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Revision as of 02:17, 13 March 2001 by Larry Sanger (talk | contribs)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)The lightest chemical element and the most abundant in the universe, around 75% of normal matter, consisting of a single proton and electron. The most common isotope has no neutrons, although there are two others - deuterium with one, and radioactive tritium with two.
At normal STP-like conditions, hydrogen forms a diatomic gas, H2, with a boiling point of only 20.28 K and a melting point of 13.81 K. Under exceedingly high pressures, like those found at the center of gas giants, the molecules lose their identity and the hydrogen becomes a liquid metal. Under the exceedingly low pressure conditions found in space, hydrogen tends to exist as individual atoms, simply because there is no way for them to combine; clouds of H2 form and are associated with star formation.
Hydrogen has an electronegativity of 2.1, so it forms compounds where it is the more non-metallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H ions or just as a solute within the other element (as in Palladium hydride). The latter tend to be covalent, since the H ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. Thus even in an acidic solution one sees ions like H3O as the protons latch on to something.
Hydrogen combines with oxygen to produce water, H2O, and releases a lot of energy doing so, burning explosively in air. The name hydrogen, which comes from the French, in fact means water-maker, ultimately from the Greek.