Isoelectronic molecules

Two species are isoelectronic if they possess the same total number of electrons. If two species contain the same number of electrons they are isoelectronic; CH₄, [BH₄]^- and [NH₄]⁺‏ are isoelectronic, since each contains a total of 10 electrons. Two other series of isoelectronic species are N₂, CO and [NO]‏⁺ , and [SiF₆]^2-, [PF₆] ^- and SF₆.  The word isoelectronic is often used in the context of meaning ‘same number of valence electrons’, although strictly such usage should always be qualified; e.g. HF, HCl and HBr are isoelectronic with respect to their valence electrons.

  The isoelectronic principle is simple but important. Often, species that are isoelectronic possess the same structure and are isostructural. However, if the term is used loosely and only the valence electrons are considered, this expectation may not hold. With respect to their valence electrons, CO₂ with SiO₂ are isoelectronic, but whereas CO₂ is a linear molecule, SiO₂ possesses an infinite lattice in which Si is 4- coordinate.