Hydrogen From Wikipedia, the free encyclopedia This article is about the chemistry of hydrogen. For the physics of atomic hydrogen, see Hydrogen atom. For other meanings, see Hydrogen (disambiguation). Page semi-protected Hydrogen 1H Hydrogen (diatomic nonmetal) Helium (noble gas) Lithium (alkali metal) Beryllium (alkaline earth metal) Boron (metalloid) Carbon (polyatomic nonmetal) Nitrogen (diatomic nonmetal) Oxygen (diatomic nonmetal) Fluorine (diatomic nonmetal) Neon (noble gas) Sodium (alkali metal) Magnesium (alkaline earth metal) Aluminium (other metals) Silicon (metalloid) Phosphorus (polyatomic nonmetal) Sulfur (polyatomic nonmetal) Chlorine (diatomic nonmetal) Argon (noble gas) Potassium (alkali metal) Calcium (alkaline earth metal) Scandium (transition metal) Titanium (transition metal) Vanadium (transition metal) Chromium (transition metal) Manganese (transition metal) Iron (transition metal) Cobalt (transition metal) Nickel (transition metal) Copper (transition metal) Zinc (transition metal) Gallium (other metals) Germanium (metalloid) Arsenic (metalloid) Selenium (polyatomic nonmetal) Bromine (diatomic nonmetal) Krypton (noble gas) Rubidium (alkali metal) Strontium (alkaline earth metal) Yttrium (transition metal) Zirconium (transition metal) Niobium (transition metal) Molybdenum (transition metal) Technetium (transition metal) Ruthenium (transition metal) Rhodium (transition metal) Palladium (transition metal) Silver (transition metal) Cadmium (transition metal) Indium (other metals) Tin (other metals) Antimony (metalloid) Tellurium (metalloid) Iodine (diatomic nonmetal) Xenon (noble gas) Caesium (alkali metal) Barium (alkaline earth metal) Lanthanum (lanthanide) Cerium (lanthanide) Praseodymium (lanthanide) Neodymium (lanthanide) Promethium (lanthanide) Samarium (lanthanide) Europium (lanthanide) Gadolinium (lanthanide) Terbium (lanthanide) Dysprosium (lanthanide) Holmium (lanthanide) Erbium (lanthanide) Thulium (lanthanide) Ytterbium (lanthanide) Lutetium (lanthanide) Hafnium (transition metal) Tantalum (transition metal) Tungsten (transition metal) Rhenium (transition metal) Osmium (transition metal) Iridium (transition metal) Platinum (transition metal) Gold (transition metal) Mercury (transition metal) Thallium (other metals) Lead (other metals) Bismuth (other metals) Polonium (other metals) Astatine (metalloid) Radon (noble gas) Francium (alkali metal) Radium (alkaline earth metal) Actinium (actinide) Thorium (actinide) Protactinium (actinide) Uranium (actinide) Neptunium (actinide) Plutonium (actinide) Americium (actinide) Curium (actinide) Berkelium (actinide) Californium (actinide) Einsteinium (actinide) Fermium (actinide) Mendelevium (actinide) Nobelium (actinide) Lawrencium (actinide) Rutherfordium (transition metal) Dubnium (transition metal) Seaborgium (transition metal) Bohrium (transition metal) Hassium (transition metal) Meitnerium (unknown chemical properties) Darmstadtium (unknown chemical properties) Roentgenium (unknown chemical properties) Copernicium (transition metal) Ununtrium (unknown chemical properties) Flerovium (unknown chemical properties) Ununpentium (unknown chemical properties) Livermorium (unknown chemical properties) Ununseptium (unknown chemical properties) Ununoctium (unknown chemical properties) - ↑ H ↓ Li - ← hydrogen → helium Hydrogen in the periodic table Appearance colorless gas Purple glow in its plasma state Spectral lines of hydrogen General properties Name, symbol, number hydrogen, H, 1 Pronunciation /ˈhaɪdrədʒən/ hy-drə-jən[1] Element category diatomic nonmetal Group, period, block 1, 1, s Standard atomic weight 1.008(1) Electron configuration 1s1 1 Physical properties Color colorless Phase gas Density (0 °C, 101.325 kPa) 0.08988 g/L Liquid density at m.p. 0.07 (0.0763 solid)[2] g·cm−3 Liquid density at b.p. 0.07099 g·cm−3 Melting point 13.99 K, −259.16 °C, −434.49 °F Boiling point 20.271 K, −252.879 °C, −423.182 °F Triple point 13.8033 K, 7.041 kPa Critical point 32.938 K, 1.2858 MPa Heat of fusion (H2) 0.117 kJ·mol−1 Heat of vaporization (H2) 0.904 kJ·mol−1 Molar heat capacity (H2) 28.836 J·mol−1·K−1 Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 15 20 Atomic properties Oxidation states 1, −1 (amphoteric oxide) Electronegativity 2.20 (Pauling scale) Ionization energies 1st: 1312.0 kJ·mol−1 Covalent radius 31±5 pm Van der Waals radius 120 pm Miscellanea Crystal structure hexagonal Hydrogen has a hexagonal crystal structure Magnetic ordering diamagnetic[3] Thermal conductivity 0.1805 W·m−1·K−1 Speed of sound (gas, 27 °C) 1310 m·s−1 CAS registry number 1333-74-0 History Discovery Henry Cavendish[4][5] (1766) Named by Antoine Lavoisier[6] (1783) Most stable isotopes Main article: Isotopes of hydrogen iso NA half-life DM DE (MeV) DP 1H 99.985% 1H is stable with 0 neutrons 2H 0.015% 2H is stable with 1 neutron 3H trace 12.32 y β− 0.01861 3He v t e · references Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass.[7][note 1] Non-remnant stars are mainly composed of hydrogen in its plasma state. The most common isotope of hydrogen, termed protium (name rarely used, symbol 1H), has a single proton and zero neutrons. The universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, highly combustible diatomic gas with the molecular formula H2. Since hydrogen readily forms covalent compounds with most non-metallic elements, most of the hydrogen on Earth exists in molecular forms such as in the form of water or organic compounds. Hydrogen plays a particularly important role in acid–base reactions. In ionic compounds, hydrogen can take the form of a negative charge (i.e., anion) known as a hydride, or as a positively charged (i.e., cation) species denoted by the symbol H+. The hydrogen cation is written as though composed of a bare proton, but in reality, hydrogen cations in ionic compounds are always more complex species than that would suggest. As the simplest atom known, the hydrogen atom has had considerable theoretical application. For example, the hydrogen atom is the only neutral atom with an analytic solution to the Schrödinger equation. 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,[8] 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.[9] 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. Hydrogen is a concern in metallurgy as it can embrittle many metals,[10] complicating the design of pipelines and storage tanks.[11] Contents [hide] 1 Properties 1.1 Combustion 1.2 Electron energy levels 1.3 Elemental molecular forms 1.4 Phases 1.5 Compounds 1.5.1 Covalent and organic compounds 1.5.2 Hydrides 1.5.3 Protons and acids 1.6 Isotopes 2 History 2.1 Discovery and use 2.2 Role in quantum theory 3 Natural occurrence 4 Production 4.1 Metal-acid 4.2 Steam reforming 4.3 Thermochemical 4.4 Anaerobic corrosion 4.5 Geological occurrence: the serpentinization reaction 4.6 Formation in transformers 4.7 Xylose 5 Applications 5.1 Consumption in processes 5.2 Coolant 5.3 Energy carrier 5.4 Semiconductor industry 6 Biological reactions 7 Safety and precautions 8 See also 9 Notes 10 References 11 Further reading 12 External links Properties Combustion A black cup-like object hanging by its bottom with blue glow coming out of its opening. The Space Shuttle Main Engine burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust. Hydrogen gas (dihydrogen or molecular hydrogen)[12] is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.[13] The enthalpy of combustion for hydrogen is −286 kJ/mol:[14] 2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)[note 2] Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures may be ignited by spark, heat or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C (932 °F).[15] Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine compared to the highly visible plume of a Space Shuttle Solid Rocket Booster. The detection of a burning hydrogen leak may require a flame detector; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames.[16] The destruction of the Hindenburg airship was an infamous example of hydrogen combustion; the cause is debated, but the visible orange flames were the result of a rich mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with chlorine and fluorine to form the corresponding hydrogen halides, hydrogen chloride and hydrogen fluoride, which are also potentially dangerous acids.[17]