THE ELEMENTS OF GROUP II! 139
OXIDATION STATE +3Summation of the first three ionisation energies of any Group III
element indicates that the formation of an E^3 + (g) ion is difficult. In
the case of boron the energy required is so large that under normal
circumstances B^3 +(g). (s) or (aq) is never formed. The energy required
is slightly less for aluminium but the simple ion Al3+(s) is found
only in anhydrous aluminium fluoride and chlorate(VII). and even
here there may be partial covalent bonding. Oxidation state +3
compounds of other Group III elements are largely covalent.
With the one exception of boron, all Group III elements form 4- 3
ions in aqueous solution; these ions exist only as complexes, often
with water, for example [A1(H 2 O) 6 ]^3 ^. and are usually extensively
hydrolysei p. 45. The large hydration energy which helps to stabilise
the ion is a major factor contributing to the low standard electrode
potential of aluminium which, in view of the energy required to
form Al^3 + (g). is rather unexpected Since hydration energy decreases
with increasing ionic size we can correctly predict that the standard
electrode potential will decrease with increasing atomic number of
the element. In the case of boron, however, the very small B3+ (g) ion
is unable to coordinate a sufficient number of water molecules to
compensate for the high ionisation energy; it can be stabilised by
tetra-coordination of certain ligands to form the boromum cation,
for example""•V"'
/ \
H 3 N HOXIDATION STATE +1
The outer electronic configuration of the Group III elements is
ns^2 npj and as we have seen on p. 32 the energy required to remove
the first p electron from a given quantum level is less than that
needed to remove one of a pair of s electrons occupying the same
quantum level. This would indicate the possible existence of a + 1
oxidation state when only the p electron was removed. However, as
was seen in Chapter 4 several factors are involved in the stabilisation
of any oxidation state. It is found, in this case, that the stability of
the 4- 1 oxidation state increases regularly with increasing atomic
number from aluminium to thallium, being (so far) unknown for
boron but being generally the most stable oxidation state for