The inert-pair effect is the tendency of the two electrons in the outermost atomic s-orbital to remain 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 outermost s electron pairs are more tightly bound to the nucleus in these atoms, and therefore more difficult to ionize or share. 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 bond formation. Description Consider as an example thallium (Tl) in group 13. The +1 oxidation state of Tl is the most stable, while Tl3+ compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence: Al+ < Ga+ < In+ < Tl+. The same trend in stability is noted in groups 14, 15 and 16. The heaviest members of each group, i.e. 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 two valence electrons in s orbitals. A partial explanation is 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. If the total ionization energies (IE) (see below) of the two electrons in s orbitals (the 2nd + 3rd ionization energies) are examined, it can be seen that there is an expected decrease from B to Al associated with increased atomic size, but the values for Ga, In and Tl are higher than expected. The high ionization energy (IE) (2nd + 3rd) of gallium is explained by d-block contraction, and the higher IE (2nd + 3rd) of thallium relative to indium, has been explained by relativistic effects. The higher value for thallium compared to indium is partly attributable to the influence of the lanthanide contraction and the ensuing poor shielding