Nitrogen ( ) is a
chemical element that has the symbol
N, the
atomic number
of 7 and an
atomic mass 14.00674 u.
Elemental nitrogen is a colorless, odorless, tasteless and mostly
inert diatomic gas at
standard conditions,
constituting 78% by volume of
Earth's
atmosphere.
Many industrially important compounds, such as
ammonia,
nitric acid,
organic nitrates (
propellants and
explosives), and
cyanides, contain nitrogen. The extremely strong
bond in elemental nitrogen dominates nitrogen chemistry, causing
difficulty for both organisms and industry in converting the into
useful compounds, and releasing large amounts of energy when these
compounds burn or decay back into nitrogen gas.
The element nitrogen was discovered by
Daniel Rutherford, a Scottish physician,
in 1772. Nitrogen occurs in all living organisms. It is a
constituent element of
amino acids and
thus of
proteins, and of
nucleic acids (
DNA and
RNA). It resides in the
chemical structure of almost all
neurotransmitters, and is a defining
component of
alkaloids, biological
molecules produced by many organisms.
History
Nitrogen (
Latin nitrogenium, where
nitrum (from
Greek
nitron νιτρον) means "
saltpetre"
(see
nitre), and
genes γενης means
"forming") is formally considered to have been discovered by
Daniel Rutherford in 1772, who
called it
noxious air or
fixed air. That there
was a fraction of air that did not support
combustion was well known to the late 18th
century chemist. Nitrogen was also studied at about the same time
by
Carl Wilhelm Scheele,
Henry Cavendish, and
Joseph Priestley, who referred to it as
burnt air or
phlogisticated air. Nitrogen gas was
inert enough that
Antoine Lavoisier referred to it as
"mephitic air" or
azote, from the
Greek word (
azotos) meaning
"lifeless". Animals died in it, and it was the principal component
of air in which animals had suffocated and flames had burned to
extinction.Lavoisier's name for nitrogen is used in many languages
(French, Russian, etc.) and still remains in English in the common
names of many compounds, such as hydrazine and compounds of the
azide ion. Compounds of nitrogen were known in the
Middle Ages. The
alchemists knew
nitric
acid as
aqua fortis (strong water). The mixture of
nitric and
hydrochloric acids was
known as
aqua regia (royal
water), celebrated for its ability to dissolve
gold (the
king of metals). The earliest
military, industrial and
agricultural
applications of nitrogen compounds involved uses of
saltpeter (
sodium nitrate or
potassium nitrate), notably in
gunpowder, and much later, as
fertilizer.
Properties
Nitrogen is a
nonmetal, with an
electronegativity of 3.04. It has five
electrons in its
outer shell and is therefore
trivalent in most compounds. The
triple bond in molecular nitrogen ( ) is
the strongest in nature. The resulting difficulty of converting
into other compounds, and the ease (and associated high energy
release) of converting nitrogen compounds into elemental , have
dominated the role of nitrogen in both nature and human economic
activities.
At
atmospheric pressure
molecular nitrogen
condenses (
liquifies) at 77
K
(−195.8 °
C) and
freezes at 63 K (−210.0 °C) into the beta
hexagonal close-packed
crystal
allotropic form. Below 35.4 K
(−237.6 °C) nitrogen assumes the alpha
cubic crystal allotropic form.
Liquid nitrogen, a fluid resembling water in
appearance, but with 80.8% of the density (the density of liquid
nitrogen at its boiling point is 0.808 g/mL), is a common
cryogen.
Unstable allotropes of nitrogen consisting of more than two
nitrogen atoms have been produced in the laboratory, like and
. Under extremely high pressures
(1.1 million
atm) and
high temperatures (2000 K), as produced using a
diamond anvil cell, nitrogen polymerizes
into the single-bonded cubic gauche crystal structure. This
structure is similar to that
diamond, and
both have extremely strong
covalent
bonds. is nicknamed "nitrogen diamond."
Isotopes
There are two stable
isotopes of nitrogen:
14N and
15N. By far the most common is
14N (99.634%), which is produced in the
CNO cycle in
stars. Of the ten
isotopes produced synthetically,
13N has a
half-life of ten minutes and the remaining
isotopes have half-lives on the order of seconds or
less.Biologically-mediated reactions (e.g.,
assimilation,
nitrification, and
denitrification) strongly control nitrogen
dynamics in the soil. These reactions typically result in
15N enrichment of the
substrate and depletion of the
product.
0.73% of the molecular nitrogen in Earth's atmosphere is the
isotopologue
14N
15N and almost all the rest is
14N
2.
Radioisotope
16N is the dominant radionuclide in the
coolant of
pressurized water
reactors during normal operation. It is produced from
16O (in water) via (n,p) reaction. It has a short
half-life of about 7.1 s, but during its decay back to
16O produces high-energy gamma radiation (5 to 7 MeV).
Because of this, the access to the primary coolant piping must be
restricted during reactor power operation.
16N is one of
the main means used to immediately detect even small leaks from the
primary coolant to the secondary steam cycle.
Electromagnetic spectrum
Molecular nitrogen (
14N
2) is largely
transparent to
infrared and
visible radiation because it is a
homonuclear molecule and thus has no
dipole moment to couple to
electromagnetic radiation at these
wavelengths. Significant
absorption occurs at
extreme
ultraviolet wavelengths,
beginning around 100 nanometers. This is associated with electronic
transitions in the molecule to states in which charge is not
distributed evenly between nitrogen atoms. Nitrogen absorption
leads to significant absorption of ultraviolet radiation in the
Earth's upper atmosphere as well as in the atmospheres of other
planetary bodies. For similar reasons, pure molecular
nitrogen lasers typically emit light in the
ultraviolet range.
Nitrogen also makes a contribution to visible
air glow from the Earth's upper atmosphere, through
electron impact excitation followed by emission. This visible blue
air glow (seen in the polar
aurora and in the re-entry glow of
returning spacecraft) typically results not from molecular
nitrogen, but rather from free nitrogen atoms combining with oxygen
to form
nitric oxide (NO).
Reactions
Nitrogen is generally unreactive at standard temperature and
pressure. N
2 reacts spontaneously with few
reagents, being resilient to
acids and
bases as well
as oxidants and most reductants. When nitrogen reacts spontaneously
with a reagent, the net transformation is often called
nitrogen fixation.
Nitrogen reacts with elemental
lithium at
STP.
Lithium burns in an atmosphere of N
2 to give
lithium nitride:
- 6 Li + N2 → 2 Li3N
Magnesium also burns in nitrogen, forming
magnesium nitride.
- 3 Mg + N2 → Mg3N2
N
2 forms a variety of
adducts with
transition metals. The first example of a
dinitrogen complex is
[Ru(NH
3)
5(N
2)]
2+ (see
figure at right). Such compounds are now numerous, other examples
include IrCl(N
2)(PPh
3)
2,
W(N
2)
2(
Ph2CH2CH2PPh2)
2,
and
[(η
5-C
5Me
4H)
2Zr]
2(
μ2,
η²,η²-N
2).
These
complexes illustrate how
N
2 might bind to the metal(s) in
nitrogenase and the
catalyst for the
Haber
process. A catalytic process to
reduce
N
2 to ammonia with the use of a
molybdenum complex in the presence of a proton
source was published in 2005. (see
nitrogen fixation)
The starting point for industrial production of nitrogen compounds
is the Haber process, in which nitrogen is fixed by reacting and
over an
iron oxide ( ) catalyst at
about 500 °C and 200 atmospheres pressure. Biological nitrogen
fixation in free-living
cyanobacteria
and in the
root nodules of plants also
produces ammonia from molecular nitrogen. The reaction, which is
the source of the bulk of nitrogen in the
biosphere, is catalyzed by the
nitrogenase enzyme complex
which contains Fe and Mo atoms, using energy derived from
hydrolysis of
adenosine
triphosphate (ATP) into
adenosine diphosphate and
inorganic phosphate
(−20.5 kJ/mol).
Occurrence
Nitrogen is the largest single constituent of the
Earth's
atmosphere
(78.082% by volume of dry air, 75.3% by weight in dry air). It is
created by
fusion processes
in
stars, and is estimated to be the 7th most
abundant
chemical element by mass
in the universe.
Molecular nitrogen and nitrogen
compound have been detected in
interstellar space by astronomers using
the
Far
Ultraviolet Spectroscopic Explorer. Molecular nitrogen is a
major constituent of the
Saturnian moon
Titan's thick atmosphere, and occurs in
trace amounts in other planetary atmospheres.
Nitrogen is present in all living organisms, in proteins, nucleic
acids and other molecules. It typically makes up around 4% of the
dry weight of plant matter, and around 3% of the weight of the
human body. It is a large component of animal waste (for example,
guano), usually in the form of
urea,
uric acid,
ammonium compounds and derivatives of these
nitrogenous products, which are essential nutrients for all plants
that are unable to
fix atmospheric
nitrogen.
Nitrogen occurs naturally in a number of minerals, such as
saltpetre (potassium nitrate),
Chile saltpetre (sodium nitrate) and
sal ammoniac (ammonium chloride). Most of these
are relatively uncommon, partly because of the minerals' ready
solubility in water. See also
Nitrate minerals and
Ammonium minerals.
Compounds
The main neutral
hydride of
nitrogen is
ammonia ( ), although
hydrazine ( ) is also commonly used. Ammonia is
more
basic than
water by 6 orders of magnitude. In
solution ammonia forms the
ammonium ion ( ). Liquid ammonia
(boiling point 240 K) is
amphiprotic (displaying either
Brønsted-Lowry acidic or basic
character) and forms ammonium and the less common
amide ions ( ); both amides and
nitride ( )
salts are known, but
decompose in water. Singly,
doubly, triply and quadruply substituted alkyl compounds of ammonia
are called
amines (four substitutions, to form
commercially and biologically important quaternary amines, results
in a positively charged nitrogen, and thus a water-soluble, or at
least
amphiphilic, compound). Larger
chains, rings and structures of nitrogen hydrides are also known,
but are generally unstable. is another polyatomic cation as in
hydrazine.
Other classes of nitrogen
anions (negatively
charged ions) are the poisonous
azides ( ),
which are linear and
isoelectronic
to
carbon dioxide, but which bind to
important iron-containing enzymes in the body in a manner more
resembling
cyanide. Another
molecule of the same structure is the colorless and
relatively inert anesthetic gas
Nitrous
oxide (dinitrogen monoxide, ), also known as laughing gas. This
is one of a variety of nitrogen
oxides that
form a family often abbreviated as
NOx.
Nitric oxide (
nitrogen monoxide, NO), is a natural
free radical used in
signal transduction in both plants and
animals, for example in
vasodilation by
causing the smooth muscle of blood vessels to relax. The reddish
and poisonous
nitrogen dioxide
contains an unpaired
electron and is an
important component of
smog. Nitrogen molecules
containing unpaired electrons show an understandable tendency to
dimerize (thus pairing the electrons), and
are generally highly reactive. The corresponding acids are
nitrous and
nitric
acid , with the corresponding salts called
nitrites and
nitrates.
The higher oxides
dinitrogen
trioxide ,
dinitrogen
tetroxide and
dinitrogen
pentoxide , are fairly unstable and explosive, a consequence of
the chemical stability of . Nearly every
hypergolic rocket engine uses as the oxidizer;
their fuels, various forms of
hydrazine,
are also nitrogen compounds. These engines are extensively used on
spacecraft such as the
space shuttle
and those of the
Apollo Program
because their propellants are liquids at room temperature and
ignition occurs on contact without an ignition system, allowing
many precisely controlled burns. Some launch vehicles, such as the
Titan II and
Ariane 1 through 4 also use
hypergolic fuels, although the trend is away from such engines for
cost and safety reasons. is an intermediate in the manufacture of
nitric acid , one of the few acids stronger than
hydronium and a fairly strong
oxidizing agent.
Nitrogen is notable for the range of explosively unstable compounds
that it can produce. Nitrogen triiodide is an extremely sensitive
contact explosive.
Nitrocellulose, produced by nitration of
cellulose with nitric acid, is also known as guncotton.
Nitroglycerin, made by nitration of
glycerin, is the dangerously unstable explosive
ingredient of
dynamite. The comparatively
stable, but more powerful explosive
trinitrotoluene (TNT) is the standard
explosive against which the power of nuclear explosions are
measured.
Nitrogen can also be found in
organic
compounds. Common nitrogen
functional groups include:
amines,
amides,
nitro groups,
imines, and
enamines. The amount of nitrogen in a
chemical substance can be
determined by the
Kjeldahl
method.
Applications
Nitrogen gas is an
industrial gas
produced by the fractional
distillation
of liquid
air, or by mechanical means using
gaseous air (i.e. pressurized reverse
osmosis membrane or
Pressure swing adsorption).
Commercial nitrogen is often a byproduct of air-processing for
industrial concentration of
oxygen for
steelmaking and other purposes. When supplied compressed in
cylinders it is often referred to as OFN (oxygen-free
nitrogen).
Nitrogen gas has a wide variety of applications, including serving
as an
inert replacement for
air where
oxidation is
undesirable;
Nitrogen is commonly used during sample preparation procedures for
chemical analysis. Specifically, it is used as a means of
concentrating and reducing the volume of liquid samples. Directing
a pressurized stream of nitrogen gas perpendicular to the surface
of the liquid allows the solvent to evaporate while leaving the
solute(s) and un-evaporated solvent behind.
Nitrogen tanks are also replacing carbon dioxide as the main power
source for paintball guns. The downside is that nitrogen must be
kept at higher pressure than CO
2, making N
2
tanks heavier and more expensive.
Nitrogenated beer
A further example of its versatility is its use as a preferred
alternative to
carbon dioxide to
pressurize kegs of some
beers, particularly
stouts and
British
ales, due to the smaller
bubbles it produces, which make the dispensed beer
smoother and headier. A modern application of a pressure sensitive
nitrogen capsule known commonly as a "
widget" now allows nitrogen charged beers to
be packaged in
cans and
bottles.
Liquid nitrogen
Liquid nitrogen is a
cryogenic liquid. At
atmospheric pressure, it boils at −195.8 °C. When insulated in
proper containers such as
Dewar flasks,
it can be transported without much
evaporative loss.
Like
dry ice, the main use of liquid
nitrogen is as a
refrigerant. Among
other things, it is used in the
cryopreservation of
blood, reproductive cells (
sperm
and
egg), and other biological samples and
materials. It is used in
cold traps for
certain laboratory equipment and to cool
x-ray detectors. It has also been used to
cool
central processing
units and other devices in computers which are
overclocked, and which produce more heat than
during normal operation.
Applications of nitrogen compounds
Molecular nitrogen (N
2) in the atmosphere is relatively
non-reactive due to its strong bond, and N
2 plays an
inert role in the human body, being neither produced nor destroyed.
In nature, nitrogen is converted into biologically (and
industrially) useful compounds by lightning, and by some living
organisms, notably certain
bacteria (i.e.
nitrogen fixing bacteria –
see
Biological role
below). Molecular nitrogen is released into the atmosphere in the
process of
decay, in dead plant and
animal tissues.
The ability to combine or
fix molecular nitrogen
is a key feature of modern industrial chemistry, where nitrogen and
natural gas are converted into
ammonia via the
Haber
process. Ammonia, in turn, can be used directly (primarily as a
fertilizer, and in the synthesis of
nitrated fertilizers), or as a precursor of many other important
materials including
explosives, largely
via the production of
nitric acid by the
Ostwald process.
The organic and inorganic
salts of
nitric acid have been important historically as convenient stores
of chemical energy. They include important compounds such as
potassium nitrate (or
saltpeter used in
gunpowder) and
ammonium nitrate, an important fertilizer
and explosive (see
ANFO). Various other
nitrated organic compounds, such as
nitroglycerin and
trinitrotoluene, and
nitrocellulose, are used as explosives and
propellants for modern firearms.
Nitric
acid is used as an
oxidizing
agent in liquid fueled
rockets.
Hydrazine and hydrazine derivatives find use as
rocket
fuels and
monopropellants. In most of these compounds,
the basic instability and tendency to burn or explode is derived
from the fact that nitrogen is present as an oxide, and not as the
far more stable nitrogen molecule (N
2) which is a
product of the compounds' thermal decomposition. When nitrates burn
or explode, the formation of the powerful triple bond in the
N
2 produces most of the energy of the reaction.
Nitrogen is a constituent of molecules in every major drug class in
pharmacology and medicine.
Nitrous
oxide (N
2O) was discovered early in the 19th century
to be a partial anesthetic, though it was not used as a surgical
anesthetic until later. Called "
laughing
gas", it was found capable of inducing a state of social
disinhibition resembling drunkenness. Other notable
nitrogen-containing drugs are drugs derived from plant
alkaloids, such as
morphine
(there exist many alkaloids known to have pharmacological effects;
in some cases they appear natural chemical defenses of plants
against predation). Nitrogen containing drugs include all of the
major classes of antibiotics, and organic nitrate drugs like
nitroglycerin and
nitroprusside which regulate blood pressure
and heart action by mimicking the action of
nitric oxide.
Biological role
Nitrogen is an essential building block of
amino and
nucleic
acids, essential to life on Earth.
Elemental nitrogen in the atmosphere cannot be used directly by
either plants or animals, and must be converted to a reduced (or
'fixed') state in order to be useful for higher plants and animals.
Precipitation often
contains substantial quantities of
ammonium
and
nitrate, thought to result from
nitrogen fixation by
lightning and other atmospheric electric
phenomena. This was first proposed by
Liebig in 1827 and later confirmed.
However, because
ammonium is preferentially
retained by the
forest canopy relative
to atmospheric nitrate, most fixed nitrogen reaches the
soil surface under trees as nitrate. Soil nitrate is
preferentially assimilated by these tree
roots
relative to soil ammonium.
Specific
bacteria (e.g.
Rhizobium trifolium) possess
nitrogenase enzymes which
can fix atmospheric nitrogen (see
nitrogen fixation) into a form (
ammonium ion) that is chemically useful to higher
organisms. This process requires a large amount of energy and
anoxic conditions. Such bacteria may live
freely in soil (e.g.
Azotobacter) but normally exist in a
symbiotic relationship in the
root nodules of
leguminous
plants (e.g.
clover,
Trifolium, or
soybean
plant,
Glycine max).
Nitrogen-fixing bacteria are also symbiotic with a number of
unrelated plant species such as alders (
Alnus) spp., lichens (
Casuarina),
Myrica,
liverwort, and
Gunnera.
As part of the symbiotic relationship, the plant converts the
'fixed' ammonium ion to nitrogen oxides and amino acids to form
proteins and other molecules, (e.g.
alkaloids). In return for the 'fixed' nitrogen,
the plant secretes sugars to the symbiotic bacteria.
Plants are able to assimilate nitrogen directly in the form of
nitrates which may be present in soil from natural mineral
deposits, artificial fertilizers, animal waste, or organic decay
(as the product of bacteria, but not bacteria specifically
associated with the plant). Nitrates absorbed in this fashion are
converted to nitrites by the enzyme
nitrate reductase, and then
converted to ammonia by another enzyme called
nitrite reductase.
Nitrogen compounds are basic building blocks in animal biology as
well. Animals use nitrogen-containing
amino
acids from plant sources, as starting materials for all
nitrogen-compound animal biochemistry, including the manufacture of
proteins and
nucleic
acids. Plant-feeding insects are dependent on nitrogen in their
diet, such that varying the amount of nitrogen fertilizer applied
to a plant can affect the reproduction rate of insects feeding on
fertilized plants.
Soluble nitrate is an important limiting factor in the growth of
certain bacteria in ocean waters . In many places in the world,
artificial
fertilizers applied to
crop-lands to increase yields result in run-off delivery of soluble
nitrogen to oceans at river mouths. This process can result in
eutrophication of the water, as
nitrogen-driven bacterial growth depletes water oxygen to the point
that all higher organisms die. Well-known
"dead zone" areas in the U.S.
Gulf Coast and the Black Sea
are due to this important polluting
process.
Many saltwater fish manufacture large amounts of
trimethylamine oxide to protect them
from the high
osmotic effects of their
environment (conversion of this compound to
dimethylamine is responsible for the early
odor in not fresh saltwater fish . In animals,
free radical nitric
oxide (
NO) (derived from an
amino acid), serves as an important
regulatory molecule for
circulation.
Animal metabolism of
NO results in production of
nitrite. Animal
metabolism
of nitrogen in proteins generally results in
excretion of
urea, while
animal metabolism of
nucleic acids
results in excretion of
urea and
uric acid. The characteristic odor of animal flesh
decay is caused by the creation of long-chain, nitrogen-containing
amines, such as
putrescine and
cadaverine.
Decay of organisms and their waste products may produce small
amounts of nitrate, but most decay eventually returns nitrogen
content to the atmosphere, as molecular nitrogen. The circulation
of nitrogen from atmosphere to organic compounds and back is
referred to as the
nitrogen
cycle.
Safety
Rapid release of nitrogen gas into an enclosed space can displace
oxygen, and therefore represents an
asphyxiation hazard. This may happen
with few warning symptoms, since the human
carotid body is a relatively slow and a poor
low-oxygen (hypoxia) sensing system. An example occurred shortly
before the launch of the first Space Shuttle mission in 1981, when
two technicians lost consciousness and died after they walked into
a space located in the Shuttle's
Mobile Launcher Platform that was
pressurized with pure nitrogen as a precaution against fire. The
technicians would have been able to exit the room if they had
experienced early symptoms from nitrogen-breathing.
When inhaled at high
partial
pressures (more than about 4 bar, encountered at depths below
about 30 m in
scuba diving) nitrogen
begins to act as an anesthetic agent. It can cause
nitrogen narcosis, a temporary
semi-anesthetized state of mental impairment similar to that caused
by
nitrous oxide.
Nitrogen also dissolves in the
bloodstream and body fats. Rapid
decompression (particularly in the case of divers ascending too
quickly, or astronauts decompressing too quickly from cabin
pressure to spacesuit pressure) can lead to a potentially fatal
condition called
decompression
sickness (formerly known as caisson sickness or more commonly,
the "bends"), when nitrogen bubbles form in the bloodstream,
nerves, joints, and other sensitive or vital areas. Other "inert"
gases (those gases other than carbon dioxide and oxygen) cause the
same effects from bubbles composed of them, so replacement of
nitrogen in
breathing gases may
prevent nitrogen narcosis, but does not prevent decompression
sickness.
Direct skin contact with
liquid
nitrogen will eventually cause severe
frostbite (cryogenic burns). This may happen
almost instantly on contact, depending on the form of liquid
nitrogen. Bulk liquid nitrogen causes less rapid freezing than a
spray of nitrogen mist (such as is used to freeze certain skin
growths in the practice of
dermatology).
The extra surface area provided by nitrogen-soaked materials is
also important, with soaked clothing or cotton causing far more
rapid damage than a spill of direct liquid to skin. Full "contact"
between naked skin and large droplets or pools of undisturbed
liquid nitrogen may be prevented for a few seconds by a layer of
insulating gas from the
Leidenfrost
effect. However, liquid nitrogen applied to skin in mists, and
on fabrics, bypasses this effect.
See also
References
Further reading
External links