Editing Metallic hydrogen (section)

===Hydrogen under pressure===
Though often placed at the top of the [[alkali metal]] column in the [[periodic table]], hydrogen does not, under ordinary conditions, exhibit the properties of an alkali metal. Instead, it forms [[Diatomic molecule|diatomic]] {{chem2|H2}} molecules, similar to [[halogens]] and some [[Nonmetal (chemistry)|nonmetal]]s in the second period of the periodic table, such as [[nitrogen]] and [[oxygen]]. Diatomic hydrogen is a gas that, at [[atmospheric pressure]], [[Liquid hydrogen|liquefies]] and [[Solid hydrogen|solidifies]] only at very low temperature (20&nbsp;[[kelvin (unit)|K]] and 14&nbsp;K respectively). 

In 1935, physicists [[Eugene Wigner]] and [[Hillard Bell Huntington]] predicted that under an immense [[pressure]] of around {{convert|25|GPa|atm psi|abbr=on}}, hydrogen would display [[metal]]lic properties: instead of discrete {{chem2|H2}} molecules (which consist of two electrons bound between two protons), a bulk phase would form with a solid lattice of protons and the electrons [[Delocalized electron|delocalized]] throughout.<ref name=Wigner1935/> Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics".<ref>{{cite press release |date=6 May 1998 |title=High-pressure scientists 'journey' to the center of the Earth, but can't find elusive metallic hydrogen |url=https://www.sciencedaily.com/releases/1998/05/980512080541.htm |publisher=[[ScienceDaily]] |access-date=28 January 2017}}</ref>

The initial prediction about the amount of pressure needed was eventually shown to be too low.<ref>{{cite journal |last1=Loubeyre |first1=P. |display-authors=etal |year=1996 |title=X-ray diffraction and equation of state of hydrogen at megabar pressures |journal=[[Nature (journal)|Nature]] |volume=383 |issue=6602 |pages=702–704 |bibcode=1996Natur.383..702L |doi=10.1038/383702a0|s2cid=4372789 }}</ref> Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metallization pressures of around {{convert|400|GPa|atm psi|abbr=on}}.<ref>{{cite journal |last1=Azadi |first1=S. |last2=Monserrat |first2=B. |last3=Foulkes |first3=W.M.C. |last4=Needs |first4=R.J. |year=2014 |title=Dissociation of High-Pressure Solid Molecular Hydrogen: A Quantum Monte Carlo and Anharmonic Vibrational Study |journal=[[Physical Review Letters]] |volume=112 |issue=16 |pages=165501 |doi=10.1103/PhysRevLett.112.165501 |pmid=24815656 |arxiv=1403.3681 |bibcode=2014PhRvL.112p5501A|s2cid=28888820 }}</ref><ref>{{cite journal |last1=McMinis |first1=J. |last2=Clay |first2=R.C. |last3=Lee |first3=D. |last4=Morales |first4=M.A. |year=2015 |title=Molecular to Atomic Phase Transition in Hydrogen under High Pressure |journal=[[Physical Review Letters]] |volume=114 |issue=10 |pages=105305 |doi=10.1103/PhysRevLett.114.105305 |pmid=25815944 |bibcode=2015PhRvL.114j5305M|doi-access=free }}</ref>