Isoelectronicity

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Two or more molecular entities (atoms, molecules, ions) are described as being isoelectronic^[1] with each other if they have the same number of valence electrons and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.

[edit] Examples

The N atom and the O⁺ radical ion are isoelectronic because each has 5 electrons in the outer electronic shell. Similarly, the cations K⁺, Ca²⁺, and Sc³⁺, the anions Cl⁻, S²⁻, and P³⁻ are all isolectronic with the Ar atom. In such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius decreasing as charge increases.

CO, N₂ and NO⁺ are isoelectronic because each have 2 nuclei and 10 valence electrons (4+6, 5+5, and 5+5, respectively).

The uncharged H₂C=C=O molecule and the zwitterionic CH₂=N⁺=N^- molecule are isoelectronic.

CH₃COCH₃ and CH₃N₂CH₃ are not isoelectronic. They do have the same number of nuclei and the same number of valence electrons, but the atoms' connectivity is different: the first one has both methyl (CH₃) groups attached to carbonyl's (CO's) carbon atom, forming a trigonal planar structure: H₃C-C(=O)-CH₃; the second molecule's structure is linear: H₃C-N=N-CH₃ and its methyl groups are not connected to the same nitrogen atom.