CRC Handbook of Chemistry and Physics, 86th Edition

The Elements 4-5
and can be stored for a considerable time, but a true chemical
bond does not exist. Van der Waals’ forces act to hold the argon.
In August 2000, researchers at the University of Helsinki,
Finland reported they made a new argon compound HArF
by shining UV light on frozen argon that contained a small
amount of HF. Naturally occurring argon is a mixture of three
isotopes. Seventeen other radioactive isotopes are now known
to exist. Commercial argon is priced at about $70/300 cu. ft.
or 8.5 cu. meters.
Arsenic — (L. arsenicum, Gr. arsenikon, yellow orpiment, identified
with arsenikos, male, from the belief that metals were
different sexes; Arabic, Az-zernikh, the orpiment from Persian
zerni-zar, gold), As; at. wt. 74.92160(2); at. no. 33; valence –3,
0, +3 or +5. Elemental arsenic occurs in two solid modifications:
yellow, and gray or metallic, with specific gravities of
1.97, and 5.73, respectively. Gray arsenic, the ordinary stable
form, has a triple point of 817°C and sublimes at 614°C and
has a critical temperature of 1400°C. Several other allotropic
forms of arsenic are reported in the literature. It is believed
that Albertus Magnus obtained the element in 1250 A.D.
In 1649 Schroeder published two methods of preparing the
element. It is found native, in the sulfides realgar and orpiment,
as arsenides and sulfarsenides of heavy metals, as the
oxide, and as arsenates. Mispickel, arsenopyrite, (FeSAs) is the
most common mineral, from which on heating the arsenic
sublimes leaving ferrous sulfide. The element is a steel gray,
very brittle, crystalline, semimetallic solid; it tarnishes in air,
and when heated is rapidly oxidized to arsenous oxide (As 2 O 3 )
with the odor of garlic. Arsenic and its compounds are poisonous.
Exposure to arsenic and its compounds should not
exceed 0.2 mg/m 3 as elemental As during an 8-h work day.
These values, however, are being studied, and may be lowered.
Arsenic is also used in bronzing, pyrotechny, and for hardening
and improving the sphericity of shot. The most important
compounds are white arsenic (As 2 O 3 ), the sulfide, Paris
green 3Cu(AsO 2 ) 2 · Cu(C 2 H 3 O 2 ) 2 , calcium arsenate, and lead
arsenate; the last three have been used as agricultural insecticides
and poisons. Marsh’s test makes use of the formation
and ready decomposition of arsine (AsH 3 ). Arsenic is available
in high-purity form. It is finding increasing uses as a doping
agent in solid-state devices such as transistors. Gallium
arsenide is used as a laser material to convert electricity directly
into coherent light. Natural arsenic is made of one isotope
75 As. Thirty other radioactive isotopes and isomers are
known. Arsenic (99%) costs about $75/50g. Purified arsenic
(99.9995%) costs about $50/g.
Astatine — (Gr. astatos, unstable), At; at. wt. (210); at. no. 85;
m.p. 300°C (est.); valence probably 1, 3, 5, or 7. Synthesized
in 1940 by D. R. Corson, K. R. MacKenzie, and E. Segre at
the University of California by bombarding bismuth with alpha
particles. The longest-lived isotope, 210 At, has a half-life
of only 8.1 hours. Thirty-six other isotopes and isomers are
now known. Minute quantities of 215 At, 218 At, and 219 At exist in
equilibrium in nature with naturally occurring uranium and
thorium isotopes, and traces of 217 At are equilibrium with 233 U
and 239 Np resulting from interaction of thorium and uranium
with naturally produced neutrons. The total amount of astatine
present in the earth’s crust, however, is probably less than
1 oz. Astatine can be produced by bombarding bismuth with
energetic alpha particles to obtain the relatively long-lived
209–211 At, which can be distilled from the target by heating it in
air. Only about 0.05 µg of astatine has been prepared to date.
The “time of flight” mass spectrometer has been used to confirm
that this highly radioactive halogen behaves chemically
very much like other halogens, particularly iodine. The interhalogen
compounds AtI, AtBr, and AtCl are known to form,
but it is not yet known if astatine forms diatomic astatine molecules.
HAt and CH 3 At (methyl astatide) have been detected.
Astatine is said to be more metallic that iodine, and, like iodine,
it probably accumulates in the thyroid gland. Workers
at the Brookhaven National Laboratory have recently used
reactive scattering in crossed molecular beams to identify and
measure elementary reactions involving astatine.
Barium — (Gr. barys, heavy), Ba; at. wt. 137.327(7), at. no. 56; m.p.
727°C; b.p. 1897°C; sp. gr. 3.5 (20°C); valence 2. Baryta was
distinguished from lime by Scheele in 1774; the element was
discovered by Sir Humphrey Davy in 1808. It is found only in
combination with other elements, chiefly in barite or heavy
spar (sulfate) and witherite (carbonate) and is prepared by
electrolysis of the chloride. Large deposits of barite are found
in China, Germany, India, Morocco, and in the U.S. Barium
is a metallic element, soft, and when pure is silvery white like
lead; it belongs to the alkaline earth group, resembling calcium
chemically. The metal oxidizes very easily and should be
kept under petroleum or other suitable oxygen-free liquids to
exclude air. It is decomposed by water or alcohol. The metal is
used as a “getter” in vacuum tubes. The most important compounds
are the peroxide (BaO 2 ), chloride, sulfate, carbonate,
nitrate, and chlorate. Lithopone, a pigment containing barium
sulfate and zinc sulfide, has good covering power, and does
not darken in the presence of sulfides. The sulfate, as permanent
white or blanc fixe, is also used in paint, in X-ray diagnostic
work, and in glassmaking. Barite is extensively used as
a weighting agent in oilwell drilling fluids, and also in making
rubber. The carbonate has been used as a rat poison, while
the nitrate and chlorate give green colors in pyrotechny. The
impure sulfide phosphoresces after exposure to the light. The
compounds and the metal are not expensive. Barium metal
(99.2 + % pure) costs about $3/g. All barium compounds that
are water or acid soluble are poisonous. Naturally occurring
barium is a mixture of seven stable isotopes. Thirty six other
radioactive isotopes and isomers are known to exist.
Berkelium — (Berkeley, home of the University of California), Bk;
at. wt. (247); at. no. 97; m.p. 1050°C; valence 3 or 4; sp. gr.
14 (est.). Berkelium, the eighth member of the actinide transition
series, was discovered in December 1949 by Thompson,
Ghiorso, and Seaborg, and was the fifth transuranium element
synthesized. It was produced by cyclotron bombardment of
milligram amounts of 241 Am with helium ions at Berkeley,
California. The first isotope produced had a mass number of
243 and decayed with a half-life of 4.5 hours. Thirteen isotopes
are now known and have been synthesized. The existence
of 249 Bk, with a half-life of 320 days, makes it feasible to
isolate berkelium in weighable amounts so that its properties
can be investigated with macroscopic quantities. One of the
first visible amounts of a pure berkelium compound, berkelium
chloride, was produced in 1962. It weighed 3 billionth
of a gram. Berkelium probably has not yet been prepared in
elemental form, but it is expected to be a silvery metal, easily
soluble in dilute mineral acids, and readily oxidized by air
or oxygen at elevated temperatures to form the oxide. X-ray
diffraction methods have been used to identify the following
compounds: BkO 2 , BkO 3 , BkF 3 , BkCl, and BkOCl. As with
other actinide elements, berkelium tends to accumulate in the