Hydrogen

Hydrogen is a chemical element with symbol "H" and atomic number 1. With an atomic weight of 1.00794 u (1.007825 u for hydrogen-1), hydrogen is the lightest element and its non-atomic form (H₁) is the most abundant chemical substance, constituting roughly 75% of the Universe's baryonic mass. Non-remnant stars are mainly composed of hydrogen in its plasma state.

At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H₂. Most of the hydrogen on Earth is in molecules such as water and organic compounds because hydrogen readily forms covalent compounds with most non-metallic elements.

Hydrogen plays a particularly important role in acid-base chemistry with many reactions exchanging protons between soluble molecules. In ionic compounds, it can take a negative charge (an anion known as a hydride and written as H⁻), or as a positively charged species H⁺. The latter cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds always occur as more complex species.

The most common isotope of hydrogen is protium (name rarely used, symbol ¹H) with a single proton and no neutrons. As the simplest atom known, the hydrogen atom has been of theoretical use. For example, as the only neutral atom with an analytic solution to the Schrödinger equation, the study of the energetics and bonding of the hydrogen atom played a key role in the development of quantum mechanics.

Hydrogen gas was first artificially produced in the early 16th century, via the mixing of metals with acids. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, a property which later gave it its name: in Greek, hydrogen means "water-former".

Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive hydrogen production methods like the electrolysis of water. Most hydrogen is employed near its production site, with the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production, mostly for the fertilizer market.