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  molecules, similar to halogens and some nonmetals in the second period of the periodic table, such as nitrogen and oxygen. Diatomic hydrogen is a gas that, at atmospheric pressure, liquefies and solidifies only at very low temperature (20 K and 14 K respectively). 
In 1935, physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around , hydrogen would display metallic properties: instead of discrete  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 throughout. Since then, producing metallic hydrogen in the laboratory has been described as "the holy grail of high-pressure physics".
The initial prediction about the amount of pressure needed was eventually shown to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations point toward higher but potentially achievable metalization pressures of around .