The isoelectronic principle states that molecules with the same number of electrons and atoms will have similar structures and chemical properties. The word "isoelectronic" itself means "the same number of electrons," and should not be confused with the term "isoelectric," which concerns electrical potential.

Most generally, the valence electrons of molecules (those in the outer shell that form chemical bonds), are isoelectronic. However, the state can also apply only to the inner electrons of molecules (those closer to the nucleus), or both. If two molecules are isoelectronic for both the inner and outer electrons, they are likely to be even more similar in most ways.Carbon dioxide and nitrous oxide (laughing gas) are isoelectronic molecules (22 electrons for both), so their properties are remarkably similar. They currently share the dubious distinction of being two of the most potent greenhouse gases in the atmosphere.

Just as the Periodic...