Isoelectronicity

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
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&minus;, S2&minus;, and P3&minus; 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.