Hydrogen (Latin: hydrogenium) is
the chemical element in the periodic table that has the symbol
H and atomic number 1. At standard temperature
and pressure it is a colorless, odorless, non-metallic, univalent,
highly flammable diatomic gas. Hydrogen is the lightest and
most abundant element in the universe. It is present in water
and in all organic compounds and living organisms. Hydrogen
is able to react chemically with most other 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.
|Name, Symbol, Number
||Hydrogen, H, 1
|Group, Period, Block
||1 (IA), 1 , s
||0.0899 kg/m3, NA
|Atomic radius (calc)
||25 (53) pm
|van der Waals radius
|e- 's per energy level
|Oxidation states (Oxide)
||14.025 K ("434 °F)
||20.268 K ("423 °F)
||11.42 ×10-6 m3/mol
| Heat of vaporization
| Heat of fusion
| Vapor pressure
||209 Pa at 23 K
|Speed of sound
||1270 m/s at 298.15 K
||2.2 (Pauling scale)
|Specific heat capacity
||__ 106/m ohm
||H is stable with 0 neutrons
||H is stable with 1 neutron
units & STP are used except where noted.
In the laboratory, hydrogen is prepared by reaction of acids
on metals such as zinc. For production in large scale commercial
bulk hydrogen is usually manufactured by decomposing natural
gas. Electrolysis of water is a simple although inefficient
method. Scientists are now researching new methods for hydrogen
production. One of them involves use of green algae. Another
promising method involves the conversion of biomass derivatives
such as glucose or sorbitol, which can be done at low temperatures
through the use of a new catalyst.
Hydrogen is 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.
The hydrogen atom
A hydrogen atom is an atom
of the element hydrogen. It is composed of
a single negatively charged electron, moving around the positively
charged proton which is the nucleus of the hydrogen atom. The
electron is bound to the proton by the Coulomb force.
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. Hydrogen is
used in hydrodealkylation, hydrodesulfurization, and hydrocracking.
- The element is used in the manufacture of hydrochloric acid,
welding, and the reduction of metallic ores.
- It is used in rocket fuels.
- Liquid hydrogen is used to perform cryogenic research, includingsuperconductivity
- Since hydrogen is fourteen and a half times lighter than
air, it was once widely used as a lifting agent in balloons
and airships. However this use was curtailed when the Hindenburg
disaster convinced the public that the gas was too dangerous
for this purpose. Deuterium, an isotope (hydrogen-2) of hydrogen,
is used in nuclear fission applications as a moderator to
slow down neutrons, and is also used in nuclear fusion reactions.
Deuterium compounds have applications in chemistry and biology
in studies of reaction isotope effects.
- Tritium (hydrogen-3), produced in nuclear reactors, is used
to construct hydrogen bombs. It is also used as an isotopic
label in the biosciences and as a radiation source in luminous
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 in 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 include most forms of organic matter (currently
all known life forms), coal, fossil fuels and natural gas. Methane
(CH4), which is a byproduct of organic decay, is
an increasingly important source of hydrogen.
Hydrogen can be prepared in several different ways: steam on
heated carbon, hydrocarbon decomposition with heat, reaction
of a strong base in an aqueous solution with aluminium, water
electrolysis, or displacement from acids with certain metals.
Commercial bulk hydrogen is usually manufactured by decomposing
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 hydronium (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
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 relative spin of the nuclei.
These two forms are known as ortho- and para-hydrogen (this
is different from isotopes, see below). In ortho-hydrogen the
nuclear spins are parallel (form a triplet), while in para they
are antiparallel (form a singlet). At standard conditions hydrogen
is composed of about 25% of the para form and 75% of the ortho
form (the so-called "normal" form). The equilibrium ratio of
these two forms depend on temperature but since the ortho form
has higher energy (is an excited state), it cannot be stable
in its pure form. In low temperatures (around boiling point),
the equilibrium state is comprised of almost only para form.
The conversion process between the forms is slow and if hydrogen
is cooled down and condensed rapidly, it contains large quantities
of the ortho form. It is important in preparation and storage
of liquid hydrogen since the ortho-para conversion produces
more heat than the heat of its evaporation and a lot of hydrogen
can be lost by evaporation in this way during several days after
liquifying. Therefore, some catalysts of the ortho-para conversion
process are used during hydrogen cooling. The two forms have
also slightly different physical properties. For example, the
melting and boiling points of parahydrogen are about 0.1 K lower
than of the "normal" form.
The most common isotope of hydrogen is 1H. This
stable isotope has a nucleus consisting of a single proton;
hence the descriptive, although rarely used, name protium for
The other stable isotope is deuterium, 2H, with
an extra neutron in the nucleus. Deuterium comprises 0.0184-0.0082%
of all hydrogen (IUPAC); ratios of deuterium to protium are
reported relative to the VSMOW standard reference water.
The third hydrogen isotope is the radioactive tritium, 3H.
The tritium nucleus contains two neutrons in addition to the
Hydrogen is the only element that has different names for its
isotopes. The symbols D and T (instead of 2H and
3H) are sometimes used for deuterium and tritium,
although this is not officially sanctioned. (The symbol P is
already in use for phosphorus
and is not available for protium.)
Hydrogen is a highly flammable gas burning at concentrations
as low as 4%. It also reacts violently with chlorine and
when mixed with oxygen and lit, burns with an explosion.