Survey of the group 13 elements

aluminum, gallium, indium, and thallium are members of group 13 of the Periodic Table. Some important properties of these elements are given in Table 13.1.1. 

Electronic configuration [He]2s22p1 [Ne]3s23p1 [Ar]3d104s24p1 [Kr]4d105s25p1 [Xe]4f 145d106s26p1 

The +3 oxidation state is characteristic of group 13 elements. However, as in the later elements of this groups, the trend in h and I3 shows increases at Ga and Tl, leading to a marked increase in stability of their +1 oxidation state. In the case of Tl, this is termed the 6s inert-pair effect. Similar effects are observed 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. 

Because of the 6s inert-pair effect, many T1+ compounds are found to be more stable than the corresponding Tl3+ compounds. The progressive weakening of M3+-X bonds from B to Tl partly accounts for this. Furthermore, the relativistic effect (as described in Section 2.4.3) is another factor which contributes to the inert-pair effect. 

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