus is called the “1st shell” (also called “K shell”), followed by the “2nd shell” (or “L shell”), then the “3rd shell” (or “M shell”), and then the “4th shell” (or N shell) and so on further and further from the nucleus. The shells are also represented by principal quantam number (n = 1, 2, 3, 4 …) or are labeled alphabetically with letters(K, L, M, N, …).

           Each shell can contain only a fixed number of electrons: The first shell can hold up to two (only contain s subshell) electrons, the second shell can hold up to eight (2 + 6) electrons (contain s and p subshell), the third shell can hold up to 18 (2 + 6 + 10) electrons (contains s, p and d subshells) the fourth shell can hold up to 32 (2 + 6 + 10+14) electrons (contains s, p, d and f subshells) and so on. The general formula to find the number of electrons in an atom is 2(n2) where n is number of shell.Since electrons are electrically attracted to the nucleus, an atom’s electrons will generally occupy outer shells only if the more inner shells have already been completely filled by other electrons. However, this is not a strict requirement: atoms may have two or even three incomplete outer shells.

         The electrons in the outermost occupied shell (or shells) determine the chemical properties of the atom; it is called the valence shell.