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
N and the O+ ion are isoelectronic because each has 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.
