Description
As an example in group 13 the +1 oxidation state of Tl is the most stable and TlIII compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence:
- AlI < GaI < InI < TlI.
The same trend in stability is noted in groups 14, 15 and 16. As such the heaviest members of the groups, e.g. lead, bismuth and polonium are comparatively stable in oxidation states +2, +3, and +4 respectively.
The lower oxidation state in each of the elements in question has 2 valence electrons in s orbitals. On the face of it a simple explanation could be that the valence electrons in an s orbital are more tightly bound are of higher energy than electrons in p orbitals and therefore less likely to be involved in bonding. Unfortunately this explanation does not stand up. If the total ionization potentials (see below) of the 2 electrons in s orbitals (the 2nd + 3rd ionization potentials), are examined it can be seen that they increase in the sequence:
- In < Al < Tl < Ga.
| IP | Boron | Aluminium | Gallium | Indium | Thallium |
|---|---|---|---|---|---|
| 1st | 800.6 | 577.5 | 578.8 | 558.3 | 589.4 |
| 2nd | 2427.1 | 1816.7 | 1979.3 | 1820.6 | 1971 |
| 3rd | 3659.7 | 2744.8 | 2963 | 2704 | 2878 |
| (2nd + 3rd) | 6086.8 | 4561.5 | 4942.3 | 4524.6 | 4849 |
The high IP (2nd + 3rd) of gallium is explained by d-block contraction, and the higher IP (2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects.
An important consideration is that the compounds in the lower oxidation state are ionic, whereas in the higher oxidation state they tend to be covalent. Therefore covalency effects must also be taken into account. In fact an alternative explanation of the inert pair effect by Drago in 1958 attributed the effect to low M-X bond enthalpies for the heavy p-block elements and the fact that it requires less energy to oxidize an element to a low oxidation state than to a higher oxidation state. This energy has to be supplied by ionic or covalent bonds, so if bonding to a particular element is weak, the high oxidation state may be inaccessible. Further work involving relativistic effects confirms this. In view of this it has been suggested that the term inert pair effect should be viewed as a description rather than as an explanation.
Read more about this topic: Inert Pair Effect
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