Isoelectronic series: a fundamental periodic property
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- Rayner-Canham, G. Found Chem (2009) 11: 123. doi:10.1007/s10698-008-9055-4
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The usefulness of isoelectronic series (same number of total electrons and atoms and of valence electrons) across Periods is often overlooked. Here we show the ubiquitousness of isoelectronic sets by means of matrices, arrays, and sequential series. Some of these series have not previously been identified. In addition, we recommend the use of the term valence-isoelectronic for species which differ in the number of core electrons and pseudo-isoelectronic for matching (n) and (n + 10) species.
Patterns and trends in the periodic table continue to fascinate (Rouvray and King 2004). As part of a series of studies on periodic patterns, we recently confirmed the validity of the ‘knight’s move’ relationship (Rayner-Canham and Oldford 2007). Another periodic trend is that of isoelectronic series across periods, yet these seem to have been little studied. This paper is intended to provide such a review using arrays of isoelectronic species to illustrate a range of periodic patterns.
A brief background
For example, since phosphorus and nitrogen atoms contain the same number of electrons in their shells, the simple octet theory … indicates that nitrogen compounds corresponding to all known phosphorus compounds could exist and vice versa. Thus we might expect the following compounds: H3NO4, Na4N2O7, P2O.
However, Langmuir, like most chemists, seemed to be interested in isoelectronic species within groups (same E, different N) rather than patterns across periods.
The definition of isoelectronic
Humpty Dumpty, in Alice Through the Looking Glass, remarked (Carroll et al. 1990): “When I use a word, it means just what I choose it to mean-neither more nor less.” In the chemical context, isoelectronic seems to mean whatever a chemist wishes it to mean. Two particular definitions are used widely, as was bought to this author’s attention by Penny LeCouteur, of Capilano College, North Vancouver, Canada: species having the same number of valence electrons are isoelectronic while the second definition adds the narrowing criteria that the species also have to have the same total number of electrons.
Species (atoms, molecules, ions) are isoelectronic with each other if they have the same total number of atoms and electrons and of valence electrons.
True isoelectronic species will usually have matching valence-level molecular orbitals which will undergo systematic energy changes across the series as Sima has shown (1995).
Species (atoms, molecules, ions) are valence-isoelectronic with each other if they have the same number of valence electrons.
For example, OCO and NCO− would be identified as isoelectronic, while OCO, OCS, and SCS would be considered valence-isoelectronic. Here we will address the significance of true isoelectronic series by demonstrating patterns among compounds of elements of the same period and neighbouring groups.
The isoelectronic principle
Two or more polyatomic species (ions and/or molecules) that are isoelectronic have a high probability of being isostructural.
Three-atom isoelectronic arrays
# of F/O
# of F/O
Sequential isoelectronic series
It is also possible to construct informative series in which only the horizontal rows are isoelectronic, but successive rows are linked in some simple stepwise manner. For example, in an isoelectronic oxidation-state array, each row contains species having one more electron than the preceding row, thus the oxidation state of each atom-combination decreases down the array.
# of H
Sima (1995) has used comparisons of atomic orbital energies to examine why such series cannot be further extended, such as H4O2+ for the top series above, and HNe+ for the bottom series.
# of Cl
# of F
# of F
# of CO
Species (atoms, molecules, ions) are pseudo-isoelectronic if they differ by only a d10 set of electrons.
(n + 10)
We have endeavoured to show that the concept of isoelectronic arrays enables chemists to perceive periodic patterns across segments of the periodic table. Such isoelectronic patterns ‘lurk’ not only across the nonmetallic elements of each period but even stretch through the semi-metal members into the weak metals. There is not just one ‘type’ of table: there are matrices with group variation along both axes, then there are arrays in which there are step-wise changes in numbers of substituent atoms and/or changes in oxidation states.
These matrices and arrays are a powerful addition to the ways of demonstrating periodic behaviour. The existence of gaps in such arrays might prompt synthetic inorganic chemists to attempt to synthesize missing members of arrays.
This author would appreciate hearing from readers of additional isoelectronic series together with information on the existence of ‘missing’ members of series shown in the matrices above. The author also thanks the two anonymous referees for very constructive comments which have led to a significant improvement in the manuscript.