X rays are electromagnetic waves of very short wavelength. Alternatively, they are very high-energy photons. They are emitted by atoms with many electrons, such as those high in the periodic table. The more electrons, the greater the charge of the nucleus, and the higher the energy of the electrons that are close to the nucleus (with n = 1 or 2). Therefore, when the atom is disturbed, as it is in an X-ray tube when the anode is bombarded by high-energy electrons, one of the n = 1 electrons can be kicked out. When an n = 2 electron loses energy and takes the place of the kicked-out electron, an X ray is emitted.