Editing Inert-pair effect (section)

==Description==
Consider as an example thallium (Tl) in [[Group 13 element|group 13]]. The +1 oxidation state of Tl is the most stable, while Tl<sup>3+</sup> compounds are comparatively rare. The stability of the +1 oxidation state increases in the following sequence:<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref>
:Al<sup>+</sup> < Ga<sup>+</sup> < In<sup>+</sup> < Tl<sup>+</sup>.

The same trend in stability is noted in groups [[Group 14 element|14]], [[Group 15 element|15]] and [[Group 16 element|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.<ref>[http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Atomic_and_Molecular_Properties/Electronegativity Electronegativity] UC Davis ChemWiki by University of California, Davis</ref> If the total [[ionization energy|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. 

{| class="wikitable" style="text-align:right"
|+ Ionization energies for group 13 elements<br /> kJ/mol
! IE !! [[Boron]] !! [[Aluminium]] !! [[Gallium]] !! [[Indium]] !! [[Thallium]]
|-
! 1st
|  800 ||  577 ||  578 ||  558 ||  589
|-
! 2nd
| 2427 || 1816 || 1979 || 1820 || 1971
|-
! 3rd
| 3659 || 2744 || 2963 || 2704 || 2878
|-
! 2nd + 3rd
| 6086 || 4560 || 4942 || 4524 || 4849
|} 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 quantum chemistry|relativistic effects]].<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}}.</ref> The higher value for thallium compared to indium is partly attributable to the influence of the lanthanide contraction and the ensuing poor shielding from the nuclear charge by the intervening filled 4d and 5f subshells.<ref>{{cite journal |last1=Rodgers |first1=G. |last2=E. |title=A visually attractive "Interconnected network of ideas" for organizing the teaching and learning of descriptive inorganic chemistry |journal=Journal of Chemical Education |date=2014 |volume=91 |issue=2 |pages=216−224 (219) |doi=10.1021/ed3003258 |bibcode=2014JChEd..91..216R }}</ref>

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 be taken into account. An alternative explanation of the inert pair effect by [[Russell S. Drago|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.<ref name=drago>{{cite journal
| title =  Thermodynamic Evaluation of the Inert Pair Effect
| author = Russell S. Drago
| journal = J. Phys. Chem.
| year = 1958
| volume = 62
| issue =  3
| pages = 353–357
| doi =  10.1021/j150561a027
}}</ref> 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.<ref>{{cite journal
| title = Low valencies and periodic trends in heavy element chemistry. A theoretical study of relativistic effects and electron correlation effects in Group 13 and Period 6 hydrides and halides
|author=Schwerdtfeger P. |author2=Heath G. A. |author3=Dolg M. |author4=Bennet M. A.
| journal =Journal of the American Chemical Society
| year = 1992
| volume = 114
| issue = 19
| pages = 7518–7527
| doi =  10.1021/ja00045a027
}}</ref>

In the case of groups 13 to 15 the inert-pair effect has been further attributed to "the decrease in bond energy with the increase in size from Al to Tl so that the energy required to involve the s electron in bonding is not compensated by the energy released in forming the two additional bonds".<ref name = "Greenwood"/> That said, the authors note that several factors are at play, including relativistic effects in the case of gold, and that "a quantitative rationalisation of all the data has not been achieved".<ref name = "Greenwood"/>