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2.4B: Isoelectronic Molecules

The term "isoelectronic" means "equal electric", or more specifically, equal charge. Isoelectronic species are elements or ions that have the same, or equal number of electrons. This module explores that although isoelectronic species have the same number of electrons, they are different in their physical and chemical properties.


Although periodic trends are often overlooked and forgotten, they are very impotant to predicting the chemistry of molecules. One periodic trend most often overlooked and rarely studied is isoelectronic species, despite it's usefulness in predicting reactions or existence of compounds. The concept of isoelectrionc species comes from the root of the word isoelectronic. The prefix -iso means "same" which translates iso-electronic to mean "same electrons". Isoelectronic species that have the same number of valence electrons, but different number of total electrons are called "valence-isoelectrionc" (Geoff).

Examples of Isoelectronic Species

A series of isoelectronic ions can be made from any any number or series of elements. For example, elements coming before and after noble gases form isoelectronic ions by gaining or losing electrons until they have the same number of electrons as the noble gas (Figure 1 & 2) .

The electron configuration for Argon (Ar) is 1s2 2s2 2p6 3s2 3p6

periodic copy.jpg

                                       Figure 1


                                     Figure 2

The series of isoelectronic species in Figure 1 are: Sc+3, Ca+2, K+1, Ar, Cl-1, S-2, P-3

Figure 2 shows how Isoelectronic Species vary in atomic radii. Within a species, the radii differs depending on charge and shielding character of the electrons.

Isoelectronic Species Periodic Trends

The observation that isoelectronic species are usually isostructural, first made by Penny and Southerland in 1936, known as the isoelectronic principle (Geoff). Table 1 shows an example of isostructural isoelectronic species periodic trends. All of these molecules are octahedral and isoelectronic within their periods.

Isostructural Isoelectronic Species in Differeing Groups and Periods

Table 1 Group 13 Group 14 Group 15 Group 16 Group 17
Period 3 AlF63- SiF62- PF6- SF6 ClF6+
Period 4 GaF63- GeF62- AsF6- SeF6 BrF6+
Period 5 InF63- SnF62- SbF6- TeF6 IF6+

Other interesting trends appear in the periodic table including:

  • Isoelectronic matricies--all isoelectronic species in a matrix defined by total electrons and valence electrons vary by progression in group number. For example, a 14 electron/10 valence electron diatomic matrix would have molecules such as CN-, CO, and N2.
  • Isoelectronic arrays--an atom is replaced with another which alters the charge and continues the isoelectronic relationship. For example: BeF42-, BF4-, and CF4.


  1. Smith, Michael, and Jerry March. March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Hoboken, NJ: Wiley-Interscience, 2007.
  2. Rayner-Canham, Geoff. "Isoelectronic Series: a Fundamental Periodic Property." Foundations of Chemistry 11.2 (2009): 123-29.


1) Why is it important to understand the term "isoelectronic" and its trends throughout the periodic table?

2) Which isoelectronic species has the largest radius?

a. Mg2+ b.N3- c. O2- d. F-

3) Name 3 isostructural molecules


1) Isoelectronic can help predict chemical reactions and interactions between molecules.

2) N3-

3) AlF63-, SiF62-, PF6-