Editing Electronegativity (section)

==Trends in electronegativity==

===Periodic trends===
[[File:Periodic variation of Pauling electronegativities.svg|thumb|upright=1.5|The variation of Pauling electronegativity (''y''-axis) as one descends the main groups of the periodic table from the second period to the sixth period]]
In general, electronegativity increases on passing from left to right along a period and decreases on descending a group. Hence, [[fluorine]] is the most electronegative of the elements (not counting [[noble gas]]es), whereas [[caesium]]<!-- not francium; please don't change unless you supply a citation for published experimental results --> is the least electronegative, at least of those elements for which substantial data is available.<ref name="Fr"/>

There are some exceptions to this general rule. [[Gallium]] and [[germanium]] have higher electronegativities than [[aluminium]] and [[silicon]], respectively, because of the [[d-block contraction]]. Elements of the [[Period 4 element|fourth period]] immediately after the first row of the transition metals have unusually small atomic radii because the 3d-electrons are not effective at shielding the increased nuclear charge, and smaller atomic size correlates with higher electronegativity (see [[#Allred–Rochow electronegativity|Allred-Rochow electronegativity]] and [[#Sanderson electronegativity equalization|Sanderson electronegativity]] above). The anomalously high electronegativity of [[lead]], in particular when compared to [[thallium]] and [[bismuth]], is an artifact of electronegativity varying with oxidation state: its electronegativity conforms better to trends if it is quoted for the +2 state with a Pauling value of 1.87 instead of the +4 state.

===Variation of electronegativity with oxidation number===
In inorganic chemistry, it is common to consider a single value of electronegativity to be valid for most "normal" situations. While this approach has the advantage of simplicity, it is clear that the electronegativity of an element is ''not'' an invariable atomic property and, in particular, increases with the [[oxidation state]] of the element.<ref>{{Cite journal |last1=Li |first1=Keyan |last2=Xue |first2=Dongfeng |date=2006-10-01 |title=Estimation of Electronegativity Values of Elements in Different Valence States |url=https://pubs.acs.org/doi/10.1021/jp062886k |journal=The Journal of Physical Chemistry A |language=en |volume=110 |issue=39 |pages=11332–11337 |doi=10.1021/jp062886k |pmid=17004743 |bibcode=2006JPCA..11011332L |issn=1089-5639}}</ref>

Allred used the Pauling method to calculate separate electronegativities for different oxidation states of the handful of elements (including tin and lead) for which sufficient data were available.<ref name="Allred"/> However, for most elements, there are not enough different covalent compounds for which bond dissociation energies are known to make this approach feasible.

{| class="wikitable" style="text-align:center"
|-
! Acid
! Formula
! Chlorine<br />oxidation<br />state
! p''K''<sub>a</sub>
|-
| [[Hypochlorous acid]]
| HClO
| +1
| +7.5
|-
| [[Chlorous acid]]
| HClO<sub>2</sub>
| +3
| +2.0
|-
| [[Chloric acid]]
| HClO<sub>3</sub>
| +5
| −1.0
|-
| [[Perchloric acid]]
| HClO<sub>4</sub>
| +7
| −10
|-
|}

The chemical effects of this increase in electronegativity can be seen both in the structures of oxides and halides and in the acidity of oxides and oxoacids. Hence [[Chromium trioxide|CrO<sub>3</sub>]] and [[Dimanganese heptoxide|Mn<sub>2</sub>O<sub>7</sub>]] are [[acidic oxide]]s with low [[melting point]]s, while [[Chromium(III) oxide|Cr<sub>2</sub>O<sub>3</sub>]] is [[amphoteric oxide|amphoteric]] and [[Manganese(III) oxide|Mn<sub>2</sub>O<sub>3</sub>]] is a completely [[basic oxide]].

The effect can also be clearly seen in the [[Acid dissociation constant|dissociation constants]] p''K''<sub>a</sub> of the [[oxoacid]]s of [[chlorine]]. The effect is much larger than could be explained by the negative charge being shared among a larger number of oxygen atoms, which would lead to a difference in p''K''<sub>a</sub> of log<sub>10</sub>({{frac|1|4}})&nbsp;= −0.6 between [[hypochlorous acid]] and [[perchloric acid]]. As the oxidation state of the central chlorine atom increases, more electron density is drawn from the oxygen atoms onto the chlorine, diminishing the partial negative charge of individual oxygen atoms. At the same time, the positive partial charge on the hydrogen increases with a higher oxidation state. This explains the observed increased acidity with an increasing oxidation state in the oxoacids of chlorine.

=== Electronegativity and hybridization scheme ===
The electronegativity of an atom changes depending on the hybridization of the orbital employed in bonding.  Electrons in s orbitals are held more tightly than electrons in p orbitals. Hence, a bond to an atom that employs an sp''<sup>x</sup>'' hybrid orbital for bonding will be more heavily polarized to that atom when the hybrid orbital has more s character.  That is, when electronegativities are compared for different hybridization schemes of a given element, the order {{math|χ(sp<sup>3</sup>) < χ(sp<sup>2</sup>) < χ(sp)}} holds (the trend should apply to [[Isovalent hybridization|non-integer hybridization indices]] as well). 
{| class="wikitable" style="text-align:center"
|-
! Hybridization
! {{math|χ}} (Pauling)<ref>{{Cite book |title=Molecular orbitals and organic chemical reactions |last=Fleming |first=Ian |date=2009 |publisher=Wiley |isbn=978-0-4707-4660-8 |edition=Student |location=Chichester, West Sussex, U.K. |oclc=424555669}}</ref>
|-
| C(sp<sup>3</sup>)
| 2.3
|-
| C(sp<sup>2</sup>)
| 2.6
|-
| C(sp)
| 3.1
|-
|'generic' C
| 2.5
|-
|}