Briefly, Boron is a Group 13 element that has properties
which are borderline between metals and non-metals (semi metallic). It is a
semiconductor rather than a metallic conductor. Chemically it is closer to
silicon than to aluminum, gallium, indium, and thallium.
Crystalline boron is inert chemically and is resistant to
attack by boiling HF or HCl. When finely divided it is attacked slowly by
ho...t concentrated nitric acid.
•Name: Boron
•Symbol: B
•Atomic number: 5
•Atomic weight: 10.811
•Standard state: solid at 298 K
•CAS Registry ID: 7440-42-8
•Group in periodic table: 13
•Period in periodic table: 2
•Block in periodic table: p-block
•Color: black
•Classification: Semi-metallic
Physical properties
•Melting point: 2349 [or 2076 °C (3769 °F)] K
•Boiling point: 4200 [or 3927 °C (7101 °F)] K
•Density of solid: 2460 kg m-3
Orbital properties
•Ground state electron configuration: [He].2s2.2p1
•Shell structure: 2.3
•Term symbol: 2P1/2
Boron is a chemical element with chemical symbol B and atomic number 5. Because
boron is produced entirely by cosmic ray spallation and not by stellar
nucleosynthesis, it is a low-abundance element in both the solar system and the
Earth's crust. Boron is concentrated on Earth by the water-solubility of its
more common naturally occurring compounds, the borate minerals. These are mined
industrially as evaporites, such as borax and kernite.
Chemically uncombined boron, which is classed as a
metalloid, is not found naturally on Earth. Industrially, very pure boron is
produced with difficulty, as boron tends to form refractory materials
containing small amounts of carbon or other elements. Several allotropes of
boron exist: amorphous boron is a brown powder and crystalline boron is black,
extremely hard (about 9.5 on the Mohs scale), and a poor conductor at room
temperature. Elemental boron is used as a dopant in the semiconductor industry.
The major industrial-scale uses of boron compounds are in
sodium perborate bleaches, and the borax component of fiberglass insulation.
Boron polymers and ceramics play specialized roles as high-strength lightweight
structural and refractory materials. Boron compounds are used in silica-based glasses
and ceramics to give them resistance to thermal shock. Boron-containing
reagents are used for as intermediates in the synthesis of organic fine
chemicals. A few boron-containing organic pharmaceuticals are used, or are in
study. Natural boron is composed of two stable isotopes, one of which
(boron-10) has a number of uses as a neutron-capturing agent.
In biology, borates have low toxicity in mammals (similar to
table salt), but are more toxic to arthropods and are used as insecticides.
Boric acid is mildly antimicrobial, and a natural boron-containing organic
antibiotic is known. Boron is essential to life. Small amounts of boron
compounds play a strengthening role in the cell walls of all plants, making
boron necessary in soils. Experiments indicate a role for boron as an
ultratrace element in animals, but its role in animal physiology is unknown.
History and etymology
The name boron originates from the Arabic word بورق buraq or the Persian word بوره burah; which are names for
the mineral borax.
Boron compounds were known thousands of years ago. Borax was
known from the deserts of western Tibet, where it received the name of tincal,
derived from the Sanskrit. Borax glazes were used in China from AD300, and some
tincal even reached the West, where the Persian alchemist Jābir ibn Hayyān
seems to mention it in 700. Marco Polo brought some glazes back to Italy in the
13th century. Agricola, around 1600, reports the use of borax as a flux in
metallurgy. In 1777, boric acid was recognized in the hot springs (soffioni)
near Florence, Italy, and became known as sal sedativum, with mainly medical
uses. The rare mineral is called sassolite, which is found at Sasso, Italy.
Sasso was the main source of European borax from 1827 to 1872, at which date
American sources replaced it. Boron compounds were relatively rarely used
chemicals until the late 1800s when Francis Marion Smith's Pacific Coast Borax
Company first popularized these compounds and made them in volume and hence
cheap.
Boron was not recognized as an element until it was isolated
by Sir Humphry Davy and by Joseph Louis Gay-Lussac and Louis Jacques Thénard.
In 1808 Davy observed that electric current sent through a solution of borates
produced a brown precipitate on one of the electrodes. In his subsequent
experiments he used potassium to reduce boric acid instead of electrolysis. He
produced enough boron to confirm a new element and named the element boracium.
Gay-Lussac and Thénard used iron to reduce boric acid at high temperatures.
They showed by oxidizing boron with air that boric acid is an oxidation product
of boron. Jöns Jakob Berzelius identified boron as an element in 1824. Pure
boron was arguably first produced by the American chemist Ezekiel Weintraub in
1909.
Characteristics
Allotropes
Boron is similar to carbon in its capability to form stable
covalently bonded molecular networks. Even nominally disordered (amorphous)
boron contains regular boron icosahedra which are, however, bonded randomly to
each other without long-range order. Crystalline boron is a very hard, black
material with a high melting point of above 2000 °C. It exists in four major
polymorphs: α, β, γ and T. whereas α, β and T phases are based on B12
icosahedra, the γ-phase can be described as a rocksalt-type arrangement of the icosahedra
and B2 atomic pairs. It can be produced by compressing other boron phases to
12–20 GPa and heating to 1500–1800 °C; it remains stable after releasing the
temperature and pressure. The T phase is produced at similar pressures, but
higher temperatures of 1800–2200 °C. As to the α and β phases, they might both
coexist at ambient conditions with the β phase being more stable. Compressing
boron above 160 GPa produces a boron phase with an as yet unknown structure,
and this phase is a superconductor at temperatures 6–12 K.
Chemistry of the
element
Elemental boron is rare and poorly studied because the material is extremely
difficult to prepare. Most studies on "boron" involve samples that
contain small amounts of carbon. Chemically, boron behaves more similarly to
silicon than to aluminium. Crystalline boron is chemically inert and resistant
to attack by boiling hydrofluoric or hydrochloric acid. When finely divided, it
is attacked slowly by hot concentrated hydrogen peroxide, hot concentrated
nitric acid, hot sulfuric acid or hot mixture of sulfuric and chromic acids.
The rate of oxidation of boron depends upon the
crystallinity, particle size, purity and temperature. Boron does not react with
air at room temperature, but at higher temperatures it burns to form boron
trioxide:
4 B + 3 O2 → 2 B2O3
Boron undergoes halogenation to give trihalides, for
example,
2 B + 3 Br2 → 2 BBr3
The trichloride in practice is usually made from the oxide.
Chemical compounds
In its most familiar compounds, boron has the formal
oxidation state III. These include oxides, sulfides, nitrides, and halides.
The trihalides adopt a planar trigonal structure. These
compounds are Lewis acids in that they readily form adducts with electron-pair
donors, which are called Lewis bases. For example, fluoride (F-) and boron
trifluoride (BF3) combined to give the tetrafluoroborate anion, BF4-. Boron
trifluoride is used in the petrochemical industry as a catalyst. The halides
react with water to form boric acid.
Boron is found in nature on Earth entirely as various oxides
of B(III), often associated with other elements. The more than one hundred
borates all feature boron in oxidation state +3. These minerals resemble
silicates in some respect, although boron is often found not only in a tetrahedral
coordination with oxygen, but also in a trigonal planar configuration. Unlike
silicates, the boron minerals never feature boron with coordination number
greater than four. A typical motif is exemplified by the tetraborate anions of
the common mineral borax, shown at left. The formal negative charge of the
tetrahedral borate centers is balanced by metal cations in the minerals, such
as the sodium (Na+) in borax.
The boron nitrides are notable for the variety of structures
that they adopt. They adopt structures analogous to various allotropes of
carbon, including graphite, diamond, and nanotubes. In the diamond-like
structure called cubic boron nitride (tradename Borazon), boron atoms exist in
the tetrahedral structure of carbons atoms in diamond, but one in every four
B-N bonds can be viewed as a coordinate covalent bond, wherein two electrons
are donated by the nitrogen atom which acts as the Lewis base to a bond to the
Lewis acidic boron(III) centre. Cubic boron nitride, among other applications,
is used as an abrasive, as it has a hardness comparable with diamond (the two
substances are able to produce scratches on each other). In the BN compound
analogue of graphite, hexagonal boron nitride (h-BN), the positively-charged
boron and negatively-charged nitrogen atoms in each plane lie adjacent to the
oppositely charged atom in the next plane. Consequently graphite and h-BN have
very different properties, although both are lubricants, as these planes slip
past each other easily. However, h-BN is a relatively poor electrical and
thermal conductor in the planar directions.
Organoboron chemistry
A large number of organoboron compounds are known and many
are useful in organic synthesis. Organoboron(III) compounds are usually
tetrahedral or trigonal planar, for example, tetraphenylborate (B(C6H5)4-) vs
triphenylborane (B(C6H5)3). Many are produced from hydroboration, which employs
diborane (B2H6).
Compounds of B(I) and
B(II)
Although these are not found on Earth naturally, boron forms
a variety of stable compounds with formal oxidation state less than three. As
for many covalent compounds, formal oxidation states are often of little
meaning in boron hydrides and metal borides. The halides also form derivatives
of B(I) and B(II). BF, isoelectronic with N2, is not isolable in condensed
form, but B2F4 and B4Cl4 are well characterized.
Binary metal-boron compounds, the metal borides, feature
boron in oxidation state less than III. Illustrative is magnesium diboride
(MgB2). Each boron atom has a formal −1 charge and magnesium is assigned a
formal charge of 2+. In this material, the boron centers are trigonal planar,
with an extra double bond for each boron, with the boron atoms forming sheets
akin to the carbon in graphite. However, unlike the case with hexagonal boron
nitride which by comparison lacks electrons in the plane of the covalent atoms,
the delocalized electrons in the plane of magnesium diboride allow it to
conduct electricity similar to isoelectronic graphite. In addition, in 2001
this material was found to be a high-temperature superconductor.
Certain other metal borides find specialized applications as
hard materials for cutting tools.
From the structural perspective, the most distinctive
chemical compounds of boron are the hydrides. Included in this series are the
cluster compounds dodecaborate (B12H122-), decaborane (B10H14), and the
carboranes such as C2B10H12. Characteristically such compounds feature boron
with coordination numbers greater than four.
Isotopes
Boron has two naturally occurring and stable isotopes, 11B
(80.1%) and 10B (19.9%). The mass difference results in a wide range of δ11B
values, which are defined as a fractional difference between the 11B and 10B
and traditionally expressed in parts per thousand, in natural waters ranging
from −16 to +59. There are 13 known isotopes of boron, the shortest-lived
isotope is 7B which decays through proton emission and alpha decay. It has a
half-life of 3.5×10−22 s. Isotopic fractionation of boron is controlled by the
exchange reactions of the boron species B(OH)3 and [B(OH)4]−. Boron isotopes
are also fractionated during mineral crystallization, during H2O phase changes
in hydrothermal systems, and during hydrothermal alteration of rock. The latter
effect results in preferential removal of the 10B(OH)4 ion onto clays. It
results in solutions enriched in 11B(OH)3 and therefore may be responsible for
the large 11B enrichment in seawater relative to both oceanic crust and
continental crust; this difference may act as an isotopic signature. The exotic
17B exhibits a nuclear halo, i.e. its radius is appreciably larger than that
predicted by the liquid drop model.
The 10B isotope is good at capturing thermal neutrons.
Natural boron is about 20% 10B and 80% 11B. The nuclear industry enriches
natural boron to nearly pure 10B. The less-valuable by-product, depleted boron,
is nearly pure 11B.
Commercial isotope
enrichment
Because of its high neutron cross-section, boron-10 is often
used to control fission in nuclear reactors as a neutron-capturing substance.
Several industrial-scale enrichment processes have been developed, however only
the fractionated vacuum distillation of the dimethyl ether adduct of boron
trifluoride (DME-BF3) and column chromatography of borates are being used.
Enriched boron
(boron-10)
Enriched boron or 10B is used in both radiation shielding
and is the primary nuclide used in neutron capture therapy of cancer. In the
latter ("boron neutron capture therapy" or BNCT), a compound
containing 10B is incorporated into a pharmaceutical which is selectively taken
up by a malignant tumor and tissues near it. The patient is then treated with a
beam of either thermal neutrons, or else neutrons of low energy, at a
relatively low neutron radiation dose. The neutrons, however, trigger energetic
and short-range secondary alpha particle and lithium-7 heavy ion radiation that
are products of the boron + neutron nuclear reaction, and this ion radiation
additionally bombards the tumor, especially from inside the tumor cells.
In nuclear reactors, 10B is used for reactivity control and
in emergency shutdown systems. It can serve either function in the form of
borosilicate control rods or as boric acid. In pressurized water reactors,
boric acid is added to the reactor coolant when the plant is shut down for
refueling. It is then slowly filtered out over many months as fissile material
is used up and the fuel becomes less reactive.
In future manned interplanetary spacecraft, 10B has a
theoretical role as structural material (as boron fibers or BN nanotube
material) which would also serve a special role in the radiation shield. One of
the difficulties in dealing with cosmic rays, which are mostly high energy
protons, is that some secondary radiation from interaction of cosmic rays and
spacecraft materials is high energy spallation neutrons. Such neutrons can be
moderated by materials high in light elements such as polyethylene, but the
moderated neutrons continue to be a radiation hazard unless actively absorbed in
the shielding. Among light elements that absorb thermal neutrons, 6Li and 10B
appear as potential spacecraft structural materials which serve both for
mechanical reinforcement and radiation protection.
Depleted boron
(boron-11)
Cosmic radiation will produce secondary neutrons if it hits
spacecraft structures. Those neutrons will be captured in 10B, if it is present
in the spacecraft's semiconductors, producing a gamma ray, an alpha particle,
and a lithium ion. These resultant decay products may then irradiate nearby
semiconductor 'chip' structures, causing data loss (bit flipping, or single
event upset). In radiation hardened semiconductor designs, one countermeasure
is to use depleted boron which is greatly enriched in 11B and contains almost
no 10B. 11B is largely immune to radiation damage. Depleted boron is a
by-product of the nuclear industry.
11B is also a candidate as a fuel for aneutronic fusion.
When struck by a proton with energy of about 500 keV, it produces three alpha
particles and 8.7 MeV of energy. Most other fusion reactions involving hydrogen
and helium produce penetrating neutron radiation, which weakens reactor
structures and induces long term radioactivity thereby endangering operating
personnel. Whereas, the alpha particles from 11B fusion can be turned directly
into electric power, and all radiation stops as soon as the reactor is turned
off.
NMR spectroscopy
Both 10B and 11B possess nuclear spin. The nuclear spin of
10B is 3 and that of 11B is 3/2. These isotopes are, therefore, of use in
nuclear magnetic resonance spectroscopy; and spectrometers specially adapted to
detecting the boron-11 nuclei are available commercially. The 10B and 11B
nuclei also cause splitting in the resonances of attached nuclei.
Occurrence
Boron is a relatively rare element in the Earth's crust,
representing only 0.001%. The worldwide commercial borate deposits are
estimated at 10 million tons. Turkey and the United States are the world's
largest producers of boron. Turkey has 63% of the world’s boron reserves. Boron
does not appear on Earth in elemental form but is found combined in borax,
boric acid, colemanite, kernite, ulexite and borates. Boric acid is sometimes
found in volcanic spring waters.
Ulexite is one of over a hundred borate minerals; it is a
fibrous crystal where individual fibers can guide light like optical
fibers.[61]
Economically important sources of boron are rasorite
(kernite) and tincal (borax ore). They are both found in the Mojave Desert of
California where the Rio Tinto Borax Mine (also known as the U.S. Borax Boron
Mine) near Boron, CA is California's largest open-pit mine and the largest
borax mine in the world, producing nearly half the world's borates from this
single site. However, the largest borax deposits known, many still untapped are
in Central and Western Turkey including the provinces of Eskişehir, Kütahya and
Balıkesir.
Production
The production of boron compounds does not involve formation
of elemental boron, but exploits the convenient availability of borates.
The earliest routes to elemental boron involved reduction of
boric oxide with metals such as magnesium or aluminum. However the product is
almost always contaminated with metal borides. Pure boron can be prepared by
reducing volatile boron halides with hydrogen at high temperatures. Ultrapure
boron, for the use in semiconductor industry, is produced by the decomposition
of diborane at high temperatures and then further purified with the zone
melting or Czochralski processes.
Applications
Nearly all boron ore extracted from the Earth is destined
for refinement into boric acid and sodium tetraborate pentahydrate. In the
United States, 70% of the boron is used for the production of glass and
ceramics.
Glass and ceramics
Borosilicate glass, which is typically 12–15% B2O3, 80%
SiO2, and 2% Al2O3, has a low coefficient of thermal expansion giving it a good
resistance to thermal shock. Duran and Pyrex are two major brand names for this
glass, used both in laboratory glassware and in consumer cookware and bakeware,
chiefly for this resistance.
Boron filaments are high-strength, lightweight materials
that are used chiefly for advanced aerospace structures as a component of
composite materials, as well as limited production consumer and sporting goods
such as golf clubs and fishing rods. The fibers can be produced by chemical
vapor deposition of boron on a tungsten filament.
Boron fibers and sub-millimeter sized crystalline boron
springs are produced by laser-assisted chemical vapor deposition. Translation
of the focused laser beam allows it to produce even complex helical structures.
Such structures show good mechanical properties (elastic modulus 450 GPa,
fracture strain 3.7%, fracture stress 17 GPa) and can be applied as
reinforcement of ceramics or in micromechanical systems.
Detergent formulations
and bleaching agents
Borax is used in various household laundry and cleaning
products, including the well-known "20 Mule Team Borax" laundry
booster and "Boraxo" powdered hand soap. It is also present in some
tooth bleaching formulas.
Sodium perborate serves as a source of active oxygen in many
detergents, laundry detergents, cleaning products, and laundry bleaches.
However, despite its name, "Borateem" laundry bleach no longer
contains any boron compounds, using sodium percarbonate instead as a bleaching
agent.
Insecticides
Boric acid is used as an insecticide, notably against ants,
fleas, and cockroaches.
Semiconductors
Boron is a useful dopant for such semiconductors as silicon,
germanium, and silicon carbide. Having one fewer valence electron than the host
atom, it donates a hole resulting in p-type conductivity. Traditional method of
introducing boron into semiconductors is via its atomic diffusion at high
temperatures. This process uses either solid (B2O3), liquid (BBr3), or gaseous
boron sources (B2H6 or BF3). However, after 1970s, it was mostly replaced by
ion implantation, which relies mostly on BF3 as a boron source. Boron
trichloride gas is also an important chemical in semiconductor industry,
however not for doping but rather for plasma etching of metals and their
oxides. Triethylborane is also injected into vapor deposition reactors as a
boron source. Examples are the plasma deposition of boron-containing hard
carbon films, silicon nitride-boron nitride films, and for doping of diamond
film with boron.
Magnets
Boron is a component of neodymium magnets (Nd2Fe14B), which
are the strongest type of permanent magnet. They are found in a variety of
domestic and professional electromechanical and electronic devices, such as
magnetic resonance imaging (MRI), various motors and actuators, computer HDDs,
CD and DVD players, mobile phones, timer switches, speakers, and so on.
Boron carbide
Boron carbide is a ceramic material which is obtained by
decomposing B2O3 with carbon in the electric furnace:
2 B2O3 + 7 C → B4C + 6 CO
Boron carbide's structure is only approximately B4C, and it
shows a clear depletion of carbon from this suggested stoichiometric ratio.
This is due to its very complex structure. The substance can be seen with
empirical formula B12C3 (i.e., with B12 dodecahedra being a motif), but with
less carbon as the suggested C3 units are replaced with B-C chains, and there
are smaller (B6) octahedra present as well. (See the article for structural
analysis).
The repeating polymer plus semi-crystalline structure of
boron carbide gives it great structural strength per weight. It is used in tank
armor, bulletproof vests, and numerous other structural applications.
Boron carbide's ability to absorb neutrons without forming
long-lived radionuclides (especially when doped with extra boron-10) makes the
material attractive as an absorbent for neutron radiation arising in nuclear
power plants. Nuclear applications of boron carbide include shielding, control
rods and shut-down pellets. Within control rods, boron carbide is often
powdered, to increase its surface area.
Shielding in nuclear
reactors
Boron shielding is used as a control for nuclear reactors,
taking advantage of its high cross-section for neutron capture.
Natural biological
role
There is a boron-containing natural antibiotic, boromycin,
isolated from streptomyces. Boron is an essential plant nutrient, required
primarily for maintaining the integrity of cell walls. Conversely, high soil
concentrations of > 1.0 ppm can cause marginal and tip necrosis in leaves as
well as poor overall growth performance. Levels as low as 0.8 ppm can cause
these same symptoms to appear in plants particularly sensitive to boron in the
soil. Nearly all plants, even those somewhat tolerant of boron in the soil,
will show at least some symptoms of boron toxicity when boron content in the
soil is greater than 1.8 ppm. When this content exceeds 2.0 ppm, few plants
will perform well and some may not survive. When boron levels in plant tissue
exceed 200 ppm symptoms of boron toxicity are likely to appear.
As an ultratrace element, boron is necessary for the optimal
health of rats, although it is necessary in such small amounts that
ultrapurified foods and dust filtration of air is necessary to induce boron
deficiency, which manifest as poor coat or hair quality. Presumably, boron is
necessary to other mammals. No deficiency syndrome in humans has been
described. Small amounts of boron occur widely in the diet, and the amounts
needed in the diet would, by analogy with rodent studies, be very small. The
exact physiological role of boron in the animal kingdom is poorly understood.
Boron occurs in all foods produced from plants. Since 1989
its nutritional value has been argued. It is thought that boron plays several
biochemical roles in animals, including humans. The U.S. Department of
agriculture conducted an experiment in which postmenopausal women took 3 mg of
boron a day. The results showed that supplemental boron reduced excretion of
calcium by 44%, and activated estrogen and vitamin D, suggesting a possible
role in the suppression of osteoporosis. However, whether these effects were
conventionally nutritional, or medicinal, could not be determined. The U.S.
National Institutes of Health states that "Total daily boron intake in
normal human diets ranges from 2.1–4.3 mg boron/day."
Analytical
quantification
For determination of boron content in food or materials the
colorimetric curcumin method is used. Boron has to be transferred to boric acid
or borates and on reaction with curcumin in acidic solution, a red colored
boron-chelate complex, rosocyanine, is formed.
Health issues and
toxicity
Elemental boron, boron oxide, boric acid, borates, and many
organoboron compounds are non-toxic to humans and animals (approximately
similar to table salt). The LD50 (dose at which there is 50% mortality) for
animals is about 6 g per kg of body weight. Substances with LD50 above 2 g are
considered non-toxic. The minimum lethal dose for humans has not been
established. An intake of 4 g/day of boric acid was reported without incidents,
but more than this is considered toxic for more than a few doses. Intakes of
more than 0.5 grams per day for 50 days cause minor digestive and other
problems suggestive of toxicity. Single medical doses of 20 g of boric acid for
neutron capture therapy have been used without undue toxicity. Fish have
survived for 30 min in a saturated boric acid solution and can survive longer
in strong borax solutions. Boric acid is more toxic to insects than to mammals,
and is routinely used as an insecticide.
The boranes (boron hydrogen compounds) and similar gaseous
compounds are quite poisonous. As usual, it is not an element that is
intrinsically poisonous, but toxicity depends on structure.
The boranes are toxic as well as highly flammable and
require special care when handling. Sodium borohydride presents a fire hazard
due to its reducing nature, and the liberation of hydrogen on contact with
acid. Boron halides are corrosive.
