In the hydrogen atom the ground state has an energy of -13.6 eV. When a hydrogen atom decays from an excited state to the ground state the photon emitted has an energy between 10.2 eV and 13.6 eV - this is in the ultraviolet part of the spectrum.

Because the ground state energy is proportional to Z², atoms higher up the periodic table give off photons of much higher energy when they decay to the ground state. For example, when Z = 45 the magnitude of the ground state energy is about 2000 times that of hydrogen, so the photons have about 2000 times the energy, enough energy to be classified as x-rays.

X-rays are usually produced by directing a beam of high-energy electrons (often about 100 keV) at a metal target (often molybdenum, with Z = 42, or tungsten, with Z = 74). When the electrons hit the target they decelerate quickly, giving off bremsstrahlung (braking) radiation that is generally made up of a wide spectrum of wavelengths.

The emitted spectrum also shows sharp peaks, with a large fraction of x-rays being emitted at particular wavelengths. These are associated with the target material. The atoms in the target get excited when impacted by the beam of electrons, and particular photons are emitted when the atoms decay back to the ground state. X-rays corresponding to the n=2 to n=1 transition make up the K_a line in the x-ray spectrum, while those corresponding to the n=3 to n=1 transition make up the K_b line.

The energy of an electron in the K-shell (i.e., the n=1 level) is approximately equal to:

EK = -13.6 eV

(Z-1)2 12

We get Z-1 here because there are two electrons in the n=1 level. For each of these two electrons, the other electron effectively cancels out one of the protons in the nucleus. This is known as screening.