Page 201 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

10 Main Group Elements

differences in properties. As mentioned above, Ga 2 O 3 is gas in the atmosphere. Surprisingly, carbon is not plentiful
compared with many elements; it ranks only seventeenth
more acidic than Al 2 O 3 . The β-crystalline form of Ga 2 O 3
is the more stable, in contrast to Al 2 O 3 . The Lewis acidity in order of abundance.
3+
of Ga 3+ is similar to that of Fe , a consequence of the Although six allotropic forms of pure carbon exist,
similar size and the same charge. Gallium (III) has been only two are considered: diamond and hexagonal (or α)
found useful, therefore, in studies of the role of iron in bio- graphite. These two crystalline forms are drastically dif-
chemistrywhenadiamagneticanalogueoftheFe(III)isre- ferent. The diamond lattice is a three-dimensional struc-
quired. GaH 3 is a viscous liquid of even more limited ther- ture in which each carbon atom is attached by covalent
mal stability than AlH 3 , decomposing to the elements at bonds to four neighbors tetrahedrally arranged around it
˚
room temperature. LiGaH 4 can be prepared from LiH and at a distance of 1.54 A. Graphite is composed of planar
GaCl 3 (LiAlH 4 can be prepared analogously) but slowly hexagonal rings in which each carbon atom is attached
˚
decomposes at room temperature and is a much weaker to three neighbors at a distance of 1.42 A. The planes in
˚
reducing agent than is LiAlH 4 . In contrast to aluminum, graphite lie at a distance of 3.35 A, and since these planes
however, gallium does exhibit some variation in oxidation are not held by chemical bonds, they readily slide past one
number, several Ga(I) species having been characterized another and give the substance a greasy feel.
in crystals and solution as well as in the gas phase. The properties of diamond and α-graphite are compared
in the following tabulation:
F. Indium and Thallium
Property Diamond α-Graphite
Both indium and thallium are rare metals, and only indium
has achieved a significant role in technology. Indium finds Hardness 10 1–2
use in low-melting alloys and solders and has a place in (Mohs scale)
semiconductor technology, like gallium, in Group 13–15 Color Transparent Black, opaque
3
(IIIA–VA) semiconductors and as a dopant for germanium Density (g/cm ) 3.51 2.27
p–n junction devices. Thallium is so far of mainly aca- Electrical Negligible Good in the direction
demic interest, though there is potential in that TlBr and conductivity along planes
◦
TlI are transparent to infrared radiation. Like its neighbor, H (kJ/mol) 1.90 0.00 (standard state)
f
mercury, the metal and its compounds are extremely toxic. Chemical Relatively insert, Relatively reactive, burns at
◦
reactivity burns at ∼800 C 400 C; undergoes attack by
◦
Chemically, neither indium nor thallium has been as
oxidizing agents and oxo
thoroughly explored as aluminum and gallium. Notewor- acids at room temperature
thy is the variable valence exhibited by both. Indium and,
especially, thallium are observed in the I and III oxida- At room temperature graphite is more stable than dia-
tion states. In fact, the preferred oxidation state for thal- mond; however, at high pressures and temperatures, gra-
lium is I. Indium and gallium are the only members of the phite can be transformed into diamond if molten chro-
2−
family yet observed in the II oxidation state, in In 2 X ,
6 mium, iron, or nickel is present as a catalyst. Although
which have an ethane-like structure with an In In bond, these synthetic diamonds are not gem quality, they are
and in the corresponding gallium anions. Even here, the useful in drills and saw blades.
atoms are trivalent. Other apparently divalent compounds Less glamorous than the synthesis of diamonds but far
consist of mixtures of I and III species, for example, more important is the industrial manufacture of three im-
−
Tl [TlCl ] = “TlCl 2 .”
+
4 pure graphite forms of carbon: carbon black (soot), coke,
and activated charcoal.
Carbon black, which consistently ranks about thirty-
III. CARBON
fifth among the top 50 chemicals produced in the United
States, is manufactured by partially oxidizing the residual
A. Allotropes
hydrocarbons from petroleum refining; coke is made by
Only two nonmetallic elements, carbon and sulfur, were heating coal in the absence of air to remove volatile com-
known to the ancients; however, carbon was not recog- ponents; and activated charcoal is made by heating saw-
nized as an element until the eighteenth century, by which dust or peat in the presence of certain metal salts. More
time diamond and graphite were known to be different than 90% of the 1.14 billion kg of carbon black produced
forms of the same element. annually is used by the rubber industry, mostly for rein-
Carbon exists free in nature as graphite and diamond; it forcing the rubber in tires. The demand for carbon black
occurs in combined form in carbonate rocks (chalk, lime- is easily understood when it is pointed out that the 4 tires
stone, marble, calcite, and dolomite) and in carbon dioxide on an automobile require more than 15 kg of carbon black