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 form water, H2O, and releases a lot of energy in doing so, burning explosively in air. The name hydrogen, which comes from the French, in fact means water-maker, ultimately from the Greek.
See: Periodic table