Two or more molecular entities (atoms, molecules, or ions) are described as being isoelectronic with each other if they have the same number of electrons or a similar electron configuration and the same structure (number and connectivity of atoms), regardless of the nature of the elements involved.
The term valence isoelectronic is used when these molecular entities have the same number of valence electrons or a similar electron configuration, but may have a different number of atoms or a different bonding.
The statement "These compounds or molecules are isoelectronic" is not just an implementation of the above definition. It has significance by the fact that calculations on molecules and electron density, and therefore capability of reaction, have been performed on many common substances. Identifying a new, rare or odd compound as being isoelectronic with an already known one offers clues to possible properties and reactions.
The Template:Chem/link atom and the Template:Chem/link radical ion are isoelectronic because each has five electrons in the outer electronic shell. Similarly, the cations Template:Chem/link, Template:Chem/link, and Template:Chem/link and the anions Template:Chem/link, Template:Chem/link, and Template:Chem/link are all isoelectronic with the Template:Chem/link atom. In such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius decreasing as charge increases.
CO, Template:Chem/link and Template:Chem/link are isoelectronic because each has two nuclei and 10 valence electrons, with each atom considered to have 5 of them (a lone-pair and a triple-bond). Isoelectronicity does not relate to formal charge on the atoms in a structure: these all have the same configuration even though carbon monoxide has formal charges that are balanced (–:C≡O:+) whereas dinitrogen has each atom neutral (:N≡N:) and nitrosonium has an overall net charge.
Template:Chem/link (acetone) and Template:Chem/link (dimethyldiazene) 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 (Template:Chem/link) groups attached to carbonyl's (CO's) carbon atom, forming a branched trigonal planar shape: H3C-C(=O)-CH3; the second molecule's structure has a consecutive attachment of the main atoms: H3C-N=N-CH3 and its methyl groups are not connected to the same nitrogen atom.