Hydrogen is the chemical element with the symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium (name rarely used, symbol 1H), has one proton and no neutrons.

The universal emergence of atomic hydrogen first occurred during the recombination epoch (Big Bang). 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 nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds.


Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. In 1766–81, Henry Cavendish was the first to recognize that hydrogen gas was a discrete substance, and that it produces water when burned, the property for which it was later named: in Greek, hydrogen means “water-former”.

Natural occurence

Occurence in the universe

Shortly after the creation of the universe, protons and neutrons were already present in overwhelming numbers. At the prevailing high temperatures, these combined to form light atomic nuclei, such as D and 4He. However, most protons remained unchanged and represented the future 1H nuclei. After about 380,000 years, when the radiation density of the universe had become small enough, hydrogen atoms could be formed simply by combining the nuclei with the electrons, without being torn apart again by a photon. Since then, there is the (unscattered!) cosmic background radiation and the universe is filled with hydrogen.

With the further cooling of the universe the mass split up asymmetrically and formed clouds of hydrogen gas. Under the influence of gravity, these clouds increasingly condensed first into galaxies and later the gas of the galaxies condensed into protostars, and under the enormous pressure of gravity the fusion of the H atoms into He atoms began. Thus the first stars and suns were formed. Later, however, especially in very large stars, heavier elements such as carbon, nitrogen and oxygen, which are the basic building blocks of all known forms of life, were also formed - also by fusion.

Occurence on earth

On earth, the mass fraction is much lower, about 0.12 % of the total weight, and 2.9 % of the earth’s crust. In addition, in contrast to occurrences in space, terrestrial hydrogen is predominantly bound and almost never pure (i.e. as unmixed gas). No other element is known to contain so many compounds, the most common being water.

Earth’s crust

But the element is also found in all living things, in oil, natural gas and many minerals. Other natural occurrences are natural gases such as methane (CH4).

Salt and fresh water

The largest proportion of terrestrial hydrogen occurs in the compound water. In this form it covers over two thirds of the earth’s surface. The total water resources of the earth amount to about 1386 billion km³. Of this, 1338 billion km³ (96.5 %) are salt water in the oceans. The remaining 3.5 % are available as fresh water. Most of it is in its solid state: in the form of ice in the Arctic and Antarctic as well as in the permafrost soils, especially in Siberia. The small remaining part is liquid fresh water and is mostly found in lakes and rivers, but also in underground deposits, for example as groundwater.


In the earth’s atmosphere hydrogen is almost exclusively chemically bound, mainly in the form of water. The percentage frequency of molecular hydrogen in the air is only 0.55 ppm. The proportion of water vapor is between about 1 and 4 percent. This value is strongly dependent on humidity and temperature.

The low percentage of molecular hydrogen in the atmosphere can be explained by the high thermal velocity of the molecules and the high percentage of oxygen in the atmosphere. At the mean temperature of the atmosphere, the H2 particles move at an average of almost 7,000 km/h. That is about one sixth of the escape velocity on Earth. However, due to the Maxwell-Boltzmann distribution of the velocities of the H2 molecules, there are still a considerable number of molecules that reach the escape velocity nevertheless. However, the molecules have only an extremely small free path length, so that only molecules in the upper layers of the atmosphere actually escape. Further H2 molecules follow from layers below, and a certain amount escapes again until finally only traces of the element are left in the atmosphere. Presumably the hydrogen in the lower layers of the atmosphere is largely burned to water. With a small proportion, a balance is established between consumption and new production (by bacteria and photonic splitting of the water).


The annual hydrogen production is currently over 500 billion standard cubic metres. Most of this comes from fossil sources (natural gas, crude oil), from the chemical industry, where it is a by-product of chlorine production, and from crude oil refinery processes.

If hydrogen is to be used on a large scale for energy generation or storage in the sense of a hydrogen energy economy, production by conventional steam reforming is not practical. However, it may still be possible as an entry point, for example in the automotive sector.

In the meantime, some hydrogen production processes have been developed up to series production readiness, others are still in the development stage:

  • Steam reformer (natural gas)
  • Partial oxidation (oil gasification)
  • Autothermal reformer (methanol reforming)
  • Electrolysis of water
  • Biomass (gasification, fermentation)
  • Kværner procedure
  • Hydrogen from green algae

Economic use

Every year more than 600 billion cubic metres of hydrogen (approx. 30 million tonnes) are produced worldwide for countless applications in industry and technology. Important fields of application are:

  • Energy carriers: In welding, as rocket fuel. Its use as a fuel for jet engines, in hydrogen combustion engines or via fuel cells is expected to replace the use of petroleum products in the foreseeable future (see hydrogen propulsion), because combustion produces mainly water, but no soot or carbon dioxide. However, unlike oil, hydrogen is not a primary energy source.
  • Hydrogenation of coal: Through various chemical reactions, coal is converted into liquid hydrocarbons with H2. In this way gasoline, diesel and heating oil can be produced artificially. At the moment, both of the above-mentioned processes have no economic significance due to higher costs. However, this could change drastically as soon as the earth’s oil reserves run out.
  • Reducing agent: H2 can react with metal oxides and extract oxygen from them. Water and the reduced metal are formed. The process is used in the smelting of metallic ores, especially to extract metals as pure as possible.
  • The Haber-Bosch process is used to produce ammonia from nitrogen and hydrogen and from this ammonia important fertilisers and explosives are produced.
  • Fat hardening: Hardened fats are obtained from vegetable oil by hydrogenation. During this process the double bonds in unsaturated fatty acid residues of the glycerides are saturated with hydrogen. The resulting fats have a higher melting point, making the product solid. Margarine is produced in this way. As a by-product, trans fats that are harmful to health can also be produced.
  • Food additive: Hydrogen is approved as E 949 and is used as a propellant gas, packing gas, etc.
  • Coolant: Due to its high heat capacity, (gaseous) hydrogen is used as a coolant in power plants and the turbo generators used there. In particular, H2 is used where liquid cooling can be problematic. The heat capacity comes into play where the gas cannot be circulated, or only slowly. Since the thermal conductivity is also high, flowing H2 is also used to transport thermal energy to large reservoirs (e.g. rivers). In these applications, hydrogen protects the equipment from overheating and increases efficiency. The advantage of hydrogen is that, due to its low density, which is included in the Reynolds number, it flows laminar with low resistance up to higher speeds than other gases.
  • Cryogenic: Because of its high heat capacity and low boiling point, liquid hydrogen is suitable as a cryogen, i.e. as a coolant for extremely low temperatures. Even larger amounts of heat can be absorbed well by liquid hydrogen before a noticeable increase in its temperature occurs. In this way, the low temperature is maintained even when there are external fluctuations.
  • Carrier gas: Hydrogen found one of its first uses in balloons and airships. However, due to the highly flammable nature of H2-air mixtures, this repeatedly led to accidents. The biggest catastrophe in this context is probably the accident of the “Dixmude” in 1923, the best known was certainly the “Hindenburg catastrophe” in 1937. Hydrogen as a carrier gas has meanwhile been replaced by helium and fulfils this purpose only in very special applications.

The two natural isotopes of Hydrogen, Deuterium and Tritium, have special applications:


Deuterium is used (in the form of heavy water) in heavy water reactors as a moderator, i.e. to slow down the fast neutrons produced during nuclear fission to thermal speed.

Deuterated solvents are used in nuclear magnetic resonance spectroscopy because deuterium has a nuclear spin of one and is not visible in the NMR spectrum of the normal hydrogen isotope.

In chemistry and biology, deuterium compounds help to study reaction processes and metabolic pathways (isotope labeling), since compounds with deuterium usually behave chemically and biochemically almost identically to the corresponding compounds with hydrogen. The reactions are not disturbed by the labeling, but the fate of deuterium in the final products can still be determined.

Furthermore, the considerable difference in mass between hydrogen and deuterium provides a clear isotope effect in the mass-dependent properties. For example, heavy water has a measurably higher boiling point than water.


The radioactive isotope tritium is produced in nuclear reactors in industrially usable quantities. Besides deuterium, it is also a starting material in nuclear fusion to helium. In civil use, it serves as a radioactive marker in biology and medicine. For example, it can be used to detect tumor cells. In physics, it is itself an object of research on the one hand, while on the other hand, highly accelerated tritium nuclei are used to study heavy nuclei or produce artificial isotopes.

Water samples can be dated very precisely with the help of the tritium method. With a half-life of about twelve years, it is particularly suitable for measuring relatively short periods of time (up to several hundred years). Among other things, it can be used to determine the age of a wine.

It is used as a long-lasting, reliable energy source for luminous paints (in a mixture with a fluorescent dye), mainly in military applications, but also in wristwatches. The isotope is also used for military purposes in the hydrogen bomb and certain types of nuclear weapons whose effect is based on fission.


Hydrogen in the form of various compounds is essential for all known living organisms. The most important of these is water, which serves as a medium for all cellular processes and for all mass transport. Together with carbon, oxygen, nitrogen (and more rarely also other elements) it is a component of those molecules from organic chemistry without which any form of life known to us is simply impossible.

Hydrogen also plays an active role in the organism, for example in some coenzymes such as nicotinamide adenine dinucleotide (NAD/NADH), which serve as reduction equivalents (or “proton transporters”) in the body and participate in redox reactions. In the mitochondria, the power stations of the cell, the transfer of hydrogen cations (protons) between different molecules of the so-called respiratory chain serves to provide a proton gradient through chemiosmotic membrane potential to generate energy-rich compounds such as adenosine triphosphate (ATP). During photosynthesis in plants and bacteria, hydrogen from water is needed to convert the fixed carbon dioxide into carbohydrates.

In terms of mass, hydrogen is the third most important element in the human body: For a person with a body weight of 70 kg, about 7 kg (= 10 % by weight) can be attributed to the hydrogen contained. Only carbon (approx. 20 wt.%) and oxygen (approx. 63 wt.%) make up an even greater proportion of the weight. In relation to the number of atoms, the very light hydrogen is by far the most common atom in the body of any living being. (The 7 kg in humans correspond to 3.5-103 moles of hydrogen with 2-6-1023 atoms each, that is about 4.2-1027 hydrogen atoms).


Hydrogen is extremely flammable. It burns with pure oxygen or air as well as with other gaseous oxidants such as chlorine or fluorine with a hot flame. Since the flame is hardly visible, one can get into it unintentionally Mixtures with chlorine or fluorine are flammable even by ultraviolet radiation (see oxyhydrogen chlorine gas). In addition to the labelling prescribed by GHS (see info box), H2 compressed gas cylinders must be provided with a red cylinder shoulder and red cylinder body in accordance with DIN EN 1089-3.

Hydrogen is non-toxic and does not harm the environment. Therefore no MAK value is specified. Breathing or skin protection is not required. Only when high concentrations are inhaled can movement disorders, unconsciousness and suffocation occur due to the lack of oxygen from about 30 % by volume .

Mixtures of air and 4 to 76 % by volume hydrogen are combustible. Above a concentration of 18 % in air, the mixture is explosive (oxyhydrogen gas). The ignition temperature in air is 560 °C. During handling, the hydrogen must be kept away from ignition sources, including electrostatic discharges. The containers should be stored away from oxidizing gases (oxygen, chlorine) and other oxidizing substances.

Because of its small atomic size, hydrogen can diffuse through many solids, i.e. gas can slowly escape through unsuitable materials (e.g. plastics). The materials and thicknesses used for gas tanks and pipes take this into account, so that there are no greater risks than with petrol, for example. Hydrogen vehicles with pressure tanks can be parked in multi-storey and underground car parks without any problems. There is no legal provision restricting this.