Hydrogen
Hydrogen is a chemical element; it has the symbolH and atomic number1. It is the lightest and most abundant chemical element in the universe, constituting about 75% of all normal matter. Under standard conditions, hydrogen is a gas of diatomic molecules with the formula, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible. Stars, including the Sun, mainly consist of hydrogen in a plasma state, while on Earth, hydrogen is found as the gas and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, H, consists of one proton, one electron, and no neutrons.
Hydrogen gas was first produced artificially in the 17thcentury by the reaction of acids with metals. Henry Cavendish, in1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means . Understanding the colors of light absorbed and emitted by hydrogen was a crucial part of the development of quantum mechanics.
Hydrogen, typically nonmetallic except under extreme pressure, readily forms covalent bonds with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in acid–base reactions, which mainly involve proton exchange among soluble molecules. In ionic compounds, hydrogen can take the form of either a negatively-charged anion, where it is known as hydride, or as a positively-charged cation,, hydron. Although tightly bonded to water molecules, hydrons strongly affect the behavior of aqueous solutions, as reflected in the importance of pH. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding.
In the early universe, neutral hydrogen atoms formed about 370,000 years after the Big Bang as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars began to form, most of the hydrogen in the intergalactic medium was re-ionized.
Nearly all hydrogen production is done by transforming fossil fuels, particularly steam reforming of natural gas. It can also be produced from water or saline by electrolysis, but this process is more expensive. Its main industrial uses include fossil fuel processing and ammonia production for fertilizer. Emerging uses for hydrogen include the use of fuel cells to generate electricity.
Properties
Atomic hydrogen
Electron energy levels
The ground state energy level of the electron in a hydrogen atom is −13.6electronvolts, equivalent to an ultraviolet photon of roughly 91nanometers wavelength. The energy levels of hydrogen are referred to by consecutive quantum numbers, with being the ground state. The hydrogen spectral series corresponds to emission of light due to transitions from higher to lower energy levels. Each energy level is further split by spin interactions between the electron and proton into four hyperfine levels.High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The eigenvalue from the Dirac equation is the largest contribution. Other terms include relativistic recoil, the self-energy, and the vacuum polarization terms.
Nomenclature
The standards organization for chemical names, IUPAC, gives general names when the context assumes natural isotope abundance or ignores the isotope. These general names are hydrogen for the neutral atom, hydron for the positive cation, H+, hydride for the negative cation, H-. The name proton is often used for the positive cation but this strictly correct only for the cation of the dominant isotope.Isotopes
Hydrogen has three naturally-occurring isotopes, denoted, and. Other, highly-unstable nuclides have been synthesized in laboratories but not observed in nature.' is the most common hydrogen isotope, with an abundance of >99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium. It is the only stable isotope with no neutrons.
', the other stable hydrogen isotope, is known as deuterium and contains one proton and one neutron in the nucleus. Nearly all deuterium nuclei in the universe are thought to have been produced in Big Bang nucleosynthesis, and have endured since then. Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called heavy water. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for -NMR spectroscopy. Heavy water is used as a neutron moderator and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial nuclear fusion.
is known as tritium and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into helium-3 through beta decay with a half-life of 12.32years. It is radioactive enough to be used in luminous paint to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case. Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in nuclear weapons tests. It is used in nuclear fusion, as a tracer in isotope geochemistry, and in specialized self-powered lighting devices. Tritium has also been used in chemical and biological labeling experiments as a radiolabel.
Unique among the elements, distinct names are assigned to hydrogen's isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D andT are sometimes used for deuterium and tritium, but the symbolP was already used for phosphorus and thus was not available for protium. In its nomenclatural guidelines, the International Union of Pure and Applied Chemistry allows any of D, T,, and to be used, though and are preferred.
Antihydrogen is the antimatter counterpart to hydrogen. It consists of an antiproton| with a positron. The exotic atom muonium, composed of an antimuon and an electron, is the analogue of hydrogen; nomenclature incorporates such hypothetical compounds as muonium chloride and sodium muonide, analogous to hydrogen chloride and sodium hydride respectively.
Dihydrogen
Under standard conditions, hydrogen is a gas of diatomic molecules with the formula, officially called "dihydrogen", but also called "molecular hydrogen", or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.Combustion
Hydrogen gas is highly flammable, reacting with oxygen in air to produce liquid water:The amount of heat released per mole of hydrogen is, or for a mass.
Hydrogen gas forms explosive mixtures with air in concentrations from and with chlorine at. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is. In a high-pressure hydrogen leak, the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.
Hydrogen flames emit faint blue and ultraviolet light. Flame detectors are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.
Spin isomers
Molecular exists as two nuclear isomers that differ in the spin states of their nuclei. In the ' form, the spins of the two nuclei are parallel, forming a spin triplet state having a total molecular spin ; in the ' form the spins are and form a spin singlet state having spin. 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. The ortho form is an excited state, having higher energy than the para form by, and it converts to the para form over the course of several minutes when cooled to low temperature. The thermal properties of these isomers differ because each has distinct rotational quantum states.The ortho-to-para ratio in is an important consideration in the liquefaction and storage of liquid hydrogen: the conversion from ortho to para is exothermic, and produces sufficient heat to evaporate most of the liquid if the conversion to does not occur during the cooling process. Catalysts for the ortho-para, such as ferric oxide and activated carbon compounds, are therefore used during hydrogen cooling to avoid this loss of liquid.
Phases
can exist at temperatures below hydrogen's critical point of. However, for it to be in a fully liquid state at atmospheric pressure, H2 needs to be cooled to. Hydrogen was liquefied by James Dewar in1898 by using regenerative cooling and his invention, the vacuum flask.Liquid hydrogen becomes solid hydrogen at standard pressure below hydrogen's melting point of. Distinct solid phases exist, known as PhaseI through PhaseV, each exhibiting a characteristic molecular arrangement. Liquid and solid phases can exist in combination at the triple point; this mixture is known as slush hydrogen.
Metallic hydrogen, a phase obtained at extremely high pressures, is an electrical conductor. It is believed to exist deep within giant planets like Jupiter.
When ionized, hydrogen becomes a plasma. This is the form in which hydrogen exists within stars.
Thermal and physical properties
History
18th century
In 1671, Irish scientist Robert Boyle discovered and described the reaction between iron filings and dilute acids, which results in the production of hydrogen gas.Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the phlogiston theory of combustion.
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 "phlogiston" and further finding in1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.
In 1783, identified the element that came to be known as hydrogen when he and Pierre-Simon Laplace| reproduced Cavendish's finding that water is produced when hydrogen is burned. produced hydrogen for his experiments on mass conservation by treating metallic iron with a stream of water 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:
Many metals react similarly with water, leading to the production of hydrogen. In some situations, this H2-producing process is problematic, for instance in the case of zirconium cladding on nuclear fuel rods.