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Metallic hydrogen is an alkali metal made from hydrogen gas. This is formed when the hydrogen gas is under compression and results with the gas changing phase. It is an example of degenerate matter. The matter has a crystal structure of proton atoms, with spacing that is extremely small, smaller than a Bohr radius (which is 52.918 x 10-12 m). The electrons in the molecular orbital are unbounded and they behave like the conduction electrons in a metal. Metallic hydrogen is unusual as the element itself is not an alkali metal, but a diatomic gas.
Metallic hydrogen is known to exist in the cores of Jupiter and Saturn. Jupiter has the most metallic hydrogen. Metallic hydrogen is a dense liquid. Metallic hydrogen acts like a metal and has crazy properties.