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)
|State of matter
||14.025 K ("434 °F)
||20.268 K ("423 °F)
||11.42 ×10-6 m3/mol
Heat of vaporization
Heat of fusion
||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. Other
- 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 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 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
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
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
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
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 1H.
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 proton.
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.