Hydrogen

General properties
Name, symbol, number	hydrogen, H, 1
Element category	nonmetal
Group, period, block	1, 1, s
Standard atomic weight	1.008(1)
Electron configuration	1s1 1
History
Discovery	Henry Cavendish (1766)
Named by	Antoine Lavoisier (1783)

Hydrogen is a chemical element with symbol Hand atomic number 1. With an average atomic weight of 1.00794 u(1.007825 u for hydrogen-1), hydrogen is the lightest element and its monatomic 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. Naturally occurring atomic hydrogen is rare on Earth because hydrogen readily forms covalent compounds with most elements and is present in the water molecule and in most organic compounds. 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 strong 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., hydro cracking) and ammonia production, mostly for the fertilizer market. Hydrogen is a concern in metallurgy as it can embrittle metals, complicating the design of pipelines and storage tanks.

Return to Periodic Table