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I would like to make a few comments on the recent
article by Gillespie, Spencer, and Moog (1).
First of all, the authors should be complemented
for trying to simplify the introduction to electron shells
in chemistry courses and for emphasizing the dangers of
too much quantum mechanics at the beginning.
I also liked the attempt to tie the presentation
strictly to what can be observed experimentally, namely,
ionization energies. However, there is an error in the discussion
concerning the filling of shells in transition metal atoms.
On page 621 the authors state that the highest ionization
energy of scandium involves the removal of a
3d electron, whereas all experimental evidence points to the removal
of a 4s electron (2).
Secondly, it is somewhat surprising to see the
authors supporting Allen's scale of electronegativity and his
claimed "third dimension" to the periodic table, which, as
several authors have argued, has very little to commend it
(3, 4). Gillespie et al. say that they choose to work with this
scale, rather than better-established ones, because Allen's
scale is the only one that involves directly measurable
quantities. This statement too is incorrect, since the
Mulliken scale, which takes the average of ionization energy and
electron affinity, also shares this
virtue.1 The unstated rationale for favoring Allen's scale of electronegativity seems
to be that it maintains the connection with ionization
energy that is used by the authors to develop shell structure,
but this is hardly a compelling reason for choosing Allen's
particular function of ionization energies.
Note
1. I thank Bill Jensen for drawing my attention to this point.
Literature Cited
- Gillespie, R. J.; Spencer, J. N.; Moog, R. S.
J. Chem. Educ. 1996, 73, 617 and 627.
- Melrose, M. P.; Scerri, E. R.
J. Chem. Educ. 1996, 73, 498.
- Scerri, E. R. J. Phy. Chem.
1993, 97, 5786.
- 4. Rouvray, D. Chem.
Intell. 1996, 2(3), 39.
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