Hydrogen - Helium

Name, Symbol, NumberHydrogen, H, 1
Chemical series nonmetals
Group, Period, Block1 (IA), 1 , s
Density, Hardness 0.0899 kg/m3, NA
Appearance colorless
Atomic Properties
Atomic weight 1.00794 amu
Atomic radius (calc) 25 (53) pm
Covalent radius 37 pm
van der Waals radius 120 pm
Electron configuration 1s1
e- 's per energy level1
Oxidation states (Oxide) 1 (amphoteric)
Crystal structure hexagonal
Physical Properties
State of matter gas
Melting point 14.025 K (-434 F)
Boiling point 20.268 K (-423 F)
Molar volume 11.42 ×1010-3 m3/mol
Heat of vaporization 0.44936 kJ/mol
Heat of fusion 0.05868 kJ/mol
Vapor pressure 209 Pa at 23 K
Speed of sound 1270 m/s at 298.15 K
Electronegativity 2.2 (Pauling scale)
Specific heat capacity 14304 J/(kg*K)
Electrical conductivity __ 106/m ohm
Thermal conductivity 0.1815 W/(m*K)
Ionization potential 1312 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
1H99.985%H is stable with 0 neutrons
2H0.015%H is stable with 1 neutron
3H{syn.}12.33 yβ-0.0193He
SI units & STP are used except where noted.
Hydrogen (Wiktionary:Hydrogen) is a chemical element in the periodic table that has the symbol H and atomic number 1. A colorless, odorless, non-metal, univalent, highly flammable diatomic gas, hydrogen is the lightest and most abundant element in the universe and is present in water and in all organic compounds and living organisms. Hydrogen is able to chemically react with most elements. Stars in their main sequence are overwhelmingly composed of hydrogen in its plasma state. This element is used in ammonia production, as a lifting gas, an alternative fuel, and more recently as a power-source of fuel cells.

In laboratory, it is prepared by reaction of acids on metals like zinc. For production in large scale, electrolysis of water is a widely used method. Scientists are now trying to develop new methods that involve use of green algae for hydrogen production.

Table of contents
1 Notable Characteristics
2 Applications
3 History
4 Occurrence
5 Compounds
6 Forms
7 Isotopes
8 Precautions
9 See also
10 External Links

Notable Characteristics

Hydrogen was the lightest chemical element with its most common isotope consisting of just a single proton and electron. At standard temperature and pressure conditions, hydrogen forms a diatomic gas, H2, with a boiling point of only 20.27 K and a melting point of 14.02 K. Under exceedingly high pressures, like those found at the center of gas giants, the molecules lose their identity and the hydrogen becomes a liquid metal (see metallic hydrogen). Under the exceedingly low pressure conditions found in space, hydrogen tends to exist as individual atoms, simply because there is no way for them to combine; clouds of H2 form and are associated with star formation.

This element plays a vital role in powering the universe through the proton-proton reaction and carbon-nitrogen cycle (these are nuclear fusion processes that release huge amounts of energy through combining two hydrogen atoms into one helium).


Large quantities of hydrogen are needed industrially, notably in the 
Haber process for the production of ammonia, the hydrogenation of fats and oils, and the production of methanol. Other uses that require hydrogen:

  • hydrodealkylation, hydrodesulfurization, and hydrocracking.
  • manufacture of hydrochloric acid, welding, rocket fuels, and the reduction of metallic ores.
  • liquid hydrogen is used in cryogenic research including superconductivity studies,
  • tritium is produced in nuclear reactors and is used in hydrogen bomb construction.
  • It is fourteen and a half times lighter than air and at one time was widely used as a lifting agent in balloons and zeppelins until the Hindenburg disaster convinced the public that the gas was too dangerous for this purpose.
  • Deuterium is used in nuclear applications as a moderator to slow down neutrons, and deuterium compounds have applications in chemistry and biology in studies of reaction isotope effects.
  • Tritium is used as an isotopic label in the biosciences, as a radiation source in luminous paints.

Hydrogen can be burned in internal combustion engines, and a fleet of hydrogen burning cars is maintained by Chrysler-BMW. Hydrogen fuel cells are being looked into as a way to provide potentially cheap, pollution-free power.


Hydrogen (French for water-maker, from Greek hudôr, "water" and gennen, "generate") was first recognized as a distinct substance in 1776 by Henry Cavendish. Antoine Lavoisier gave the element its name.


Hydrogen is the most abundant element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is found in great abundance stars and gas giant planets. Relative to its great abundance elsewhere, hydrogen is very rare in the earth's atmosphere (1 ppm by volume). The most common source for this element on earth is water which is composed two parts hydrogen to one part oxygen (H2O). Other sources are; most forms of organic matter which includes all known life forms, coal, fossil fuels and natural gas. Methane (CHH4), which is a byproduct of organic decay, is an increasingly important source of hydrogen.

Hydrogen is prepared in several different ways; steam on heated carbon, hydrocarbon decomposition with heat, action of sodium or potassium hydroxide on aluminum, water electrolysis, or by displacement from acids with certain metals.

Commercial bulk hydrogen is usually manufactured by decomposing natural gas.


The lightest of all gases, hydrogen combines with most other elements to form compounds. 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 (as in Palladium hydride). 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 H3O+ as the protons latch on to something.

Hydrogen combines with oxygen to form water, H2O, and releases a lot of energy in doing so, burning explosively in air. 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.


Under normal conditions hydrogen gas is a mix of two different kinds of molecules which differ from one another by the "direction" that their electrons' and nuclei spin. These two forms are known as ortho- and para-hydrogen (this is different than isotopes, see below). At standard conditionss normal hydrogen is comprised of 25% of the para form and 75% of the ortho form. The ortho form can't be prepared in its pure state. The two forms of hydrogen differ in energy and this results in slightly different physical properties. For example, the melting and boiling points of parahydrogen are about 0.1 ° K lower than orthohydrogen (the so-called "normal" form).


The most common hydrogen isotope, protium, has no neutrons, although there are two others - deuterium with one, and radioactive tritium with two neutrons. The two stable isotopes are protium (H-1) and deuterium (H-2, D). Deuterium comprises 0.0184-0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium are reported relative to the VSMOW standard reference water. A radioactive isotope, tritium (T or H-3) has one proton and two neutrons.

Hydrogen is the only element that has different names for its isotopes.


Hydrogen is a highly flammable gas. It also reacts violently with chlorine and fluorine. D2O, or heavy water, is toxic to many species. The quantity required to kill a human, however, is substantial.

See also

periodic table, hydrogen bond, hydrogen atom, antihydrogen, hydrogen car, photohydrogen.

External Links