Physical properties of the group 13 elements

Table 1.1 lists selected physical properties of the group 13 elements. Despite the discussion of ionization energies that follows, there is no evidence for the formation of free M³⁺‏ ions in compounds of the group 13 elements under normal conditions, other than, perhaps, some trifluorides.  Electronic configurations and oxidation States While the elements have an outer electronic configuration ns²np¹ and a larger difference between IE₁ and IE₂ than between IE₂ and IE₃ (i.e. comparing the removal of a p with that of an s electron), the relationships between the electronic structures of the group 13 elements and those of the preceding noble gases are more complex than for the group 1 and 2 elements discussed in Chapters 10 and 11. For Ga

and In, the electronic structures of the species formed after the removal of three valence electrons are [Ar]3d¹⁰ and [Kr]4d¹⁰ respectively, while for Tl, the corresponding species has the configuration [Xe]4f ¹⁴5d¹⁰. Thus, whereas for B and Al, the value of IE₄ (Table 1.1) refers to the removal of an electron from a noble gas configuration, this is not the case for the three later elements; the difference between IE₃ and IE₄ is not nearly so large for Ga, In and Tl as for B and Al. On going down group 13, the observed discontinuities in values of IE₂ and IE₃, and the differences between them (Table 1.1), originate in the failure of the d and f electrons (which have a low screening power, see Section 1.7) to compensate for the increase in nuclear charge. This failure is also reflected in the relatively small difference between values of rion for Al³⁺‏ and Ga³⁺‏. For Tl, relativistic effects are also involved.

On descending group 13, the trend in IE₂ and IE₃ shows increases at Ga and Tl (Table 1.1), and this leads to a marked increase in stability of the ‏+1 oxidation state for these elements. In the case of Tl (the only salt-like trihalide of which is TlF₃), this is termed the thermodynamic 6s inert pair effect . Similar effects are seen for Pb (group 14) and Bi (group 15), for which the most stable oxidation states are  +‏2 and +‏3 respectively, rather than ‏4 and 5. The inclusion in Table 1.1 of E^o values for the M³⁺‏/M and M‏/M redox couples for the later group 13 elements reflects the variable accessibility of the M‏ state within the group.

   Although an oxidation state of ‏+3 (and for Ga and Tl, ‏1) is characteristic of a group 13 element, most of the group 13 elements also form compounds in which a formal oxidation state of +‏2 is suggested, e.g. B₂Cl₄ and GaCl₂. However, caution is needed. In B₂Cl₄, the ‏+2 oxidation state arises because of the presence of a B^_B bond, and GaCl₂ is the mixed oxidation state species Ga[GaCl₄].