Sodium is a chemical element with symbol Na
(from Latin natrium) and atomic number 11.
It is a soft, silvery-white, highly reactive
metal. Sodium is an alkali metal, being in
group 1 of the periodic table, because it
has a single electron in its outer shell that
it readily donates, creating a positively
charged ion—the Na+ cation. Its only stable
isotope is 23Na. The free metal does not occur
in nature, but must be prepared from compounds.
Sodium is the sixth most abundant element
in the Earth's crust and exists in numerous
minerals such as feldspars, sodalite, and
rock salt (NaCl). Many salts of sodium are
highly water-soluble: sodium ions have been
leached by the action of water from the Earth's
minerals over eons, and thus sodium and chlorine
are the most common dissolved elements by
weight in the oceans.
Sodium was first isolated by Humphry Davy
in 1807 by the electrolysis of sodium hydroxide.
Among many other useful sodium compounds,
sodium hydroxide (lye) is used in soap manufacture,
and sodium chloride (edible salt) is a de-icing
agent and a nutrient for animals including
humans.
Sodium is an essential element for all animals
and some plants. Sodium ions are the major
cation in the extracellular fluid (ECF) and
as such are the major contributor to the ECF
osmotic pressure and ECF compartment volume.
Loss of water from the ECF compartment increases
the sodium concentration, a condition called
hypernatremia. Isotonic loss of water and
sodium from the ECF compartment decreases
the size of that compartment in a condition
called ECF hypovolemia.
By means of the sodium-potassium pump, living
human cells pump three sodium ions out of
the cell in exchange for two potassium ions
pumped in; comparing ion concentrations across
the cell membrane, inside to outside, potassium
measures about 40:1, and sodium, about 1:10.
In nerve cells, the electrical charge across
the cell membrane enables transmission of
the nerve impulse—an action potential—when
the charge is dissipated; sodium plays a key
role in that activity.
== Characteristics ==
=== 
Physical ===
Sodium at standard temperature and pressure
is a soft silvery metal that combines with
oxygen in the air and forms grayish white
sodium oxide unless immersed in oil or inert
gas, which are the conditions it is usually
stored in. Sodium metal can be easily cut
with a knife and is a good conductor of electricity
and heat because it has only one electron
in its valence shell, resulting in weak metallic
bonding and free electrons, which carry energy.
Due to having low atomic mass and large atomic
radius, sodium is third-least dense of all
elemental metals and is one of only three
metals that can float on water, the other
two being lithium and potassium. The melting
(98 °C) and boiling (883 °C) points of sodium
are lower than those of lithium but higher
than those of the heavier alkali metals potassium,
rubidium, and caesium, following periodic
trends down the group. These properties change
dramatically at elevated pressures: at 1.5
Mbar, the color changes from silvery metallic
to black; at 1.9 Mbar the material becomes
transparent with a red color; and at 3 Mbar,
sodium is a clear and transparent solid. All
of these high-pressure allotropes are insulators
and electrides.
In a flame test, sodium and its compounds
glow yellow because the excited 3s electrons
of sodium emit a photon when they fall from
3p to 3s; the wavelength of this photon corresponds
to the D line at about 589.3 nm. Spin-orbit
interactions involving the electron in the
3p orbital split the D line into two, at 589.0
and 589.6 nm; hyperfine structures involving
both orbitals cause many more lines.
=== Isotopes ===
Twenty isotopes of sodium are known, but only
23Na is stable. 23Na is created in the carbon-burning
process in stars by fusing two carbon atoms
together; this requires temperatures above
600 megakelvins and a star of at least three
solar masses. Two radioactive, cosmogenic
isotopes are the byproduct of cosmic ray spallation:
22Na has a half-life of 2.6 years and 24Na,
a half-life of 15 hours; all other isotopes
have a half-life of less than one minute.
Two nuclear isomers have been discovered,
the longer-lived one being 24mNa with a half-life
of around 20.2 milliseconds. Acute neutron
radiation, as from a nuclear criticality accident,
converts some of the stable 23Na in human
blood to 24Na; the neutron radiation dosage
of a victim can be calculated by measuring
the concentration of 24Na relative to 23Na.
== Chemistry ==
Sodium atoms have 11 electrons, one more than
the extremely stable configuration of the
noble gas neon. Because of this and its low
first ionization energy of 495.8 kJ/mol, the
sodium atom is much more likely to lose the
last electron and acquire a positive charge
than to gain one and acquire a negative charge.
This process requires so little energy that
sodium is readily oxidized by giving up its
11th electron. In contrast, the second ionization
energy is very high (4562 kJ/mol), because
the 10th electron is closer to the nucleus
than the 11th electron. As a result, sodium
usually forms ionic compounds involving the
Na+ cation.The most common oxidation state
for sodium is +1. It is generally less reactive
than potassium and more reactive than lithium.
Sodium metal is highly reducing, with the
standard reduction potential for the Na+/Na
couple being −2.71 volts, though potassium
and lithium have even more negative potentials.
=== Salts and oxides ===
Sodium compounds are of immense commercial
importance, being particularly central to
industries producing glass, paper, soap, and
textiles. The most important sodium compounds
are table salt (NaCl), soda ash (Na2CO3),
baking soda (NaHCO3), caustic soda (NaOH),
sodium nitrate (NaNO3), di- and tri-sodium
phosphates, sodium thiosulfate (Na2S2O3·5H2O),
and borax (Na2B4O7·10H2O). In compounds,
sodium is usually ionically bonded to water
and anions and is viewed as a hard Lewis acid.
Most soaps are sodium salts of fatty acids.
Sodium soaps have a higher melting temperature
(and seem "harder") than potassium soaps.Like
all the alkali metals, sodium reacts exothermically
with water, and sufficiently large pieces
melt to a sphere and may explode. The reaction
produces caustic soda (sodium hydroxide) and
flammable hydrogen gas. When burned in air,
it forms primarily sodium peroxide with some
sodium oxide.
=== Aqueous solutions ===
Sodium tends to form water-soluble compounds,
such as halides, sulfates, nitrates, carboxylates
and carbonates. The main aqueous species are
the aquo complexes [Na(H2O)n]+, where n = 4–8;
with n = 6 indicated from X-ray diffraction
data and computer simulations.Direct precipitation
of sodium salts from aqueous solutions is
rare because sodium salts typically have a
high affinity for water; an exception is sodium
bismuthate (NaBiO3). Because of this, sodium
salts are usually isolated as solids by evaporation
or by precipitation with an organic solvent,
such as ethanol; for example, only 0.35 g/L
of sodium chloride will dissolve in ethanol.
Crown ethers, like 15-crown-5, may be used
as a phase-transfer catalyst.Sodium content
in bulk may be determined by treating with
a large excess of uranyl zinc acetate; the
hexahydrate (UO2)2ZnNa(CH3CO2)·6H2O precipitates
and can be weighed. Caesium and rubidium do
not interfere with this reaction, but potassium
and lithium do. Lower concentrations of sodium
may be determined by atomic absorption spectrophotometry
or by potentiometry using ion-selective electrodes.
=== Electrides and sodides ===
Like the other alkali metals, sodium dissolves
in ammonia and some amines to give deeply
colored solutions; evaporation of these solutions
leaves a shiny film of metallic sodium. The
solutions contain the coordination complex
(Na(NH3)6)+, with the positive charge counterbalanced
by electrons as anions; cryptands permit the
isolation of these complexes as crystalline
solids. Sodium forms complexes with crown
ethers, cryptands and other ligands. For example,
15-crown-5 has a high affinity for sodium
because the cavity size of 15-crown-5 is 1.7–2.2
Å, which is enough to fit sodium ion (1.9
Å). Cryptands, like crown ethers and other
ionophores, also have a high affinity for
the sodium ion; derivatives of the alkalide
Na− are obtainable by the addition of cryptands
to solutions of sodium in ammonia via disproportionation.
=== Organosodium compounds ===
Many organosodium compounds have been prepared.
Because of the high polarity of the C-Na bonds,
they behave like sources of carbanions (salts
with organic anions). Some well-known derivatives
include sodium cyclopentadienide (NaC5H5)
and trityl sodium ((C6H5)3CNa). Because of
the large size and very low polarising power
of the Na+ cation, it can stabilize large,
aromatic, polarisable radical anions, such
as in sodium naphthalenide, Na+[C10H8•]−,
a strong reducing agent.
=== Intermetallic compounds ===
Sodium forms alloys with many metals, such
as potassium, calcium, lead, and the group
11 and 12 elements. Sodium and potassium form
KNa2 and NaK. NaK is 40–90% potassium and
it is liquid at ambient temperature. It is
an excellent thermal and electrical conductor.
Sodium-calcium alloys are by-products of the
electrolytic production of sodium from a binary
salt mixture of NaCl-CaCl2 and ternary mixture
NaCl-CaCl2-BaCl2. Calcium is only partially
miscible with sodium. In a liquid state, sodium
is completely miscible with lead. There are
several methods to make sodium-lead alloys.
One is to melt them together and another is
to deposit sodium electrolytically on molten
lead cathodes. NaPb3, NaPb, Na9Pb4, Na5Pb2,
and Na15Pb4 are some of the known sodium-lead
alloys. Sodium also forms alloys with gold
(NaAu2) and silver (NaAg2). Group 12 metals
(zinc, cadmium and mercury) are known to make
alloys with sodium. NaZn13 and NaCd2 are alloys
of zinc and cadmium. Sodium and mercury form
NaHg, NaHg4, NaHg2, Na3Hg2, and Na3Hg.
== History ==
Because of its importance in human metabolism,
salt has long been an important commodity,
as shown by the English word salary, which
derives from salarium, the wafers of salt
sometimes given to Roman soldiers along with
their other wages. In medieval Europe, a compound
of sodium with the Latin name of sodanum was
used as a headache remedy. The name sodium
is thought to originate from the Arabic suda,
meaning headache, as the headache-alleviating
properties of sodium carbonate or soda were
well known in early times. Although sodium,
sometimes called soda, had long been recognized
in compounds, the metal itself was not isolated
until 1807 by Sir Humphry Davy through the
electrolysis of sodium hydroxide. In 1809,
the German physicist and chemist Ludwig Wilhelm
Gilbert proposed the names Natronium for Humphry
Davy's "sodium" and Kalium for Davy's "potassium".
The chemical abbreviation for sodium was first
published in 1814 by Jöns Jakob Berzelius
in his system of atomic symbols, and is an
abbreviation of the element's New Latin name
natrium, which refers to the Egyptian natron,
a natural mineral salt mainly consisting of
hydrated sodium carbonate. Natron historically
had several important industrial and household
uses, later eclipsed by other sodium compounds.Sodium
imparts an intense yellow color to flames.
As early as 1860, Kirchhoff and Bunsen noted
the high sensitivity of a sodium flame test,
and stated in Annalen der Physik und Chemie:
In a corner of our 60 m3 room farthest away
from the apparatus, we exploded 3 mg of sodium
chlorate with milk sugar while observing the
nonluminous flame before the slit. After a
while, it glowed a bright yellow and showed
a strong sodium line that disappeared only
after 10 minutes. From the weight of the sodium
salt and the volume of air in the room, we
easily calculate that one part by weight of
air could not contain more than 1/20 millionth
weight of sodium.
== Occurrence ==
The Earth's crust contains 2.27% sodium, making
it the seventh most abundant element on Earth
and the fifth most abundant metal, behind
aluminium, iron, calcium, and magnesium and
ahead of potassium. Sodium's estimated oceanic
abundance is 1.08×104 milligrams per liter.
Because of its high reactivity, it is never
found as a pure element. It is found in many
different minerals, some very soluble, such
as halite and natron, others much less soluble,
such as amphibole and zeolite. The insolubility
of certain sodium minerals such as cryolite
and feldspar arises from their polymeric anions,
which in the case of feldspar is a polysilicate.
=== Astronomical observations ===
Atomic sodium has a very strong spectral line
in the yellow-orange part of the spectrum
(the same line as is used in sodium vapour
street lights). This appears as an absorption
line in many types of stars, including the
Sun. The line was first studied in 1814 by
Joseph von Fraunhofer during his investigation
of the lines in the solar spectrum, now known
as the Fraunhofer lines. Fraunhofer named
it the 'D line', although it is now known
to actually be a group of closely spaced lines
split by a fine and hyperfine structure.The
strength of the D line means it has been detected
in many other astronomical environments. In
stars, it is seen in any whose surfaces are
cool enough for sodium to exist in atomic
form (rather than ionised). This corresponds
to stars of roughly F-type and cooler. Many
other stars appear to have a sodium absorption
line, but this is actually caused by gas in
the foreground interstellar medium. The two
can be distinguished via high-resolution spectroscopy,
because interstellar lines are much narrower
than those broadened by stellar rotation.Sodium
has also been detected in numerous Solar System
environments, including Mercury's atmosphere,
the exosphere of the Moon, and numerous other
bodies. Some comets have a sodium tail, which
was first detected in observations of Comet
Hale-Bopp in 1997. Sodium has even been detected
in the atmospheres of some extrasolar planets
via transit spectroscopy.
== Commercial production ==
Employed only in rather specialized applications,
only about 100,000 tonnes of metallic sodium
are produced annually. Metallic sodium was
first produced commercially in the late 19th
century by carbothermal reduction of sodium
carbonate at 1100 °C, as the first step of
the Deville process for the production of
aluminium:
Na2CO3 + 2 C → 2 Na + 3 COThe high demand
for aluminium created the need for the production
of sodium. The introduction of the Hall–Héroult
process for the production of aluminium by
electrolysing a molten salt bath ended the
need for large quantities of sodium. A related
process based on the reduction of sodium hydroxide
was developed in 1886.Sodium is now produced
commercially through the electrolysis of molten
sodium chloride, based on a process patented
in 1924. This is done in a Downs cell in which
the NaCl is mixed with calcium chloride to
lower the melting point below 700 °C. As
calcium is less electropositive than sodium,
no calcium will be deposited at the cathode.
This method is less expensive than the previous
Castner process (the electrolysis of sodium
hydroxide).The market for sodium is volatile
due to the difficulty in its storage and shipping;
it must be stored under a dry inert gas atmosphere
or anhydrous mineral oil to prevent the formation
of a surface layer of sodium oxide or sodium
superoxide.
== Applications ==
Though metallic sodium has some important
uses, the major applications for sodium use
compounds; millions of tons of sodium chloride,
hydroxide, and carbonate are produced annually.
Sodium chloride is extensively used for anti-icing
and de-icing and as a preservative; examples
of the uses of sodium bicarbonate include
baking, as a raising agent, and sodablasting.
Along with potassium, many important medicines
have sodium added to improve their bioavailability;
though potassium is the better ion in most
cases, sodium is chosen for its lower price
and atomic weight. Sodium hydride is used
as a base for various reactions (such as the
aldol reaction) in organic chemistry, and
as a reducing agent in inorganic chemistry.Metallic
sodium is used mainly for the production of
sodium borohydride, sodium azide, indigo,
and triphenylphosphine. A once-common use
was the making of tetraethyllead and titanium
metal; because of the move away from TEL and
new titanium production methods, the production
of sodium declined after 1970. Sodium is also
used as an alloying metal, an anti-scaling
agent, and as a reducing agent for metals
when other materials are ineffective. Note
the free element is not used as a scaling
agent, ions in the water are exchanged for
sodium ions. Sodium plasma ("vapor") lamps
are often used for street lighting in cities,
shedding light that ranges from yellow-orange
to peach as the pressure increases. By itself
or with potassium, sodium is a desiccant;
it gives an intense blue coloration with benzophenone
when the desiccate is dry. In organic synthesis,
sodium is used in various reactions such as
the Birch reduction, and the sodium fusion
test is conducted to qualitatively analyse
compounds. Sodium reacts with alcohol and
gives alkoxides, and when sodium is dissolved
in ammonia solution, it can be used to reduce
alkynes to trans-alkenes. Lasers emitting
light at the sodium D line are used to create
artificial laser guide stars that assist in
the adaptive optics for land-based visible-light
telescopes.
=== Heat transfer ===
Liquid sodium is used as a heat transfer fluid
in some types of nuclear reactors because
it has the high thermal conductivity and low
neutron absorption cross section required
to achieve a high neutron flux in the reactor.
The high boiling point of sodium allows the
reactor to operate at ambient (normal) pressure,
but the drawbacks include its opacity, which
hinders visual maintenance, and its explosive
properties. Radioactive sodium-24 may be produced
by neutron bombardment during operation, posing
a slight radiation hazard; the radioactivity
stops within a few days after removal from
the reactor. If a reactor needs to be shut
down frequently, NaK is used; because NaK
is a liquid at room temperature, the coolant
does not solidify in the pipes. In this case,
the pyrophoricity of potassium requires extra
precautions to prevent and detect leaks. Another
heat transfer application is poppet valves
in high-performance internal combustion engines;
the valve stems are partially filled with
sodium and work as a heat pipe to cool the
valves.
== Biological role ==
In humans, sodium is an essential mineral
that regulates blood volume, blood pressure,
osmotic equilibrium and pH; the minimum physiological
requirement for sodium is 500 milligrams per
day. Sodium chloride is the principal source
of sodium in the diet, and is used as seasoning
and preservative in such commodities as pickled
preserves and jerky; for Americans, most sodium
chloride comes from processed foods. Other
sources of sodium are its natural occurrence
in food and such food additives as monosodium
glutamate (MSG), sodium nitrite, sodium saccharin,
baking soda (sodium bicarbonate), and sodium
benzoate. The US Institute of Medicine set
its Tolerable Upper Intake Level for sodium
at 2.3 grams per day, but the average person
in the United States consumes 3.4 grams per
day. Studies have found that lowering sodium
intake by 2 g per day tends to lower systolic
blood pressure by about two to four mm Hg.
It has been estimated that such a decrease
in sodium intake would lead to between 9 and
17% fewer cases of hypertension.Hypertension
causes 7.6 million premature deaths worldwide
each year. (Note that salt contains about
39.3% sodium—the rest being chlorine and
trace chemicals; thus, 2.3 g sodium is about
5.9 g, or 2.7 ml of salt—about one US teaspoon.)
The American Heart Association recommends
no more than 1.5 g of sodium per day.One study
found that people with or without hypertension
who excreted less than 3 grams of sodium per
day in their urine (and therefore were taking
in less than 3 g/d) had a higher risk of death,
stroke, or heart attack than those excreting
4 to 5 grams per day. Levels of 7 g per day
or more in people with hypertension were associated
with higher mortality and cardiovascular events,
but this was not found to be true for people
without hypertension. The US FDA states that
adults with hypertension and prehypertension
should reduce daily intake to 1.5 g.The renin–angiotensin
system regulates the amount of fluid and sodium
concentration in the body. Reduction of blood
pressure and sodium concentration in the kidney
result in the production of renin, which in
turn produces aldosterone and angiotensin,
retaining sodium in the urine. When the concentration
of sodium increases, the production of renin
decreases, and the sodium concentration returns
to normal. The sodium ion (Na+) is an important
electrolyte in neuron function, and in osmoregulation
between cells and the extracellular fluid.
This is accomplished in all animals by Na+/K+-ATPase,
an active transporter pumping ions against
the gradient, and sodium/potassium channels.
Sodium is the most prevalent metallic ion
in extracellular fluid.Unusually low or high
sodium levels in humans are recognized in
medicine as hyponatremia and hypernatremia.
These conditions may be caused by genetic
factors, ageing, or prolonged vomiting or
diarrhea.In C4 plants, sodium is a micronutrient
that aids metabolism, specifically in regeneration
of phosphoenolpyruvate and synthesis of chlorophyll.
In others, it substitutes for potassium in
several roles, such as maintaining turgor
pressure and aiding in the opening and closing
of stomata. Excess sodium in the soil can
limit the uptake of water by decreasing the
water potential, which may result in plant
wilting; excess concentrations in the cytoplasm
can lead to enzyme inhibition, which in turn
causes necrosis and chlorosis. In response,
some plants have developed mechanisms to limit
sodium uptake in the roots, to store it in
cell vacuoles, and restrict salt transport
from roots to leaves; excess sodium may also
be stored in old plant tissue, limiting the
damage to new growth. Halophytes have adapted
to be able to flourish in sodium rich environments.
== Safety and precautions ==
Sodium forms flammable hydrogen and caustic
sodium hydroxide on contact with water; ingestion
and contact with moisture on skin, eyes or
mucous membranes can cause severe burns. Sodium
spontaneously explodes in the presence of
water due to the formation of hydrogen (highly
explosive) and sodium hydroxide (which dissolves
in the water, liberating more surface). However,
sodium exposed to air and ignited or reaching
autoignition (reported to occur when a molten
pool of sodium reaches about 290 °C) displays
a relatively mild fire. In the case of massive
(non-molten) pieces of sodium, the reaction
with oxygen eventually becomes slow due to
formation of a protective layer. Fire extinguishers
based on water accelerate sodium fires; those
based on carbon dioxide and bromochlorodifluoromethane
should not be used on sodium fire. Metal fires
are Class D, but not all Class D extinguishers
are workable with sodium. An effective extinguishing
agent for sodium fires is Met-L-X. Other effective
agents include Lith-X, which has graphite
powder and an organophosphate flame retardant,
and dry sand. Sodium fires are prevented in
nuclear reactors by isolating sodium from
oxygen by surrounding sodium pipes with inert
gas. Pool-type sodium fires are prevented
using different design measures called catch
pan systems. They collect leaking sodium into
a leak-recovery tank where it is isolated
from oxygen.
== See also
