Page 202 - 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

Main Group Elements 11

and the 18 tires on a truck require more than 160 kg of be regarded as synthetic or unnatural allotropes of carbon,
carbon black. will be systematically named the cyclo(n) carbons (where
Coke is used as a reducing agent in the pyrometallurgy n = the number of carbon atoms present). Evidence indi-
of iron. The stoichiometric process, but not the mechanis- catesthattheringsconsistofcarbonatomswithalternating
tic process, is represented by the following equation: C C and C C bonds. A general structure is shown below.
2Fe 2 O 3 + 3C (coke) → 4Fe + 3CO 2
Activated charcoal is used to decolorize sugar, to pu-
rify air and other gases, and to treat water. One gram of
charcoal provides an adsorption surface of roughly 300 to
2
2000 m —about half the area of a football field.
B. The Buckminsterfullerene

In 1985, researchers at Rice University vaporized graphite
with lasers and produced what might be regarded as a D. Carbonates and Carbonic Acid
new modification of carbon: a surprisingly stable, highly Carbon dioxide in an aqueous solution can be described
symmetric molecule containing 60 carbon atoms. The by the following equilibrium:
molecule is envisioned as an aromatic spheroidal cluster

+
CO 2 + H 2 O H 2 CO 3 H + HCO −

with 60 vertices and 32 faces, 20 of which are hexagons 3
Carbonic Bicarbonate
and 12 of which are pentagons. The proposed structure, acid ion
formally described as a truncated icosahedron, has the 2H + CO 2−
+

3
geometry of the surface of an ordinary soccer ball. In- Carbonate
ion
deed, soccerballene would be an apt name for the C 60
species, but the discoverers settled on the whimsical name Less than 1% of the dissolved carbon dioxide exists as
buckminsterfullerene (also known as buckyball) to honor carbonic acid or its dissociation products. Carbonic acid,
R. Buckminster Fuller, whose pioneering efforts with a weak acid with a pK of ∼6.38, serves as an important
polygonal structures led to the development of geodesic buffer in blood.
domes. The basic structural feature of all geodesic domes, All the C O bonds in the carbonate ion are equal in
however, is actually the triangle. length, but the three resonance structures of this flat trian-
˚
The C 60 cluster has a diameter of ∼7 A and an inner gular species show one double and two single bonds:
˚
cavity of ∼5 A. Vaporization of graphite impregnated with
O 2 O 2 O 2
lanthanum results in a species LaC 60 , which is assumed
to be a buckyball sphere inside which the lanthanum atom C C C
O O O O O O
is trapped. Other clusters containing an even number of
carbon atoms ranging from 44 to 76 also encapsulate lan-
More than 70 minerals containing the carbonate ion are
thanum, but the LaC 60 complex predominates.
recognized, and some of these are fairly abundant in na-
It has been speculated that buckyballs might occur in
outer space around carbon-rich stars, and, rather surpris- ture. Important natural carbonates include calcite, CaCO 3
(calcium carbonate); siderite, FeCO 3 (ferrous carbon-
ingly, they may even exist at the cores of mundane soot
ate); magnesite, MgCO 3 (magnesium carbonate); smith-
particles, which tend to be spherical. In recent years,
fullerenes of C 70 and C 50 have been proposed as stable sonite, ZnCO 3 (zinc carbonate); dolomite, CaMg(CO 3 ) 2
(calcium magnesium carbonate); witherite, BaCO 3 (bar-
even clusters of carbon atoms.
ium carbonate); cerussite, PbCO 3 (lead carbonate); mala-
chite, Cu 2 CO 3 (OH) 2 (basic copper carbonate); natron,
C. All-Carbon Cyclic Molecules: Na 2 CO 3 · 10H 2 O (hydrous sodium carbonate); and trona,
The Cyclo(n) Carbons
Na 3 H(CO 3 ) 2 · 2H 2 O (hydrous sodium acid carbonate).
Elegant chemical syntheses at UCLA have resulted in the Smithsonite, witherite, cerussite, and malachite are useful
unambiguous structural characterization of monocyclic ores and serve as sources of the metals present in each.
all-carbon molecules. Cyclo(18) carbon and other species Carbonates containing relatively small and highly
have been prepared and characterized. Neutral cycles sat- charged metal ions can be decomposed by heating. For
isfying the general formula 4n + 2 atoms are predicted to example, calcium carbonate can decompose as follows:
be closed-shell aromatic molecules with special stability.
CaCO 3 (s) −→ CaO (s) + CO 2 (g)
Thisnewclassofmonocyclicmolecules,whichmightwell heat