Atomic Number:


Melting Point: - 259.34 șC
Atomic Symbol: H Boiling Point:  -252.87 șC
Atomic Weight: 1.0079 amu Density: 0.0899 kg/m 3
Atomic Radius:

78 pm

Oxidation States: 1, -1
Covalent Radius: 37 pm Electron Configuration: 1s1
van der Waals Radius:

120 pm

State of Matter: gas


(Gr. hydro: water, and genes: forming) Hydrogen was prepared many years before it was recognized as a distinct substance by Cavendish in 1776. Later, Lavoisier named the element. 


Hydrogen is the lightest and most abundant element in the universe. The heavier elements were originally made from hydrogen atoms or from other elements that were originally made from hydrogen atoms. At standard temperature and pressure it is a colorless, odorless, non-metallic, univalent, highly flammable diatomic gas. Hydrogen is also able to react chemically with most other elements.

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 0.1oC lower than those of normal hydrogen.

Hydrogen combines with oxygen to form water, H2O, and releases a lot of energy in doing so. Deuterium oxide, or D2O, is commonly referred to as heavy water. Hydrogen also forms a vast array of compounds with carbon. Because of their association with living things, these compounds are called organic compounds, and the study of the properties of these compounds is called organic chemistry.


Although 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 less than 1 ppm by volume.

Hydrogen is estimated to make up more than 90% of all the atoms -- 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  release massive amounts of energy by combining hydrogens to form helium.

Production of hydrogen in the U.S. alone amounts to about 3 billion cubic feet per year. Hydrogen is prepared by

Liquid hydrogen is important in cryogenics and in the study of superconductivity, as its melting point is only 20 degrees above absolute zero.

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 g/cm3. 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. Predictions say that metallic hydrogen may be metastable; others have predicted it would be a superconductor at room temperature.


Great quantities of hydrogen are required commercially for nitrogen fixation using 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 1 cubic foot of hydrogen gas is about 0.07 lb at 0C, 760 mm pressure.

The Hydrogen Fuel cell is a developing technology that will allow great amounts of electrical power to be obtained using a source of hydrogen gas. 

Consideration is being given to an entire economy based on solar- 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. 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.


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.

Hydrogen has an electronegativity of 2.2, so it forms compounds where it is the more non-metallic and where it is the more metallic element. The former are called hydrides, where hydrogen either exists as H- ions or just as a solute within the other element. The latter tend to be covalent, since the H+ ion would be a bare nucleus and so has a strong tendency to pull electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like hydronium (H3O+) as the protons latch on to something.


Due to its high flammability at concentrations as low as 4%, hydrogen gas is quite dangerous.  Reactions with chlorine and fluorine are very violent and when hydrogen is mixed with oxygen and lit, it burns explosively.