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Isoelectronicity is a phenomenon observed when two or more have the same structure (positions and connectivities among ) and the same electronic configurations, but differ by what specific chemical element are at certain locations in the structure. For example, , , and are isoelectronic, while and Azomethane are not.
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 electronic configuration. Isoelectronic Configurations iun.edu More usually, definitions are broader, and may extend to allowing different numbers of atoms in the chemical species being compared.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) google books link
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 atoms are with only one electron such as . These ions would be described as being isoelectronic with hydrogen.
Similarly, the , , and and the , , and are all isoelectronic with the atom.
, , , and are isoelectronic because each has two atoms triple bonded together, and due to the charge have analogous electronic configurations ( is identical in electronic configuration to so is identical electronically to ).
Molecular orbital diagrams 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 isoelectronic also. For example, the serine, cysteine, and selenocysteine are all valence isoelectronic to each other. They differ by which specific chalcogen is present at one location in the side-chain.
([[acetone]]) and ([[azomethane]]) are not isoelectronic. They do have the same number of electrons but they do not have the same structure.
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