Lanthanum is a chemical element with symbol
La and atomic number 57. It is a soft, ductile,
silvery-white metal that tarnishes rapidly
when exposed to air and is soft enough to
be cut with a knife. It is the eponym of the
lanthanide series, a group of 15 similar elements
between lanthanum and lutetium in the periodic
table, of which lanthanum is the first and
the prototype. It is also sometimes considered
the first element of the 6th-period transition
metals, which would put it in group 3, although
lutetium is sometimes placed in this position
instead. Lanthanum is traditionally counted
among the rare earth elements. The usual oxidation
state is +3. Lanthanum has no biological role
in humans but is essential to some bacteria.
It is not particularly toxic to humans but
does show some antimicrobial activity.
Lanthanum usually occurs together with cerium
and the other rare earth elements. Lanthanum
was first found by the Swedish chemist Carl
Gustav Mosander in 1839 as an impurity in
cerium nitrate – hence the name lanthanum,
from the Ancient Greek λανθάνειν
(lanthanein), meaning "to lie hidden". Although
it is classified as a rare earth element,
lanthanum is the 28th most abundant element
in the Earth's crust, almost three times as
abundant as lead. In minerals such as monazite
and bastnäsite, lanthanum composes about
a quarter of the lanthanide content. It is
extracted from those minerals by a process
of such complexity that pure lanthanum metal
was not isolated until 1923.
Lanthanum compounds have numerous applications
as catalysts, additives in glass, carbon arc
lamps for studio lights and projectors, ignition
elements in lighters and torches, electron
cathodes, scintillators, GTAW electrodes,
and other things. Lanthanum carbonate is used
as a phosphate binder in cases of renal failure.
It is also an element in the 6th period and
in the 3rd group.
== Characteristics ==
=== 
Physical ===
Lanthanum is the first element and prototype
of the lanthanide series. In the periodic
table, it appears to the right of the alkaline
earth metal barium and to the left of the
lanthanide cerium. Lanthanum is often considered
to be a group 3 element, along with its lighter
congeners scandium and yttrium and its heavier
congener, the radioactive actinium, although
this classification is sometimes disputed.
Similarly to scandium, yttrium, and actinium,
the 57 electrons of a lanthanum atom are arranged
in the configuration [Xe]5d16s2, with three
valence electrons outside the noble gas core.
In chemical reactions, lanthanum almost always
gives up these three valence electrons from
the 5d and 6s subshells to form the +3 oxidation
state, achieving the stable configuration
of the preceding noble gas xenon. Some lanthanum(II)
compounds are also known, but they are much
less stable.Among the lanthanides, lanthanum
is exceptional as it does not have any 4f
electrons; indeed, the sudden contraction
and lowering of energy of the 4f orbital that
is important for the chemistry of the lanthanides
only begins to happen at cerium. Thus it is
only very weakly paramagnetic, unlike the
strongly paramagnetic later lanthanides (with
the exceptions of the last two, ytterbium
and lutetium, where the 4f shell is completely
full). Furthermore, since the melting points
of the trivalent lanthanides are related to
the extent of hybridisation of the 6s, 5d,
and 4f electrons, lanthanum has the second-lowest
(after cerium) melting point among all the
lanthanides: 920 °C. The lanthanides become
harder as the series is traversed: as expected,
lanthanum is a soft metal. Lanthanum has a
relatively high resistivity of 615 nΩm at
room temperature; in comparison, the value
for the good conductor aluminium is only 26.50
nΩm. Lanthanum is the least volatile of the
lanthanides. Like most of the lanthanides,
lanthanum has a hexagonal crystal structure
at room temperature. At 310 °C, lanthanum
changes to a face-centered cubic structure,
and at 865 °C, it changes to a body-centered
cubic structure.
=== Chemical ===
As expected from periodic trends, lanthanum
has the largest atomic radius of the lanthanides
and the stable group 3 elements. Hence, it
is the most reactive among them, tarnishing
slowly in air and burning readily to form
lanthanum(III) oxide, La2O3, which is almost
as basic as calcium oxide. A centimeter-sized
sample of lanthanum will corrode completely
in a year as its oxide spalls off like iron
rust, instead of forming a protective oxide
coating like aluminium and lanthanum's lighter
congeners scandium and yttrium. Lanthanum
reacts with the halogens at room temperature
to form the trihalides, and upon warming will
form binary compounds with the nonmetals nitrogen,
carbon, sulfur, phosphorus, boron, selenium,
silicon and arsenic. Lanthanum reacts slowly
with water to form lanthanum(III) hydroxide,
La(OH)3. In dilute sulfuric acid, lanthanum
readily forms the aquated tripositive ion
[La(H2O)9]3+: this is colorless in aqueous
solution since La3+ has no f electrons. Lanthanum
is the strongest and hardest base among the
lanthanides and group 3 elements, which is
again expected from its being the largest
of them.
=== Isotopes ===
Naturally occurring lanthanum is made up of
two isotopes, the stable 139La and the primordial
long-lived radioisotope 138La. 139La is by
far the most abundant, making up 99.910% of
natural lanthanum: it is produced in the s-process
(slow neutron capture, which occurs in low-
to medium-mass stars) and the r-process (rapid
neutron capture, which occurs in core-collapse
supernovae). The very rare isotope 138La is
one of the few primordial odd-odd nuclei,
with a long half-life of 1.05×1011 years:
it is one of the proton-rich p-nuclei which
cannot be produced in the s- or r-processes.
138La, along with the even rarer 180mTa, is
produced in the ν-process, where neutrinos
interact with stable nuclei. All other lanthanum
isotopes are synthetic: with the exception
of 137La with a half-life of about 60,000
years, all of them have half-lives less than
a day, and most have half-lives less than
a minute. The isotopes 139La and 140La occur
as fission products of uranium.
== Compounds ==
Lanthanum oxide is a white solid that can
be prepared by direct reaction of its constituent
elements. Due to the large size of the La3+
ion, La2O3 adopts a hexagonal 7-coordinate
structure that changes to the 6-coordinate
structure of scandium oxide (Sc2O3) and yttrium
oxide (Y2O3) at high temperature. When it
reacts with water, lanthanum hydroxide is
formed: a lot of heat is evolved in the reaction
and a hissing sound is heard. Lanthanum hydroxide
will react with atmospheric carbon dioxide
to form the basic carbonate.Lanthanum fluoride
is insoluble in water and can be used as a
qualitative test for the presence of La3+.
The heavier halides are all very soluble deliquescent
compounds. The anhydrous halides are produced
by direct reaction of their elements, as heating
the hydrates causes hydrolysis: for example,
heating hydrated LaCl3 produces LaOCl.Lanthanum
reacts exothermically with hydrogen to produce
the dihydride LaH2, a black, pyrophoric, brittle,
conducting compound with the calcium fluoride
structure. This is a non-stoichiometric compound,
and further absorption of hydrogen is possible,
with a concomitant loss of electrical conductivity,
until the more salt-like LaH3 is reached.
Like LaI2 and LaI, LaH2 is probably an electride
compound.Due to the large ionic radius and
great electropositivity of La3+, there is
not much covalent contribution to its bonding
and hence it has a limited coordination chemistry,
like yttrium and the other lanthanides. Lanthanum
oxalate does not dissolve very much in alkali-metal
oxalate solutions, and [La(acac)3(H2O)2] decomposes
around 500 °C. Oxygen is the most common
donor atom in lanthanum complexes, which are
mostly ionic and often have high coordination
numbers over 6: 8 is the most characteristic,
forming square antiprismatic and dodecadeltahedral
structures. These high-coordinate species,
reaching up to coordination number 12 with
the use of chelating ligands such as in La2(SO4)3·9H2O,
often have a low degree of symmetry because
of stereo-chemical factors.Lanthanum chemistry
tends not to involve π bonding due to the
electron configuration of the element: thus
its organometallic chemistry is quite limited.
The best characterized organolanthanum compounds
are the cyclopentadienyl complex La(C5H5)3,
which is produced by reacting anhydrous LaCl3
with NaC5H5 in tetrahydrofuran, and its methyl-substituted
derivatives.
== History ==
In 1751, the Swedish mineralogist Axel Fredrik
Cronstedt discovered a heavy mineral from
the mine at Bastnäs, later named cerite.
Thirty years later, the fifteen-year-old Vilhelm
Hisinger, from the family owning the mine,
sent a sample of it to Carl Scheele, who did
not find any new elements within. In 1803,
after Hisinger had become an ironmaster, he
returned to the mineral with Jöns Jacob Berzelius
and isolated a new oxide which they named
ceria after the dwarf planet Ceres, which
had been discovered two years earlier. Ceria
was simultaneously independently isolated
in Germany by Martin Heinrich Klaproth. Between
1839 and 1843, ceria was shown to be a mixture
of oxides by the Swedish surgeon and chemist
Carl Gustaf Mosander, who lived in the same
house as Berzelius: he separated out two other
oxides which he named lanthana and didymia.
He partially decomposed a sample of cerium
nitrate by roasting it in air and then treating
the resulting oxide with dilute nitric acid.
Since lanthanum's properties differed only
slightly from those of cerium, and occurred
along with it in its salts, he named it from
the Ancient Greek λανθάνειν [lanthanein]
(lit. to lie hidden). Relatively pure lanthanum
metal was first isolated in 1923.
== Occurrence and production ==
Lanthanum is the third-most abundant of all
the lanthanides, making up 39 mg/kg of the
Earth's crust, behind neodymium at 41.5 mg/kg
and cerium at 66.5 mg/kg. It is almost three
times as abundant as lead in the Earth's crust.
Despite being among the so-called "rare earth
metals", lanthanum is thus not rare at all,
but it is historically so named because it
is rarer than "common earths" such as lime
and magnesia, and historically only a few
deposits were known. Lanthanum is considered
a rare earth metal because the process to
mine it is difficult, time-consuming, and
expensive. Lanthanum is rarely the dominant
lanthanide found in the rare earth minerals,
and in their chemical formulae it is usually
preceded by cerium. Rare examples of La-dominant
minerals are monazite-(La) and lanthanite-(La).
The La3+ ion is similarly-sized to the early
lanthanides of the cerium group (those up
to samarium and europium) that immediately
follow in the periodic table, and hence it
tends to occur along with them in phosphate,
silicate and carbonate minerals, such as monazite
(MIIIPO4) and bastnäsite (MIIICO3F), where
M refers to all the rare earth metals except
scandium and the radioactive promethium (mostly
Ce, La, and Y). Bastnäsite is usually lacking
in thorium and the heavy lanthanides, and
the purification of the light lanthanides
from it is less involved. The ore, after being
crushed and ground, is first treated with
hot concentrated sulfuric acid, evolving carbon
dioxide, hydrogen fluoride, and silicon tetrafluoride:
the product is then dried and leached with
water, leaving the early lanthanide ions,
including lanthanum, in solution.The procedure
for monazite, which usually contains all the
rare earths as well as thorium, is more involved.
Monazite, because of its magnetic properties,
can be separated by repeated electromagnetic
separation. After separation, it is treated
with hot concentrated sulfuric acid to produce
water-soluble sulfates of rare earths. The
acidic filtrates are partially neutralized
with sodium hydroxide to pH 3-4. Thorium precipitates
out of solution as hydroxide and is removed.
After that, the solution is treated with ammonium
oxalate to convert rare earths to their insoluble
oxalates. The oxalates are converted to oxides
by annealing. The oxides are dissolved in
nitric acid that excludes one of the main
components, cerium, whose oxide is insoluble
in HNO3. Lanthanum is separated as a double
salt with ammonium nitrate by crystallization.
This salt is relatively less soluble than
other rare earth double salts and therefore
stays in the residue. Care must be taken when
handling some of the residues as they contain
228Ra, the daughter of 232Th, which is a strong
gamma emitter. Lanthanum is relatively easy
to extract as it has only one neighbouring
lanthanide, cerium, which can be removed by
making use of its ability to be oxidised to
the +4 state; thereafter, lanthanum may be
separated out by the historical method of
fractional crystallization of La(NO3)3·2NH4NO3·4H2O,
or by ion-exchange techniques when higher
purity is desired.Lanthanum metal is obtained
from its oxide by heating it with ammonium
chloride or fluoride and hydrofluoric acid
at 300-400 °C to produce the chloride or
fluoride:
La2O3 + 6 NH4Cl → 2 LaCl3 + 6 NH3 + 3 H2OThis
is followed by reduction with alkali or alkaline
earth metals in vacuum or argon atmosphere:
LaCl3 + 3 Li → La + 3 LiClAlso, pure lanthanum
can be produced by electrolysis of molten
mixture of anhydrous LaCl3 and NaCl or KCl
at elevated temperatures.
== Applications ==
The first historical application of lanthanum
was in gas lantern mantles. Carl Auer von
Welsbach used a mixture of 60% magnesium oxide,
20% lanthanum oxide, and 20% yttrium oxide,
which he called Actinophor and patented in
1885. The original mantles gave a green-tinted
light and were not very successful, and his
first company, which established a factory
in Atzgersdorf in 1887, failed in 1889.Modern
uses of lanthanum include:
One material used for anodic material of nickel-metal
hydride batteries is La(Ni3.6Mn0.4Al0.3Co0.7).
Due to high cost to extract the other lanthanides,
a mischmetal with more than 50% of lanthanum
is used instead of pure lanthanum. The compound
is an intermetallic component of the AB5 type.As
most hybrid cars use nickel-metal hydride
batteries, massive quantities of lanthanum
are required for the production of hybrid
automobiles. A typical hybrid automobile battery
for a Toyota Prius requires 10 to 15 kilograms
(22 to 33 lb) of lanthanum. As engineers push
the technology to increase fuel efficiency,
twice that amount of lanthanum could be required
per vehicle.Hydrogen sponge alloys can contain
lanthanum. These alloys are capable of storing
up to 400 times their own volume of hydrogen
gas in a reversible adsorption process. Heat
energy is released every time they do so;
therefore these alloys have possibilities
in energy conservation systems.
Mischmetal, a pyrophoric alloy used in lighter
flints, contains 25% to 45% lanthanum.
Lanthanum oxide and the boride are used in
electronic vacuum tubes as hot cathode materials
with strong emissivity of electrons. Crystals
of LaB6 are used in high-brightness, extended-life,
thermionic electron emission sources for electron
microscopes and Hall-effect thrusters.
Lanthanum trifluoride (LaF3) is an essential
component of a heavy fluoride glass named
ZBLAN. This glass has superior transmittance
in the infrared range and is therefore used
for fiber-optical communication systems.
Cerium-doped lanthanum bromide and lanthanum
chloride are the recent inorganic scintillators,
which have a combination of high light yield,
best energy resolution, and fast response.
Their high yield converts into superior energy
resolution; moreover, the light output is
very stable and quite high over a very wide
range of temperatures, making it particularly
attractive for high-temperature applications.
These scintillators are already widely used
commercially in detectors of neutrons or gamma
rays.
Carbon arc lamps use a mixture of rare earth
elements to improve the light quality. This
application, especially by the motion picture
industry for studio lighting and projection,
consumed about 25% of the rare-earth compounds
produced until the phase out of carbon arc
lamps.
Lanthanum(III) oxide (La2O3) improves the
alkali resistance of glass and is used in
making special optical glasses, such as infrared-absorbing
glass, as well as camera and telescope lenses,
because of the high refractive index and low
dispersion of rare-earth glasses. Lanthanum
oxide is also used as a grain-growth additive
during the liquid-phase sintering of silicon
nitride and zirconium diboride.
Small amounts of lanthanum added to steel
improves its malleability, resistance to impact,
and ductility, whereas addition of lanthanum
to molybdenum decreases its hardness and sensitivity
to temperature variations.
Small amounts of lanthanum are present in
many pool products to remove the phosphates
that feed algae.
Lanthanum oxide additive to tungsten is used
in gas tungsten arc welding electrodes, as
a substitute for radioactive thorium.
Various compounds of lanthanum and other rare-earth
elements (oxides, chlorides, etc.) are components
of various catalysis, such as petroleum cracking
catalysts.
Lanthanum-barium radiometric dating is used
to estimate age of rocks and ores, though
the technique has limited popularity.
Lanthanum carbonate was approved as a medication
(Fosrenol, Shire Pharmaceuticals) to absorb
excess phosphate in cases of end-stage renal
failure.
Lanthanum fluoride is used in phosphor lamp
coatings. Mixed with europium fluoride, it
is also applied in the crystal membrane of
fluoride ion-selective electrodes.
Like horseradish peroxidase, lanthanum is
used as an electron-dense tracer in molecular
biology.
Lanthanum-modified bentonite (or phoslock)
is used to remove phosphates from water in
lake treatments.
== Biological role ==
Lanthanum has no known biological role in
humans. The element is very poorly absorbed
after oral administration and when injected
its elimination is very slow. Lanthanum carbonate
(Fosrenol) was approved as a phosphate binder
to absorb excess phosphate in cases of end
stage renal disease.While lanthanum has pharmacological
effects on several receptors and ion channels,
its specificity for the GABA receptor is unique
among trivalent cations. Lanthanum acts at
the same modulatory site on the GABA receptor
as zinc, a known negative allosteric modulator.
The lanthanum cation La3+ is a positive allosteric
modulator at native and recombinant GABA receptors,
increasing open channel time and decreasing
desensitization in a subunit configuration
dependent manner.Lanthanum is an essential
cofactor for the methanol dehydrogenase of
the methanotrophic bacterium Methylacidiphilum
fumariolicum SolV, although the great chemical
similarity of the lanthanides means that it
may be substituted with cerium, praseodymium,
or neodymium without ill effects, and with
the smaller samarium, europium, or gadolinium
giving no side effects other than slower growth.
== Precautions ==
Lanthanum has a low to moderate level of toxicity
and should be handled with care. The injection
of lanthanum solutions produces hyperglycemia,
low blood pressure, degeneration of the spleen
and hepatic alterations. The application in
carbon arc light led to the exposure of people
to rare earth element oxides and fluorides,
which sometimes led to pneumoconiosis. As
the La3+ ion is similar in size to the Ca2+
ion, it is sometimes used as an easily traced
substitute for the latter in medical studies.
Lanthanum, like the other lanthanides, is
known to affect human metabolism, lowering
cholesterol levels, blood pressure, appetite,
and risk of blood coagulation. When injected
into the brain, it acts as a painkiller, similarly
to morphine and other opiates, though the
mechanism behind this is still unknown.
== See also ==
== Notes
