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Hydrogen is a chemical element; it has symbol H and atomic number 1. It is the lightest element and, at standard conditions, is a gas of diatomic molecules with the formula H2. It is colorless, odorless, tasteless,[9] non-toxic, and highly combustible. Hydrogen is the most abundant chemical substance in the universe, constituting roughly 75% of all normal matter.[10][note 1] Stars such as the Sun are mainly composed of hydrogen in the plasma state. Most of the hydrogen on Earth exists in molecular forms such as water and organic compounds. For the most common isotope of hydrogen (symbol 1H) each atom has one proton, one electron, and no neutrons.

In the early universe, the formation of protons, the nuclei of hydrogen, occurred during the first second after the Big Bang. The emergence of neutral hydrogen atoms throughout the universe occurred about 370,000 years later during the recombination epoch, when the plasma had cooled enough for electrons to remain bound to protons.[11]

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Industrial production is mainly from steam reforming of natural gas, oil reforming, or coal gasification. A small percentage is also produced using more energy-intensive methods such as the electrolysis of water.[14][15] Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing (e.g., hydrocracking) and ammonia production. Hydrogen can be deployed as an energy source in fuel cells to produce electricity, or via combustion to generate heat.[16] When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapour.[16] Combustion of hydrogen can lead to the thermal formation of nitrogen oxides.[16] Hydrogen atoms may embrittle metals.[17]

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, which uses an ammonium perchlorate composite. 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.[22] The destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible flames in the photographs were the result of carbon compounds in the airship skin burning.[23]

The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. Because of the discretization of angular momentum postulated in early quantum mechanics by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.[28]

Molecular H2 exists as two spin isomers, i.e. compounds that differ only in the spin states of their nuclei.[30] In the orthohydrogen form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin S = 1 {\displaystyle S=1} ; in the parahydrogen form the spins are antiparallel and form a spin singlet state having spin S = 0 {\displaystyle S=0} . The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.[31] The ortho form is an excited state, having higher energy than the para form by 1.455 kJ/mol,[32] and it converts to the para form over the course of several minutes when cooled to low temperature.[33] The thermal properties of the forms differ because they differ in their allowed rotational quantum states, resulting in different thermal properties such as the heat capacity.[34]

The ortho-to-para ratio in H2 is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic and produces enough heat to evaporate most of the liquid if not converted first to parahydrogen during the cooling process.[35] Catalysts for the ortho-para interconversion, such as ferric oxide and activated carbon compounds, are used during hydrogen cooling to avoid this loss of liquid.[36]

While H2 is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more electronegative, such as halogens (F, Cl, Br, I), or oxygen; in these compounds hydrogen takes on a partial positive charge.[37] When bonded to a more electronegative element, particularly fluorine, oxygen, or nitrogen, hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair, a phenomenon called hydrogen bonding that is critical to the stability of many biological molecules.[38][39] Hydrogen also forms compounds with less electronegative elements, such as metals and metalloids, where it takes on a partial negative charge. These compounds are often known as hydrides.[40]

Hydrogen forms a vast array of compounds with carbon called the hydrocarbons, and an even vaster array with heteroatoms that, because of their general association with living things, are called organic compounds.[41] The study of their properties is known as organic chemistry[42] and their study in the context of living organisms is known as biochemistry.[43] By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond that gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.[41] Millions of hydrocarbons are known, and they are usually formed by complicated pathways that seldom involve elemental hydrogen.

Hydrogen is highly soluble in many rare earth and transition metals[44] and is soluble in both nanocrystalline and amorphous metals.[45] Hydrogen solubility in metals is influenced by local distortions or impurities in the crystal lattice.[46] These properties may be useful when hydrogen is purified by passage through hot palladium disks, but the gas's high solubility is a metallurgical problem, contributing to the embrittlement of many metals,[17] complicating the design of pipelines and storage tanks.[47]

A bare proton, H+, cannot exist in solution or in ionic crystals because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the electron clouds of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or cationic hydrogen attached to other species in this fashion, and as such is denoted "H+" without any implication that any single protons exist freely as a species.

To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "hydronium ion" ([H3O]+). However, even in this case, such solvated hydrogen cations are more realistically conceived as being organized into clusters that form species closer to [H9O4]+.[52] Other oxonium ions are found when water is in acidic solution with other solvents.[53]

The exotic atom muonium (symbol Mu), composed of an antimuon and an electron, can also be considered a light radioisotope of hydrogen.[68] Because muons decay with lifetime 2.2 s, muonium is too unstable to exhibit observable chemistry.[69] Nevertheless, muonium compounds are important test cases for quantum simulation, due to the mass difference between the antimuon and the proton,[70] and IUPAC nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to hydrogen chloride and sodium hydride respectively.[71]

In 1766, Henry Cavendish was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a metal-acid reaction "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance called "phlogiston"[78][79] and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.[5][6]

In 1783, Antoine Lavoisier identified the element that came to be known as hydrogen (from the Greek - hydro meaning "water" and - genes meaning "former")[80] when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned.[6] Lavoisier produced hydrogen for his experiments on mass conservation by reacting a flux of steam with metallic iron through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:

Franois Isaac de Rivaz built the first de Rivaz engine, an internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. Edward Daniel Clarke invented the hydrogen gas blowpipe in 1819. The Dbereiner's lamp and limelight were invented in 1823.[6]

The first hydrogen-filled balloon was invented by Jacques Charles in 1783.[6] Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by Henri Giffard.[6] German count Ferdinand von Zeppelin promoted the idea of rigid airships lifted by hydrogen that later were called Zeppelins; the first of which had its maiden flight in 1900.[6] Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war. 17dc91bb1f