Reactive nonmetal
Hydrogen is a chemical element; it has the symbol H and atomic number 1. 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 H2, 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 H2 (dihydrogen) and in molecules, such as in water and organic compounds. The most common isotope of hydrogen, 1H, consists of one proton, one electron, and no neutrons.
Hydrogen gas was first produced artificially in the 17th century by the reaction of acids with metals. Henry Cavendish, in 1766–1781, 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 'water-former' (from Ancient Greek: ὕδωρ, romanized: húdōr, lit. 'water', and γεννάω, gennáō, 'I bring forth'). 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. …
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 in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.
In 1783, Antoine Lavoisier identified the element that came to be known as hydrogen when he and Laplace reproduced Cavendish's finding that water is produced when hydrogen is burned. Lavoisier 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:
Fe + H2O → FeO + H2
2 Fe + 3 H2O → Fe2O3 + 3 H2
3 Fe + 4 H2O → Fe3O4 + 4 H2
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. …
The ground state energy level of the electron in a hydrogen atom is −13.6 electronvolts (eV), equivalent to an ultraviolet photon of roughly 91 nanometers wavelength. The energy levels of hydrogen are referred to by consecutive quantum numbers, with
n
=
1
{\displaystyle n=1}
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.
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 cation, H+, hydride for the anion, and H-. The name proton is often used for the positively charged cation, but this is strictly correct only for the cation of the dominant isotope 1H.
Hydrogen has three naturally-occurring isotopes, denoted 1H, 2H and 3H. …
Hydrogen, as atomic H, is the most abundant chemical element in the universe, making up 75% of normal matter by mass. and >90% by number of atoms. In the early universe, protons formed in the first second after the Big Bang; neutral hydrogen atoms formed about 370,000 years later during the recombination epoch as the universe expanded and plasma had cooled enough for electrons to remain bound to protons.
In astrophysics, neutral hydrogen in the interstellar medium is called H I and ionized hydrogen is called H II. Radiation from stars ionizes H I to H II, creating spheres of ionized H II around stars. In the chronology of the universe neutral hydrogen dominated until the birth of stars during the era of reionization, which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.
These are the source of the 21-centimeter hydrogen line, at 1420 MHz, that is detected in order to probe primordial hydrogen. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the universe up to a redshift of z = 4.
Hydrogen is found in great abundance in stars and gas giant planets. Molecular clouds of H2 are associated with star formation. Hydrogen plays a vital role in powering stars through the proton-proton reaction in lower-mass stars, and through the CNO cycle of nuclear fusion in stars more massive than the Sun. …
Large quantities of H2 are used in the "upgrading" of fossil fuels. Key consumers of H2 include hydrodesulfurization and hydrocracking. Many of these reactions can be classified as hydrogenolysis, i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:
Hydrogenation, the addition of H2 to various substrates, is done on a large scale. Hydrogenation of N2 produces ammonia by the Haber process:
This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in fertilizer production; these fertilizers have become essential feedstocks in modern agriculture. Hydrogenation is also used to convert unsaturated fats and oils to saturated fats and oils. The major application is the production of margarine. Methanol is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as syngas. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent for the conversion of some ores to the metals.
The potential for using hydrogen (H2) as a fuel has been widely discussed. Hydrogen can be used in fuel cells to produce electricity, or burned to generate heat. When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor. …
Hydrogen has three naturally-occurring isotopes, denoted 1H, 2H and 3H. Other, highly-unstable nuclides (4H to 7H) have been synthesized in laboratories but not observed in nature.
1H 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 (see diproton for a discussion of why others do not exist).
2H, 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 1H-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.
3H 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 12.32 years. …
In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing hydrogen embrittlement and leaks in the pipeline or storage vessel. Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture. However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and detonation.
Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a volumetric ratio of hydrogen to air as low as 4%. In approximately 70% of hydrogen ignition accidents, the ignition source cannot be determined.
Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns. Hydrogen is non-toxic, but like most gases it can cause asphyxiation in the absence of adequate ventilation.