Editing Isoelectronicity

{{Short description|Identical electron configuration}}
{{distinguish|text = [[Isoelectric (disambiguation)|Isoelectric]]}}
{{multiple image
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|image1 = Carbon monoxide simple.svg
|width1 = 150
|caption1 = Carbon monoxide molecule
|image2 = Nitrosonium-2D-dimensions.png
|width2 = 150
|caption2 = Nitrosonium ion
|footer = Carbon monoxide and nitrosonium are isoelectronic
}}

'''Isoelectronicity''' is a [[phenomenon]] observed when two or more [[molecule]]s have the same [[chemical structure|structure]] (positions and connectivities among [[atom]]s) and the same [[electron configuration|electronic configuration]]s, but differ by what specific [[chemical element|elements]] are at certain locations in the structure. For example, {{chem|CO|link=carbon monoxide}}, {{chem|NO|+|link=nitrosonium}}, and {{chem|N|2|link=Nitrogen}} are isoelectronic, while {{chem|CH|3|CO|CH|3|link=acetone}} and {{chem|CH|3|N}}={{chem|N|CH|3|}} are not.<ref>{{GoldBookRef|title=isoelectronic|file=I03276}}</ref>  

This definition is sometimes termed '''valence isoelectronicity'''.  Definitions can sometimes be not as strict, sometimes requiring identity of the total [[electron]] count and with it the entire [[electron configuration|electronic configuration]].<ref>[http://www.iun.edu/~cpanhd/C101webnotes/chemical-bond/isoelectronic.html Isoelectronic Configurations] {{Webarchive|url=https://web.archive.org/web/20170717073429/http://www.iun.edu/~cpanhd/C101webnotes/chemical-bond/isoelectronic.html |date=2017-07-17 }} ''iun.edu''</ref>  More usually, definitions are broader, and may extend to allowing different numbers of atoms in the [[chemical species|species]] being compared.<ref>A. A. Aradi & T. P. Fehlner, "Isoelectronic Organometallic Molecules", in F. G. A. Stone & Robert West (eds.) ''Advances in Organometallic Chemistry Vol. 30'' (1990), Chapter 5 (at p. 190) [https://books.google.com/books?id=e6R4oMRDhvsC&pg=PA190 google books link]</ref>

The importance of the concept lies in identifying significantly related species, as pairs or series.  Isoelectronic species can be expected to show useful consistency and predictability in their properties, so identifying a compound as isoelectronic with one already characterised offers clues to possible properties and reactions. Differences in properties such as [[electronegativity]] of the atoms in isolelectronic species can affect reactivity.

In [[quantum mechanics]], [[hydrogen-like atom]]s are [[ion]]s with only one electron such as {{chem|Li|2+|link=lithium}}. These ions would be described as being isoelectronic with [[hydrogen]].

==Examples==
{{multiple image
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|total_width = 315
|image1 = L-serine-2D-skeletal.png
|caption1 = [[Serine]]
|image2 = L-cysteine-2D-skeletal.png
|caption2 = [[Cysteine]]
|image3 = L-selenocysteine-2D-skeletal.png
|caption3 = [[Selenocysteine]]
|footer = Three isoelectronic amino acids
}}
{{multiple image
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|image1 = Dimethyl ether Structural Formulae.svg
|width1 = 150
|caption1 = Dimethyl ether
|image2 = (E)-Azomethane.svg
|width2 = 150
|caption2 = Azomethane
|footer = Both molecules have the same electron configuration, but due to the double bond and extra atom in azomethane, they are not isoelectronic 
}}

The {{chem|N|link=nitrogen}} atom and the {{chem|O|+|link=oxygen}} ion are isoelectronic because each has five [[valence electron]]s, or more accurately an electronic configuration of [He] 2s<sup>2</sup> 2p<sup>3</sup>.

Similarly, the [[cation]]s {{chem|K|+|link=potassium}}, {{chem|Ca|2+|link=calcium}}, and {{chem|Sc|3+|link=scandium}} and the [[anion]]s {{chem|Cl|-|link=chloride}}, {{chem|S|2-|link=sulfide}}, and {{chem|P|3-|link=phosphide}} are all isoelectronic with the {{chem|Ar|link=argon}} atom. 

{{chem|CO}}, {{chem|CN|-|link=Cyanide}}, {{chem|N|2}}, and {{chem|NO|+}} are isoelectronic because each has two atoms triple bonded together, and due to the charge have analogous electronic configurations ({{chem|N|-}} is identical in electronic configuration to {{chem|O}} so {{chem|CO}} is identical electronically to {{chem|CN|-}}).

[[Molecular orbital diagram]]s best illustrate isoelectronicity in diatomic molecules, showing how [[atomic orbital]] mixing in isoelectronic species results in identical orbital combination, and thus also bonding.

More complex molecules can be polyatomic also. For example, the [[amino acid]]s [[serine]], [[cysteine]], and [[selenocysteine]] are all isoelectronic to each other. They differ by which specific [[chalcogen]] is present at one location in the side-chain.

{{chem|CH|3|COCH|3}} ([[acetone]]) and {{chem|CH|3|N|2|CH|3}} ([[azomethane]]) are not isoelectronic. They do have the same number of electrons but they do not have the same structure.

==See also==
* [[Isolobal principle]]

==References==
{{Reflist}}

[[Category:Theoretical chemistry]]