CH226.12
•The Boron group
Elements of the Boron group (Group III or 13)
B, boron (the name is derived from the Arab buraq, which was the name for the
material borax, Na2[B4O5(OH)4]·nH2O)
Al, aluminum (the name is derived from the ancient name for alum - potassium
aluminum sulfate, which was alumen, meaning bitter salt in Latin))
Ga, gallium (the name is derived from Gallia, the Latin name for France. It was also
suggested that the French discoverer of Ga, Paul-Émile Lecoq de
Boisbaudran named it after himself. ‘Le coq‘ means ‘rooster’ in French, and
Latin for rooster is "gallus”. Boisbaudran denied this.
In, indium (the element comes from the Latin indicum, meaning violet or indigo, which
refers to the brightest line in its atomic spectrum)
Tl, thallium (the name is derived from the Greek thallos, meaning a green twig.
Apparently, the element was named after the beautiful green spectral line,
which identified the element in the atomic spectrum)
Members of Group III have electronic configuration ns2np1.
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Boron
Boron is the only non-metal in Group III. Also, boron is
the least electronegative non-metal. Amorphous boron
is a dark solid, unreactive to oxygen, water, acids and
alkalis. The element is not found free in nature. Boron is
a poor conductor of electricity at 20º.
Pure boron is little used. The most important compounds are borax (sodium borate,
Na2[B4O5(OH)4]·nH2O), boric oxide (B2O3) and boric acid B(OH)3. Boron compounds
are extensively used in the manufacture of borosilicate glasses. Pyrex glass, which
contains 12-15% of boric oxide, is tough and heat-resistant. Thousands of tones of
borate are added to fertilizers because boron is vital to plants, especially trees. The
isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear
radiation. Boron nitride has remarkable properties and can be used to make a material
as hard as diamond. Amorphous boron is used in pyrotechnic flares to provide a
distinctive green color. Boric acid is insecticidal, especially for ants and cockroaches.
Elemental boron and the borates are not considered to be toxic, and they do not
require special care in handling. However, some of the more exotic boron hydrogen
compounds are definitely toxic and do require care.
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Aluminum
Pure aluminum, a silvery-white metal, possesses many
desirable characteristics. It is light, nonmagnetic,
malleable and ductile. Aluminum is the most abundant
metal in the Earth's crust, but it is never found free in
nature.
Industrial production of Al is around 20 million tones a year. Al is extensively used for
kitchen utensils, outside building decoration, and in thousands of industrial
applications where a strong, light, easily constructed material is needed. Aluminum
alloys are of vital importance in the construction of modern aircraft and rockets. Al is
used for cables. Compounds of importance are aluminum oxide (Al2O3), the sulfate,
and the soluble sulfate with potassium (alum). The oxide, alumina, occurs naturally as
ruby, sapphire, and corundum, and is used in glassmaking. Synthetic ruby and
sapphire are used in lasers for producing coherent light.
Cut ruby
Sapphire
No living species has been discovered to need aluminum as an essential element. 4
Gallium
Gallium is a silvery-white metal that is soft enough to be
cut with a knife. It is stable in air and water. Ga will melt
on a hot summer day or when held in the hand (mp 30
°C). There is a strong tendency for gallium to super-cool
below its freezing point. The metal expands 3.1% on
solidifying; therefore, it should not be stored in glass or
metal containers, because they may break as the metal
solidifies.
Gallium is not found concentrated into minerals; it is widely dispersed. Only 30 tones
of Ga is produced as by-product of zinc and copper refining. Gallium arsenide, GaAs,
has semiconductor properties, and the crystals are used for electronic devices such as
transistors and light emitting diodes. The Gallium solar Neutrino Observatory (GNO) in
Italy has a tank filled with 100 tons of a GaCl3 solution in water and HCl, containing
30.3 tons of gallium.
Gallium has no biological role but is known to stimulate metabolism. Its salts have low
toxicity and present few health risks.
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Indium
Indium is a soft, silvery metal. It is stable in air and in water.
The pure metal gives a high-pitched "cry" when bent (I have
been unable to prove this observation)
World production of In is around 75 tones per year and the metal comes mainly from
Canada (ca. 31 tones). The application of indium-tin-oxide thin films for liquid crystal
displays (LCD) is the largest end use. The amount of indium consumed is largely a
function of worldwide LCD production. Indium finds use in low melting alloys such as
those used in fire-sprinkler systems in shops and warehouses. Indium metal will ‘glue’
itself to glass, and when evaporated and allowed to deposit on glass, it produces a
mirror. Some indium compounds such as indium antimonide (InSb), indium phosphide
(InP), and indium nitride (InN) are semiconductors with useful properties.
Pure indium is considered non-toxic by most sources. This may not be the case with
indium compounds: there is some unconfirmed evidence that suggests that indium
compounds have a low level of toxicity.
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Thallium
This metal is very soft and can be cut with a knife. Thallium
has a metallic luster but quickly tarnishes in air with a bluishgray tinge that resembles lead. A heavy layer of oxide builds
up on thallium if the metal is stored in air.
The odorless and tasteless thallium sulfate was widely used in the past as a rat poison
and ant killer. In the United States and many other countries this use is no longer
allowed due to safety concerns.
Thallium and its compounds are highly toxic and should be handled with great care.
The toxicity derives from the ability of Tl+ to replace important alkali metal cations such
as Na+ and K+ in the body. This substitution disrupts many cellular processes. Amongst
the distinctive effects of thallium poisoning are loss of hair, and damage to peripheral
nerves. Contact with skin is dangerous. Thallium was once an effective murder
weapon. The CIA is believed (by its Inspector General) to have conceived (but not
executed) a scheme to poison Fidel Castro by exposure to thallium salts (placed in his
shoes while they were being polished). The goal was to discredit him by causing him
to lose his characteristic hair and beard. The scheme progressed as far as testing on
animals, but the trip during which the poison was to be administered fell through.
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Properties of the elements from Group III/13
Some physical properties of the Group III elements are listed in the following Table.
The relative stability of +1 vs. +3 oxidation state increases down the group.
Tl+ is more stable than Tl3+ (the “inert pair” effect), for example Thallium forms Tl2O
while aluminum in the same group forms Al2O3.
Atomic number, Z
Covalent radius, rcov / Å
Melting point, mp / K
First ionization energy
Standard reduction
potential, E° (M3+/M)
Standard reduction
potential, E° (M+/M)
B
Al
Ga
In
Tl
1.44
1.48
5
13
31
49
2453
933
303
430
-1.66
0.82
191
1.18
138
1.26
138
81
576.5
-0.55
133
-0.34
141
-0.2
-0.14
-0.34
+0.72
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