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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. ==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 and are of lower energy than electrons in p orbitals and therefore less likely to be involved in bonding.〔(Electronegativity ) UC Davis ChemWiki by University of California, Davis〕 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: : In < Al < Tl < Ga. The high ionization potential (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.〔Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.〕 An important consideration is that compounds in the lower oxidation state are ionic, whereas the compounds in the higher oxidation state 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「inert pair effect」の詳細全文を読む スポンサード リンク
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