Inert pair effect

The inert pair effect is the tendency of the electrons in the outermost atomic s orbital to remain unionized or unshared in compounds of post-transition metals. The term inert pair effect is often used in relation to the increasing stability of oxidation states that are two less than the group valency for the heavier elements of groups 13, 14, 15 and 16. The term "inert pair" was first proposed by Nevil Sidgwick in 1927. The name suggests that the s electrons are more tightly bound to the nucleus and therefore more difficult to ionize.

For example, the p-block elements of the 4th, 5th and 6th period come after d-block elements, but the electrons present in the intervening d- (and f-) orbitals do not effectively shield the s-electrons of the valence shell. As a result, the inert pair of ns electrons remains more tightly held by the nucleus and hence participates less in bonding.

As an example in group 13 the +1 oxidation state of Tl is the most stable and Tl^III compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence:

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 and are of lower 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 (IP) (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:

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