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Vial of glowing ultrapure hydrogen, H2. Actual size 1x5 cm. Source: images-of-elements.com
Physics & Chemistry (main)

August 6, 2010, 5:17 pm
September 22, 2011, 11:27 am

Hydrogen has an atomic structure consisting of one proton and one electron, making it the lightest of the elements and exists as diatomic molecules. In the solid state the element has a hexagonal closest packed structure. Hydrogen, with atomic symbol H, is thought to be the most abundant of all elements in the universe.


Previous Element: Periodic table

Next Element: Helium


Physical Properties
Color colorless
Phase at Room Temp. gas
Density (g/cm3) 0.0001
Hardness (Mohs) ---

Melting Point (K)


Boiling Point (K)

Heat of Fusion (kJ/mol) 0.117
Heat of Vaporization (kJ/mol) 0.9
Heat of Atomization (kJ/mol) 218
Thermal Conductivity (J/m sec K) 0.18
Electrical Conductivity (1/mohm cm) ---

Water, Methane

Atomic Properties
Electron Configuration 1s1

Number of Isotopes

Electron Affinity (kJ/mol) 72.7711
First Ionization Energy (kJ/mol) 1312
Second Ionization Energy (kJ/mol) ---
Third Ionization Energy (kJ/mol) ---
Electronegativity 2.2
Polarizability (Å3) 0.7
Atomic Weight 1.008
Atomic Volume (cm3/mol) 14.4
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 37.1
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) ---
Common Oxidation Numbers -1,+1
Other Oxid. Numbers ---
In Earth's Crust (mg/kg) 2.825
In Earth's Ocean (mg/L) 2.2
In Human Body (%) 10.0% (Mostly Water)
Regulatory / Health
CAS Number 1333-74-0 Compressed gas
OSHA Permissible Exposure Limit (PEL) No limits
OSHA PEL Vacated 1989 No limits

NIOSH Recommended Exposure Limit (REL)

No limits

Mineral Information Institute
Jefferson Accelerator Laboratory

The name derives from the Greek hydro for "water" and genes for "forming", since it burned in air to form water. It was Henry Cavendish who collected the gas over Mercury in 1766, subjected it to systematic study, and reported his findings in 1766 to the Royal Society. He made no claim to having discovered Hydrogen. Cavendish first thought the gas was phlogiston, but then was the first to distinguish it from other gases. Probably this distinction was a major contribution to Antoine-Laurent de Lavoisier's belief that each gas is a separate element and led him to name it.

Hydrogen was observed and collected long before it was recognized as a unique gas. Robert Boyle had before 1671 dissolved iron in dilute hydrochloric acid and prepared what he described as the inflammable solution of Mars [Iron]. Johann Becher and Georg Stahl had proposed a combustible substance, phlogiston, which they felt responsible for all burning.

Hydrogen's historic use is as an industrial chemical. The heavier elements were originally made from hydrogen or from other elements that were originally made from hydrogen. Much of its current use is in improving crude oil in refineries. However, hydrogen was considered for energy even in the 19th century – by Jules Verne, as well as by contemporary scientists.


Hydrogen is estimated to make up more than 90% of all the atoms or three quarters of the mass of the universe. This element is found in the stars, and plays an important part in powering the universe through both the proton-proton reaction and carbon-nitrogen cycle – stellar hydrogen fusion processes that release massive amounts of energy by combining hydrogen to form helium.

Production of hydrogen in the USA. alone now amounts to about three billion cubic feet per year. Hydrogen is prepared by steam on heated carbon, decomposition of certain hydrocarbons with heat, action of sodium or potassium hydroxide on aluminum, electrolysis of water, or displacement from acids by certain metals. Liquid hydrogen is important in cryogenics and in the study of superconductivity, as its melting point is only 20 degrees above absolute zero.

Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen bomb. Hydrogen is the primary component of Jupiter and the other gas giant planets. At some depth in the planet's interior the pressure is so great that solid molecular hydrogen is converted to solid metallic hydrogen.

In 1973, a group of Russian experimenters may have produced metallic hydrogen at a pressure of 2.8 Mbar. At the transition the density changed from 1.08 to 1.3 grams per cubic meter. Earlier, in 1972, at Livermore, California, a group also reported on a similar experiment in which they observed a pressure-volume point centered at 2 Mbar.


Pure hydrogen is a gas we find very little of it in our atmosphere. Hydrogen gas is so light that uncombined hydrogen will gain enough velocity from collisions with other gases that they will quickly be ejected from the atmosphere. On Earth, hydrogen occurs chiefly in combination with oxygen in water, but it is also present in organic matter such as living plants, petroleum, coal, etc. It is present as the free element in the atmosphere, but only to the extent of less than one part per million by volume. The lightest of all gases, hydrogen combines with other elements—sometimes explosively—to form compounds.

Natural occurrence

Hydrogen is the universe's most abundant element, comprising three quarters of mass This element is found in particularly high occurrence in dwarf stars and gas giant planets.Molecular clouds of H2 are implicated in the process of star formation. Hydrogen has a key function in powering stars via proton-proton reactions and carbon-nitrogen-oxygen cycle nuclear fusion. Within the universe as a whole, hydrogen chiefly occurs in atom and Plasma states, the properties of which are radically distinct from molecular hydrogen. As a plasma, the hydrogen atom's electron and proton are not bound together, leading to extraordinarily high electrical conductivity and high light emissivity, which is the source of most starlight. The charged particles are strongly influenced by magnetic and electric fields. In the case of solar wind these elemental particles interact with the magnetosphere of the Earth, producing Birkeland currents and aurora phenomena. Hydrogen also occurs in a neutral atomic state in the interstellar medium. The considerable quantity of neutral hydrogen in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the universe up to redshift z=4.

Under the conditions of the natural environment on Earth, elemental hydrogen exists as a diatomic molecular gas. However, hydrogen gas is extremely rare in the Earth's atmosphere (approximately one part per million by volume), an outcome due to its light mass, which encourages atmospheric escape. However, hydrogen is the third most abundant element of the Earth crust. Hydrogen gas is produced by some bacteria and algae and is a natural component of flatulence processes of livestock and other fauna, as is methane.


Great quantities of hydrogen are required commercially for the fixation of nitrogen from the air in the Haber ammonia process and for the hydrogenation of fats and oils. It is also used in large quantities in methanol production, in hydrodealkylation, hydrocracking, and hydrodesulfurization. Other uses include rocket fuel, welding, producing hydrochloric acid, reducing metallic ores, and filling balloons.

The lifting power of one cubic foot of hydrogen gas is about 0.07 lb at 0°C, 760 mm pressure. Hydrogen fuel cells are a developing technology that could allow great amounts of electrical power to be obtained using a source of hyrogen gas.

Consideration is being given to an entire economy based on nuclear fusion and nuclear-generated hydrogen. Public acceptance, high capital investment, and the high cost of hydrogen with respect to today's fuels are but a few of the problems facing such an economy. Located in remote regions, power plants would electrolyze seawater; the hydrogen produced would travel to distant cities by pipelines. Much more attention to nuclear fusion research is warranted, based upon its long term future as a major renewable energy source, which is devoid of the radioactive waste of fission technology. Pollution-free hydrogen could replace natural gas, gasoline, etc., and could serve as a reducing agent in metallurgy, chemical processing, refining, etc. It could also be used to convert trash into methane and ethylene.

Energy Carrier

Hydrogen powered vehicle. Source: Oak Ridge National Library Review

Hydrogen is like electricity in that it is an energy carrier, not a primary source—it is derived from the conversion of some other form of energy. It can be manufactured from a variety of sources, including natural gas, coal, nuclear energy and all forms of renewable energy.

When used as a fuel in combustion processes or in fuel cells, hydrogen has minimal emissions relative to conventional fuels. Although hydrogen has about three times the energy density of gasoline per kilogram, making it ideal as a rocket fuel, it has a very low energy density on a volumetric basis. This poses significant economic and technical challenges to the transmission and storage of hydrogen. Potential end uses of hydrogen include fuel cell vehicle technology as well as stationary power generation. Research and development of hydrogen energy centers on reducing the costs associated with its manufacture and storage. As a potential complement to electricity as one of the two primary long-term energy carriers, hydrogen ultimately could offer a transition from today's energy mix that is as significant as that from wood to coal, or coal to oil.

Hydrogen energy has been the subject of increasing political interest over the past two decades. The Japanese government funded the WE-NET project to investigate its potential, while policy measures such as the California Zero Emission Vehicles (ZEV) mandate accelerated corporate involvement. The International Energy Agency (IEA) manages a number of international cooperation agreements on hydrogen. In 2003, the International Partnership for a Hydrogen Economy was formed, comprising Government stakeholders from seventeen countries and regions.


Hydrogen is essential to life for all known organisms on Earth. It is one of the six bulk elements and it is the third most common element in the human body. Hydrogen is a component of water, without which, life could not exist. Hydrogen also is a constituent of DNA and most other organic molecules. Pure hydrogen is not used by higher life forms, but is metabolized by some types of bacteria.


Quite apart from isotopes, it has been shown that under ordinary conditions hydrogen gas is a mixture of two kinds of molecules, known as ortho- and para-hydrogen, which differ from one another by the spins of their electrons and nuclei.

Normal hydrogen at room temperature contains 25% of the para form and 75% of the ortho form. The ortho form cannot be prepared in the pure state. Since the two forms differ in energy, the physical properties also differ. The melting and boiling points of parahydrogen are about 0.1°C lower than those of normal hydrogen.


The ordinary isotope of hydrogen, H, is known as Protium, the other two isotopes are Deuterium (a proton and a neutron) and Tritium (a protron and two neutrons). Hydrogen is the only element whose isotopes have been given different names. Deuterium and Tritium are both used as fuel in nuclear fusion reactors. One atom of Deuterium is found in about 6000 ordinary hydrogen atoms.

Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making luminous paints, and as a tracer.


  1. Hans Haubold and A.M.Mathai. 2007. Solar Thermonuclear Energy Generation. Columbia University
  2. Lisa J.Storrie-Lombardi and Arthur M.Wolfe. 2000. Surveys for z > 3 Damped Lyman-alpha Absorption Systems: the Evolution of Neutral Gas. Astrophysical Journal. vol. 543, pp 552–576


Hart, D., & , S. (2011). Hydrogen. Retrieved from http://editors.eol.org/eoearth/wiki/Hydrogen