2.4B: Isoelectronic Molecules

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periodic copy.jpg — 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	AlF_6³_^-	SiF_6²_^-	PF₆_^-	SF₆	ClF₆_⁺
Period 4	GaF₆_³_^-	GeF_6²_^-	AsF₆_^-	SeF₆	BrF₆_⁺
Period 5	InF_6³_^-	SnF_6²_^-	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: BeF₄_^2-, BF₄^-, and CF₄_.

References

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

Problems

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

Answers:

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

2) N3-

3) AlF_6³_^-,SiF_6²_^-,PF₆_^-