CHEM-I-PAGE

Boron                                             

Standard Sate: solid at 298k

Color: black
                                                                                                                     

Boron is a Group 13 element. Boron has properties which are borderline between metals and non-metals. It is a semiconductor rather than a metallic conductor. Chemically it is closer to silicon than to aluminium, gallium, indium, and thallium.

(B), chemical element, semimetal of main Group IIIa (boron group)
of the periodic table, essential to plant growth and of wide industrial
application.

Properties, occurrence, and uses.


Pure crystalline boron is a black, lustrous, semiconductor; i.e., it
conducts electricity like a metal at high temperatures and is almost an
insulator at low temperatures. It is hard enough (9.3 on Mohs scale)
to scratch some abrasives, such as carborundum, but too brittle for
use in tools. Constituting about 0.001 percent by weight of the Earth's
crust, boron occurs combined as borax, kernite, and tincalconite
(hydrated sodium borates), the major commercial boron minerals,
especially concentrated in the arid regions of California, and as widely
dispersed minerals such as colemanite, ulexite, and tourmaline.
Sassolite--natural boric acid--occurs especially in Italy.

Boron was first isolated (1808) by Joseph-Louis Gay-Lussac and
Louis-Jacques Thenard and independently by Sir Humphry Davy by
heating boron oxide (B2O3) with potassium metal. The impure,
amorphous product, a brownish black powder, was the only form of
boron known for more than a century. Pure crystalline boron may
be prepared with difficulty by reduction of its bromide or chloride
(BBr3, BCl3) with hydrogen on an electrically heated tantalum
filament.

Limited quantities of elemental boron are widely used to increase
hardness in making steel. Added as the iron alloy ferroboron, it is
present in many steels, usually in the range 0.001 to 0.005 percent.
Boron is also utilized in the nonferrous-metals industry, generally as a
deoxidizer, in copper-base alloys and high-conductance copper as a
degasifier, and in aluminum castings to refine the grain. In the
semiconductor industry, small, carefully controlled amounts of boron
are added as a doping agent to silicon and germanium to modify
electrical conductivity.

In the form of boric acid or borates, traces of boron are necessary
for growth of land plants and thus indirectly essential for animal life.
Vegetable "brown heart" and sugar beet "dry rot" are among the
disorders due to boron deficiency. In excess quantities, however,
borates act as unselective herbicides.

In nature, boron consists of a mixture of two stable
isotopes--boron-10 (19.8 percent) and boron-11 (80.2 percent);
slight variations in this proportion produce a range of +/-0.003 in the
atomic weight. Because of the high thermal neutron cross section of
the rarer isotope boron-10 (3,836 barns), boron and some of its
compounds have been used as neutron shields. Pure boron exists in
at least four crystalline modifications or allotropes.

Crystalline boron is almost inert chemically at ordinary temperatures.
Boiling hydrochloric acid does not affect it, and hot concentrated nitric
acid only slowly converts finely powdered boron to boric acid
(H3BO3). Boron in its chemical behaviour is nonmetallic.


Compounds.


In its compounds boron shows a valence of three. The first three
ionization energies of boron, however, are much too high to allow
formation of compounds containing the B3+ ion; thus in all its
compounds boron is covalently bonded, with boron being either three
or four-coordinated. The three coordinated derivatives are planar
molecules that readily form donor-acceptor complexes (called
adducts), with compounds containing lone pairs of electrons; in these
adducts the boron atom is four-coordinated, the four groups being
tetrahedrally disposed around it. The tetrahedral bonds result from the
formation of anions or from the reception of an unshared pair of
electrons from a donor atom. This allows a variety of structures to
form. Solid borates show five types of structures involving several
anions (i.e., BO33-, formed of boron and oxygen) and
shared-electron bonds. The most familiar borate is sodium tetraborate,
commonly known as borax, Na2B4O710H2O, which occurs naturally
in salt beds. Borax has long been used in soaps and mild antiseptics.
Because of its ability to dissolve metallic oxides, it has also found
wide applications as a soldering flux.

Another boron compound with diverse industrial applications is boric
acid, H3BO3. This white solid, also called boracic, or orthoboric, acid,
is obtained by treating a concentrated solution of borax with sulfuric
or hydrochloric acid. Boric acid is commonly used as a mild antiseptic
for burns and surface wounds and is a major ingredient in eye lotions.
Among its other important applications are its use as a fire-retardant
in fabrics, in solutions for electroplating nickel or for tanning leather,
and as a major constituent in catalysts for numerous organic chemical
reactions. Upon heating, boric acid loses water and forms metaboric
acid, HBO2; further loss of water from metaboric acid results in the
formation of boron oxide, B2O3. The latter is mixed with silica to
make heat-resistant glass (borosilicate glass) for use in cooking ware
and certain types of laboratory equipment. Boron combines with
carbon to form boron carbide (B4C), an extremely hard substance
that is used as an abrasive and as a reinforcing agent in composite
materials.

Boron combines with various metals to form a class of compounds
called borides. The borides are usually harder, chemically less
reactive, and electrically less resistive and have a higher melting point
than the corresponding pure metallic elements. Some of the borides
are among the hardest and most heat-resistant of all known
substances. Aluminum boride (AlB12), for example, is used in many
cases as a substitute for diamond dust for grinding and polishing.

With nitrogen, boron forms boron nitride (BN), which, like carbon,
can exist in two allotropic (chemically identical but physically
different) forms. One of them has a layer structure resembling that of
graphite, while the other has a cubic crystalline structure similar to
that of diamond. The latter allotropic form, called borazon, is capable
of withstanding oxidation at much higher temperatures and is
extremely hard--properties that make it useful as a high-temperature
abrasive. Boron reacts with all halogen elements to give monomeric,
highly reactive trihalides. These so-called Lewis acids form
complexes with amines, phosphines, ethers, and halide ions.

With hydrogen, boron forms a series of compounds called boranes,
the simplest being diborane (B2H6). The molecular structure and
chemical behaviour of these boron hydrides are unique among
inorganic compounds. Typically, their molecular structure reveals
some boron and hydrogen atoms closely surrounded by or bonded to
more atoms than can be explained by an electron-pair bond for each
pair of atoms. This variance led to the concept of a chemical bond
consisting of an electron pair not localized between two atoms but
shared by three atoms (three-centre bond). Diborane combines with a
wide variety of compounds to form a large number of boron or
borane derivatives, including organic boron compounds (e.g. alkyl- or
aryl-boranes and adducts with aldehydes). atomic number 5 atomic
weight 10.811 +/-0.003 melting point 2,200 C boiling point 2,550 C
specific gravity 2.34 (20 C) valence 3 electronic config. 2-3 or
1s22s22p1

"boron" Encyclop�dia Britannica Online.

Copyright � 1999 Chem-i-Page. Please do not reproduce any material found on this page or its attached sub-pages. If you have any questions please contact the webmaster at mailto:[email protected]                                                                                                                                       Ram