<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> A quantum mechanical system or particle that is bound--that is, confined spatially--can only take on certain discrete values of energy . This contrasts with classical particles, which can have any energy . These discrete values are called energy levels . The term is commonly used for the energy levels of electrons in atoms, ions, or molecules, which are bound by the electric field of the nucleus, but can also refer to energy levels of nuclei or vibrational or rotational energy levels in molecules . The energy spectrum of a system with such discrete energy levels is said to be quantized . </P> <P> In chemistry and atomic physics, an electron shell, or a principal energy level, may be thought of as an orbit followed by electrons around an atom's nucleus . The closest shell to the nucleus is called the "1 shell" (also called "K shell"), followed by the "2 shell" (or "L shell"), then the "3 shell" (or "M shell"), and so on farther and farther from the nucleus . The shells correspond with the principal quantum numbers (n = 1, 2, 3, 4 ...) or are labeled alphabetically with letters used in the X-ray notation (K, L, M, ...). </P> <P> Each shell can contain only a fixed number of electrons: The first shell can hold up to two electrons, the second shell can hold up to eight (2 + 6) electrons, the third shell can hold up to 18 (2 + 6 + 10) and so on . The general formula is that the nth shell can in principle hold up to 2 (n) electrons . 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 . (See Madelung rule for more details .) For an explanation of why electrons exist in these shells see electron configuration . </P>

The principal energy levels of an atom are