Wikipedia:Today's featured article/October 29, 2006



Hydrogen is a chemical element in the periodic table that has the symbol H and atomic number 1. At standard temperature and pressure it is a colorless, odorless, nonmetallic, univalent, tasteless, highly flammable diatomic gas. With an atomic mass of 1.00794 g/mol, hydrogen is the lightest element. It is also the most abundant, constituting roughly 75% of the universe's elemental matter. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. Elemental hydrogen is relatively rare on Earth, and is industrially produced from hydrocarbons, after which most free hydrogen is used "captively" (meaning locally at the production site), with the largest markets about equally divided between fossil fuel upgrading (e.g. hydrocracking) and in ammonia production (mostly for the fertilizer market). The most common naturally occurring isotope of hydrogen contains one electron and an atomic nucleus of one proton. Hydrogen can form compounds with most elements and is present in water and all organic compounds. It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules. (more...)

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