A

Hydrogen

---

MELTING POINT: −259.14°C
BOILING POINT: −252.87°C
DENSITY: 0.08988 g/L
MOST COMMON IONS: H⁺, H⁻

Hydrogen was first recognized as a gaseous substance in 1766 by English chemist and physicist Henry Cavendish. The abundance of hydrogen in Earth's crust is 1,520 parts per million. The abundance of hydrogen in the universe by weight is 74 percent and by number of atoms is 90 percent. Hence, hydrogen is the major constituent of the universe. Under ordinary conditions (STP) on Earth, hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water. It is the least dense gas known (0.08988 grams per liter at STP). Ordinary hydrogen gas (H₂) exists as diatomic molecules. It reacts with oxygen to form its major compound on Earth, water (H₂O). It also reacts with nitrogen, halogens, and sulfur, to form ammonia (NH₃), hydrogen monohalide compounds (e.g., HCl) and hydrogen sulfide (H₂S), respectively. It combines with several metals to form metal hydrides, and carbon to form a great many organic compounds.

Hydrogen is a mixture of three isotopes: protium (¹H; atomic mass 1.007822); deuterium, or heavy hydrogen (²H or D; atomic mass 2.0140; 1 atom of ²H to every 6,700 atoms of ¹H); and tritium (³H or T; atomic mass 3.016; has a radioactive nucleus). The fusion of protium nuclei (protons) to form helium is believed to be the major source of the Sun's energy. The extreme heat of reaction in hydrogen-oxygen burning is used in high temperature welding and melting processes. Hydrogen molecule addition reactions (hydrogenation) are widely used in industry, for example, for the hardening of animal fats or vegetable oils, for the synthesis of methanol from carbon monoxide, and in petroleum refining.

SEE ALSO CAVENDISH, HENRY; EXPLOSIONS; GASES.

Ágúst Kvaran

Bibliography

Rigden, John S. (2002). Hydrogen: The Essential Element. Cambridge, MA: Harvard University Press.