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

8 Main Group Elements

formulation: amine: BHRX (X = BH 2 CN, ·BH 2 CO 2 H, These irregularities in the periodic trend in atomic prop-
·BH 2 CONHCH 2 CH 3 ). Boron analogues have shown sig- erties are reflected in similar irregularities in chemical
nificant antiarthritic, antitumor, antiinflammatory, hy- properties. For example, Ga 2 O 3 is more acidic than Al 2 O 3 .
polipidemic, and hypocholesteremic activity in mice. Not The Lewis acidities of aluminum and gallium halides are
only are these compounds biologically active, but if they comparable, and the sequence of strengths depends on
can be selectively incorporated into a tumor, irradiation the specific Lewis base considered; nitrogen and oxy-
with thermal neutrons could lead to cellular destruction gen bases generally prefer aluminum, whereas sulfur and
in vivo. Interesting developments in boron neutron cap- phosphorus bases prefer gallium. Indium compounds are
ture theory (BNCT) include the synthesis of backbone- only slightly amphoteric, and the oxides of thallium are
boronated oligonucleotides and base-boronated nucleic clearly basic. Indium and, especially, thallium show a
acids. Another major advance is the encapsulation of cer- significant tendency to form univalent compounds (e.g.,
tain boron compounds (which by themselves do not neces- TlOH); Tl(III) salts are good oxidants in aqueous solu-
+ −
sarily exhibit tumor affinity) in phospholipid vesicles that tions. In fact, TlI 3 is best formulated as Tl I .
3
are capable of selective localization within cancer cells.
A. Aluminum
II. ALUMINUM, GALLIUM, INDIUM, Aluminum is the most abundant metal in the earth’s crust
AND THALLIUM
and has assumed a major technological role since the
development of the Hall–Herault metallurgical process.
This family of elements, with boron, constitutes one of
Among its compounds, the various aluminum oxides,
the least regularly varying families in the periodic table.
lithium aluminum hydride, and aluminum chloride are the
It is noteworthy, however, that the trends from boron and
most important, though the fluoride is important in the
aluminum through the Group III elements (scandium, yt- metallurgical process itself and organoaluminum nitrides
trium, and lanthanum) are quite regular. This points out are of current structural interest.
the importance of the underlying core of electrons to the
chemical properties of atoms, noble gas in this case, while B. Aluminum Chloride
10
gallium, indium, and thallium have a d core. In contrast
to boron, each of these elements is metallic in its physical Aluminum chloride is noteworthy as a Lewis acid and is
properties. The first two members are notoriously ampho- extensively used as such in the Friedel–Crafts alkylation
teric chemically; that is, they and their oxides are solu- and acylation of aromatic hydrocarbons. This property is
ble in either strong acid or strong base. Rather than the evidenced in the structure of the compound. While the
smooth trend in ionization energies decreasing from alu- crystal consists of an “ionic” lattice, with each aluminum
minum through thallium, which is observed for aluminum ion surrounded by six chloride ions in an octahedral ar-
through lanthanum, the values for gallium are comparable ray, the compound becomes a dimeric molecule in the
to those for aluminum followed by a substantial decrease nonconducting liquid phase and in the gas phase. This
at indium and an increase at thallium such that the first molecule, Al 2 Cl 6 , consists of tetrahedrally coordinated
ionization energy is greater than that of aluminum. (This aluminum with two chlorine atoms bridging the two alu-
parallelsthetrendforzinc,cadmium,andmercury.)Metal- minum atoms, effectively a Lewis acid–base adduct of one
lic and ionic radii vary in a similarly irregular way, with monomer to another.
gallium quite comparable to aluminum, and indium and
thallium quite similar to one another but different from C. Aluminum Oxides
aluminum and gallium. The trend in electronegativity is
Aluminum oxides and related compounds have long been
even more remarkable and shows an increase from alu-
technologically important as abrasives (corundum) and in
minum to thallium rather than the expected decrease.
refractories and ceramics in the α-crystalline modifica-
tion. In the γ modification, a more open, defect structure,
B Al Ga In Tl
aluminum oxide becomes “activated alumina” and is use-
ful in chromatography and in catalysis. A third modifica-
Ionization energy I 800.5 577.4 578.6 558.2 589.1
(kJ/mol) II 2426.5 1816.1 1978.8 1820.2 1970.5 tion occurs on the surface of the metal on exposure to air
and serves as the well-known protective oxide. A more
III 3658.7 2744.1 2962.3 2704.0 2877.4
recent technological achievement is the production of re-
Ionic radius I — — 120 140 150
(pm) III — 53.5 62.0 80.0 88.5 markably uniform cylindrical fibers of Al 2 O 3 . These fibers
can be incorporated in a variety of fabrics, papers, ropes,
Electronegativity — 2.0 1.5 1.6 1.7 1.8
(Pauling) and so on, which gain the advantage of stability to very