Quantum mechanics that is in accord with special relativity theory. The main equation of relativistic quantum mechanics is the Dirac equation. It is necessary to use relativistic quantum mechanics to describe the electronic properties of heavy atoms and all the fine structure of atomic spectra, such as the energy level splitting due to spin-orbit coupling. The colour of solid gold and mercury existing as a liquid are both due to relativistic effects in quantum mechanics. Such effects become more pronounced as the atomic number increases. It has been suggested that relativistic quantum mechanics imposes an end to the periodic table irrespective of the stability of atomic nuclei (see feynmanium).