Editing Electronegativity (section)

===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.