Editing Atomic orbital (section)

=== Relativistic effects ===
{{Main|Relativistic quantum chemistry}}
{{See also|Extended periodic table}}

For elements with high atomic number {{mvar|Z}}, the effects of relativity become more pronounced, and especially so for s&nbsp;electrons, which move at relativistic velocities as they penetrate the screening electrons near the core of high-{{mvar|Z}} atoms. This relativistic increase in momentum for high speed electrons causes a corresponding decrease in wavelength and contraction of 6s orbitals relative to 5d orbitals (by comparison to corresponding s and d electrons in lighter elements in the same column of the periodic table); this results in 6s valence electrons becoming lowered in energy.

Examples of significant physical outcomes of this effect include the lowered melting temperature of [[mercury (element)|mercury]] (which results from 6s electrons not being available for metal bonding) and the golden color of gold and [[caesium]].<ref>{{cite web|url=http://www.chem1.com/acad/webtut/atomic/qprimer/#Q26|title= Primer on Quantum Theory of the Atom|first=Stephen |last=Lower}}</ref>

In the [[Bohr model]], an {{math|1=''n'' = 1}}&nbsp;electron has a velocity given by <math>v = Z \alpha c</math>, where {{mvar|Z}} is the atomic number, <math>\alpha</math> is the [[fine-structure constant]], and {{math|''c''}} is the speed of light. In non-relativistic quantum mechanics, therefore, any atom with an atomic number greater than 137 would require its 1s electrons to be traveling faster than the speed of light. Even in the [[Dirac equation]], which accounts for relativistic effects, the wave function of the electron for atoms with <math>Z > 137</math> is oscillatory and [[unbounded function|unbounded]]. The significance of element 137, also known as [[untriseptium]], was first pointed out by the physicist [[Richard Feynman]]. Element 137 is sometimes informally called [[feynmanium]] (symbol Fy).<ref>{{cite journal|last1=Poliakoff|first1=Martyn|last2=Tang|first2=Samantha|title=The periodic table: icon and inspiration|journal=Philosophical Transactions of the Royal Society A|date=9 February 2015|volume=373|issue=2037|page=20140211|doi=10.1098/rsta.2014.0211|pmid=25666072|bibcode = 2015RSPTA.37340211P |doi-access=free}}</ref> However, Feynman's approximation fails to predict the exact critical value of&nbsp;{{mvar|Z}} due to the non-point-charge nature of the nucleus and very small orbital radius of inner electrons, resulting in a potential seen by inner electrons which is effectively less than {{mvar|Z}}. The critical {{mvar|Z}}&nbsp;value, which makes the atom unstable with regard to high-field breakdown of the vacuum and production of electron-positron pairs, does not occur until {{mvar|Z}} is about 173. These conditions are not seen except transiently in collisions of very heavy nuclei such as lead or uranium in accelerators, where such electron-positron production from these effects has been claimed to be observed.
	
There are no nodes in relativistic orbital densities, although individual components of the wave function will have nodes.<ref>{{cite journal|doi=10.1021/ed046p678|title=Contour diagrams for relativistic orbitals|year=1969|last1=Szabo|first1=Attila|journal=Journal of Chemical Education|volume=46|issue=10|pages=678|bibcode = 1969JChEd..46..678S }}</ref>