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Encyclopedia > Spin isomers of hydrogen

A hydrogen molecule (H2) is made up of two atoms of hydrogen linked by a covalent bond. If we neglect the traces of deuterium and tritium which could be present, each hydrogen atom is constituted of a single proton and a single electron (this is called light hydrogen, protium; the similar term protonium designates an exotic atomic state consisting of a proton and an antiproton). The proton has an associated magnetic moment, which we can treat as being generated by the proton's spin. The H2 molecule can thus have its protons aligned in either the same direction (this is orthohydrogen) or in opposite directions (this is parahydrogen). The ratio between the ortho and para forms is about 3:1 at standard temperature and pressure, but the para form dominates at low temperatures (approx. 99.999% at 20 K). Other molecules and functional groups containing two hydrogen atoms, such as water and methylene, also have ortho and para forms, although their ratios differ from that of the dihydrogen molecule. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... Covalent bonding is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... Tritium (symbol T or 3H) is a radioactive isotope of hydrogen. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... e- redirects here. ... Protonium is a type of exotic atom in which an antiproton orbits a proton or a proton orbits an antiproton. ... In physics, the proton (Greek proton = first) is a subatomic particle with an electric charge of one positive fundamental unit (1. ... The antiproton (aka pbar) is the antiparticle of the proton. ... A bar magnet. ... Temperature and air pressure can vary from one place to another on the Earth, and can also vary in the same place with time. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... In chemistry, methylene is a divalent functional group CH2 derived formally from methane. ...

Orthohydrogen is unstable at low temperatures, and spontaneously changes into parahydrogen with time, a process which releases heat; an undesirable occurrence when one wants hydrogen to remain liquid. At standard (room) temperature, hydrogen is made up of 75% orthohydrogen, a proportion which the liquefaction process preserves. One must therefore use a catalyst to accelerate the conversion of the liquid hydrogen into parahydrogen, or supply additional refrigeration equipment to absorb the heat that the liquid hydrogen will give off as it spontaneously converts itself to pure parahydrogen. For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another as a result of a difference in temperature. ... It has been suggested that this article or section be merged into Catalysis. ...

The first synthesis of pure parahydrogen was achieved by Paul Harteck and Karl Friedrich Bonhoeffer in 1929. Karl Friedrich Bonhoeffer (born 13 January 1899 in Breslau; died 15 May 1957 in Goettingen) was a German chemist. ...


  1. Tikhonov V. I., Volkov A. A. (2002). "Separation of water into its ortho and para isomers". Science 296 (5577): 2363. 
  2. Bonhoeffer KF, Harteck P (1929). "Para- and ortho hydrogen". Zeitschrift für Physikalische Chemie B 4 (1-2): 113-141. 
  3. A. Farakas (1935). Orthohydrogen, parahydrogen and heavy hydrogen,. 



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