The elements of Group III

Of the five Group III elements, only boron and aluminium are reasonably familiar elements. Aluminium is in fact the most abundant metal, the third most abundant element in nature, but the other elements are rare and boron is the only one so far found in concentrated deposits. 

The data in Table 7.1 show that, as expected, density, ionic radius, and atomic radius increase with increasing atomic number. However, we should also note the marked differences in m.p. and liquid range of boron compared with the other Group III elements here we have the first indication of the very large difference in properties between boron and the other elements in the group. Boron is in fact a non-metal, whilst the remaining elements are metals with closely related properties. 

The tendency of elements of higher atomic number to retain the s electrons as an inert pair is also encountered in Group IV, and in this case it is found that for lead the most stable oxidation state is + 2, achieved by loss of two p electrons. 

Boron achieves a covalency of three by sharing its three outer electrons, for example BF3 (p. 153). By accepting an electron pair from a donor molecule or ion, boron can achieve a noble gas configuration whilst increasing its covalency to four, for example H3N- BC13. K + BF4. This is the maximum for boron and the second quantum level is now complete these 4-coordinate species are tetrahedral (p. 38). 

Aluminium also has a strong tendency to achieve a noble gas configuration by electron pair acceptance as shown in dimeric aluminium chloride, 

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All the elements discussed so far, except hydrogen, have been metals. Group III contains one element, boron, that is not a metal. The elements of Group III are ... 

Hydrogen and the ammonium salts are dealt with in Volume II, along with the Elements of Group I. The position of the rare earth metals in the Periodic Classification has for many years been a source of difficulty. They have all been included in Volume IV, along with the Elements of Group III, as this was found to be the most suitable place for them. 

The problems associated with the M11 state of the elements of group III have been noted previously cf. Sections 25.1.13 and 25.2,3). The Tl2+ ion has been identified in pulse radiolysis decomposition of aqueous TI2SO4,350,351 and in y-irradiated frozen aqueous solutions.352... 

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