1. A colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known chemical elements.
2. A colorless, highly flammable gaseous element, the lightest of all gases and the most abundant element in the universe, used in the production of synthetic ammonia and methanol, in petroleum refining, in the hydrogenation of organic materials, as a reducing atmosphere, in oxyhydrogen torches, and in rocket fuels.
3. From 1791, French hydrogène, coined in 1787 by G. de Morveau from Greek hydr-, stem of hydros, "water" + French -gène, "producing". So called because it forms water when exposed to oxygen.

Source: water, most organic compounds. Use: industrial processes, production of ammonia, reduction of metal ores to metals.

Information is located at Chemical Element: hydrogen.

An electrode that absorbs hydrogen gas; used in pH measurement.

The nucleus of a hydrogen atom separated from its accompanying electron.

The hydrogen nucleus is made up of a particle carrying a unit positive electric charge, called a proton.

Because the bare nucleus can readily combine with other particles (electrons, atoms, and molecules), the isolated hydrogen ion can exist only in a nearly particle-free space (high vacuum) and in the gaseous state.

In common usage, the term hydrogen ion is used to refer to the hydrogen ion present in water solutions, in which it exists as the combined molecule H + H₂O.

The amount of hydrogen ion present in a water solution is used as a measure of the acidity of a substance; the higher the concentration of hydrogen ion the more acidic the solution and the lower the pH.

An economy in which direct solar energy would be the primary energy source and hydrogen the secondary energy carrier.

Power from wind or photovoltaic systems would drive photo-electrolytic hydrogen production.