Basic features
Hydrogen is the lightest chemical element; its most common isotope
comprises just one negatively charged electron,
distributed around a positively charged proton
(the nucleus
of the atom). The electron is bound to the proton by the Coulomb
force, the electrical force that one stationary, electrically
charged nanoparticle exerts on another. The hydrogen atom has special
significance in quantum
mechanics as a simple physical system for which there is an exact
solution to the Schrödinger
equation; from that equation, the experimentally observed
frequencies and intensities of hydrogen's spectral
lines can be calculated. Spectral lines are dark or bright lines
in an otherwise uniform and continuous spectrum, resulting from an
excess or deficiency of photons in a narrow frequency range, compared
with the nearby frequencies.
At standard
temperature and pressure, hydrogen forms a diatomic gas, H2,
with a boiling point of only 20.27 K
and a melting point of 14.02 K.[1]
Under extreme pressures, such as those at the centre of gas
giants, the molecules lose their identity and the hydrogen becomes
a metal
(metallic
hydrogen). Under the extremely low pressure in space—virtually a
vacuum—the element tends to exist as individual atoms, simply
because there is no way for them to combine. However, clouds of H2
and possibly singular hydrogen atoms are said to form in H
I and H
II regions and are associated with star
formation. Hydrogen plays a vital role in powering stars
through the proton–proton
and carbon–nitrogen
cycle. These are nuclear
fusion processes, which release huge amounts of energy in stars
and other hot celestial bodies as hydrogen atoms combine into helium
atoms.
At high temperatures, hydrogen gas can exist as a mixture of atoms,
protons, and negatively charged hydride ions. This mixture has a high emissivity
and absorptivity
in the visible
light range, and plays an important part in the emission of light
from the sun
and other stars.
H2 is highly soluble in water, alcohol, and ether. It
has a high capacity for adsorption,
in which it is attached to and held to the surface of some substances.
It is an odorless, tasteless, colorless, and highly flammable
gas that burns at concentrations as low as 4% H2 in air. It
reacts violently with chlorine
and fluorine,
forming hydrohalic
acids that can damage the lungs
and other tissues.
When mixed with oxygen, hydrogen explodes upon ignition. A unique
property of hydrogen is that its flame is completely invisible in air.
This makes it difficult to tell if a leak is burning, and carries the
added risk that it is easy to walk into a hydrogen fire inadvertently.
See also: hydrogen
atom.
Applications
Large quantities of hydrogen are needed in the chemical and
petroleum industries, notably in the Haber
process for the production of ammonia,
which by mass ranks as the world's fifth most produced industrial
compound. Hydrogen is used in the hydrogenation
of fats and
oils (found
in items such as margarine),
and in the production of methanol.
Hydrogen is used in hydrodealkylation,
hydrodesulfurization,
and hydrocracking[2].
The element has several other important uses.
There are no "hydrogen wells" or "hydrogen
mines" on Earth, so hydrogen cannot be considered a primary
energy source such as fossil
fuels or uranium.
Hydrogen can however be burned in internal
combustion engines, an approach advocated by BMW's experimental hydrogen
car. However, it is currently difficult and dangerous to store and
handle in sufficient quantity for motor fuel use. Hydrogen fuel
cells are being investigated as mobile power
sources with lower emissions than hydrogen-burning internal combustion
engines. The low emissions of hydrogen in internal combustion engines
and fuel
cells are currently offset by the pollution created by hydrogen
production. This may change if the substantial amounts of electricity
required for water electrolysis
can be generated primarily from low pollution sources such as nuclear
energy or wind. Research is being conducted on hydrogen as a
replacement for fossil fuels. It could become the link between a range
of energy sources, carriers and storage. Hydrogen can be converted to
and from electricity (solving the electricity storage and transport
issues), from biofuels,
and from and into natural
gas and diesel
fuel. All of this can theoretically be achieved with zero emissions of
CO2 and toxic pollutants.
History
Hydrogen was first produced by Theophratus Bombastus von Hohenheim
(1493–1541)—also
known as Paracelsus—by
mixing metals with acids. He was unaware that the explosive gas
produced by this chemical reaction was hydrogen. In 1671, Robert
Boyle described the reaction between two iron fillings and dilute
acids, which results in the production of gaseous hydrogen.[3]
In 1766, Henry
Cavendish was the first to recognize hydrogen as a discrete
substance, by identifying the gas from this reaction as
"inflammable" and finding that the gas produces water when
burned in air. Cavendish stumbled on hydrogen when experimenting with
acids and mercury.
Although he wrongly assumed that hydrogen was a compound of
mercury—and not of the acid—he
was still able to accurately describe several key properties of
hydrogen.
Antoine
Lavoisier gave the element its name and proved that water is
composed of hydrogen and oxygen.
One of the first uses of the element was for balloons.
The hydrogen was obtained by mixing sulfuric
acid and iron.
In 1931, Harold
C. Urey discovered deuterium,
an isotope
of hydrogen, by repeated distilling the same sample of water. For this
discovery, Urey received the Nobel
Prize in Chemistry in 1934. In the same year, the third isotope, tritium,
was discovered. Because of its relatively simple structure, hydrogen
has often been used in models of how an atom
works.
Electron energy levels
The ground
state energy
level of the electron in a hydrogen atom is 13.6 eV,
which is equivalent to an ultraviolet photon of roughly 92 nm.
With the Bohr
Model, the energy levels of hydrogen can be calculated fairly
accurately. This is done by modeling the electron as revolving around
the proton, much like the earth revolving around the sun. Except the
sun holds earth in orbit with the force of gravity,
but the proton holds the electron in orbit with the force of electromagnetism.
Another difference between the Earth-Sun system and the
electron-proton system is that, in this model, due to quantum
mechanics the electron is allowed to only be at very specific
distances from the proton. Modeling the hydrogen atom in this fashion
yields the correct energy levels and spectrum.
Occurrence
Hydrogen is the most abundant
element in the universe, making up 75% of normal matter by mass
and over 90% by number of atoms.
[4]
This element is found in great abundance in stars
and gas giant planets. It is very rare in the Earth's
atmosphere (1 ppm
by volume), because being the lightest gas causes it to escape Earth's
gravity, though when compounds
are considered, it is the tenth most abundant element on Earth. 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
including coal,
natural
gas, and other fossil
fuels. Methane
(CH4)
is an increasingly important source of hydrogen.
Throughout the universe, hydrogen is mostly found in the plasma
state whose properties are quite different to molecular hydrogen. As a
plasma, hydrogen's electron and proton are not bound together,
resulting in very high electrical conductivity, even when the gas is
only partially ionized. The charged particles are highly influenced by
magnetic and electric fields, for example, in the solar
wind they interact with the Earth's magnetosphere
giving rise to Birkeland
currents and the aurora.
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 produced by the steam
reforming of natural
gas. At high temperatures (700–1100 °C), steam reacts with
methane to yield carbon
monoxide and hydrogen.
- CH4
+ H2O
→ CO
+ 3 H2
Additional hydrogen can be recovered from the carbon monoxide
through the water-gas
shift reaction:
- CO
+ H2O
→ CO2
+ H2
Compounds
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 nonmetallic 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.
Although exotic on earth, one of the most common ions in the universe
is the H3+
ion.
Hydrogen combines with oxygen to form water,
H2O, and releases significant amounts 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.
First tracks observed in Liquid hydrogen bubble chamber.
[
Forms
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.[5]
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 and form
a triplet, while in para they are antiparallel and 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 depends on temperature, but since
the ortho form has higher energy (is an excited
state), it cannot be stable in its pure form. At low temperatures
(around boiling point), the equilibrium state is comprised almost
entirely of the 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 liquefying.
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.
Isotopes
- Main Article: Isotopes
of hydrogen
Hydrogen is the only element that has different names for its
isotopes. (During the early study of radioactivity, various heavy
radioactive isotopes were given names, but such names are no longer
used, although one element, radon,
has a name that originally applied to only one of 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.)
- 1H
The most common isotope of hydrogen, this stable isotope has a nucleus
consisting of a single proton;
hence the descriptive, although rarely used, name protium.
The spin
of a protium atom is 1/2+. [6]
- 2H
The other stable isotope is deuterium,
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 spin of a deuterium atom is 1+.
- 3H
The third naturally occurring hydrogen isotope is the radioactive tritium.
The tritium nucleus contains two neutrons in addition to the proton.
It decays through beta
decay and has a half-life of 12.32 years.
Tritium occurs naturally due to cosmic rays interacting with
atmospheric gases. Like ordinary hydrogen, tritium reacts with the
oxygen in the atmosphere to form T2O. This radioactive
"water" molecule constantly enters the Earth's seas and
lakes in the form of slightly radioactive rain, but its half-life is
short enough to prevent a buildup of hazardous radioactivity. The spin
of a tritium atom is 1/2+.
- 4H
Hydrogen-4
was synthesized by bombarding tritium with fast-moving deuterium
nuclei. It decays through neutron
emission and has a half-life of 9.93696x10-23 seconds.
The spin of a hydrogen-4 atom is 2-.
- 5H
In 2001 scientists detected hydrogen-5
by bombarding a hydrogen target with heavy ions. It decays through neutron
emission and has a half-life of 8.01930x10-23 seconds.
- 6H
Hydrogen-6
decays through triple neutron
emission and has a half-life of 3.26500x10-22 seconds.
- 7H
In 2003 hydrogen-7
was created (article)
at the RIKEN laboratory in Japan by colliding a high-energy beam of
helium-8 atoms with a cryogenic hydrogen target and detecting
tritons—the nuclei of tritium atoms—and neutrons from the breakup
of hydrogen-7, the same method used to produce and detect hydrogen-5.
Biology
Scientists from the University
of Colorado at Boulder discovered in 2005
that microbes living in the hot waters of Yellowstone
National Park gain their sustenance from molecular hydrogen.
[
See also
