Isoelectronic
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Two or more molecular entities (atoms, molecules, ions) are described as being isoelectronic 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.
Examples
N and the O+ ion are isoelectronic because each has one atom and 5 electrons in the outer electronic shell.
CO, N2 and NO+ are isoelectronic because each has 2 nuclei and 10 valence electrons (4+6, 5+5, and 5+5, respectively).
CH2=C=O and CH2=N=N 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; the second molecule's structure is linear: H3C-N=N-CH3; methyl groups are not connected to the same nitrogen atom.