Energy levels of atomic electrons
In an atom, the laws of quantum mechanics force the electrons to have one of a set of well-defined energy values. This is very different from the classical interpretation of physics, where any energy value would have been allowed. The lowest energy level (the K shell) can only be occupied by two electrons, the L shell by 8 and the M shell by 18. The K shell electrons are closest to the nucleus. On the figure above, a gap has been left on the K shell, with the former occupant having been kicked out by a passing particle. The ejected electron is immediately replaced by an L shell electron which ‘falls’ to the lower level by emitting an X-ray.