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 " shell" (also called "K shell"), followed by the " shell" (or "L shell"), then the " 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, …).