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When Hard-Pressed, Hydrogen Becomes a Metal The most active metals are found in Group 1A. In contrast to the other members of this group, hydrogen is typically a nonmetal. The very small size of the hydrogen atom means that its electron is bound quite firmly, and when hydrogen bonds to other nonmetals, it typically does so by sharing electrons rather than losing its electron to become a cation. However, recent research indicates that under extraordinarily high pressures, hydrogen changes to a metal. Ho-Kwang Mao and R. J. Hemley have performed experiments using a diamond-anvil cell in which mechanical means are used to produce millions of atmospheres of pressure in a tiny 20-m cell between two beveled, gem-quality diamonds. These researchers found that under 3 million atm of pressure, hydrogen first assumes a solid phase that is transparent and does not conduct electricity, and then it changes to an opaque solid that conducts electricity like a metal. There is a great deal of interest in this metallic form of hydrogen because theory predicts it should be a superconducting solid (zero resistance to current flow) at temperatures near 0°C, much higher than the temperatures required for any presently known superconducting material. However, so far no conditions have been found that stabilize the metallic form of hydrogen other than extraordinarily high pressure. Researchers are hoping that a way can be found to stabilize the metallic hydrogen phase so that it persists under less severe conditions. ■