Editing Inert-pair effect (section)

{{Short description|The reluctance of 'ns' pair of electrons to take part in bond formation}}
The '''inert-pair effect''' is the tendency of the two electrons in the outermost [[Atomic orbital|atomic ''s''-orbital]] to remain  unshared in compounds of [[Post-transition metal|post-transition metals]]. The term ''inert-pair effect'' is often used in relation to the increasing stability of [[oxidation state]]s that are two less than the group valency for the heavier elements of groups [[Group 13 element|13]], [[Group 14 element|14]], [[Group 15 element|15]] and [[Group 16 element|16]]. The term "inert pair" was first proposed by [[Nevil Sidgwick]] in 1927.<ref>{{cite book | last = Sidgwick | first = Nevil Vincent | author-link = Nevil Sidgwick | title = The Electronic Theory of Valency | url = https://archive.org/details/electronictheory004236mbp | year = 1927 | location = Oxford | publisher = Clarendon | pages = [https://archive.org/details/electronictheory004236mbp/page/n191 178]–181 |quote=The Inert Pair of Electrons ..... under some conditions the first two valency electrons of an element could become more like core electrons, and refuse either to ionize, or to form covalencies, or both.}}</ref> 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 ''n''s electrons remains more tightly held by the nucleus and hence participates less in bond formation.