Chemical Education Today
Letters Demystification at What Cost? 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
In at least five separate places in the article we do state our rationale for using Allen’s electronegativities. In the first paragraph we state that: “ionization energies provide a convenient method…that is simpler and more appropriate for the introductory course.” On p 628 we write: “In our view, students will better understand and appreciate this important concept if it can be developed in a manner consistent with simple models of atomic structure and properties.” On p 629: “Their [electronegativities] origin is evident and clear to the students, and their development is consistent with that for previous topics in the course.” Further on p 629: “AVEE electronegativity values have the advantage that students can see where they come from and can calculate the values themselves.” Page 631: “The origin of the Allen electronegativities values can be easily introduced in general chemistry courses, whereas other electronegativity scales have a basis beyond the comprehension of most beginning students.” Scerri has very succinctly stated our rationale: “Allen’s scale of electronegativities…maintains the connection with ionization energy that is used by the authors to develop shell structure…”. We believe that this is a compelling reason for choosing this scale. There are some 15 electronegativity scales in use, all of which, including the Allen scale, give the same qualitative interpretation. It makes little difference which is used unless there is some distinguishing feature that argues for a particular choice. We believe that the Allen scale does demystify the electronegativity concept, and that pedagogically it is consistent with the concepts already introduced in the course. We have taken no stand on the overall usefulness of the Allen scale except for the consistency it provides in our scheme of demystifying the general course.
1. I thank Bill Jensen for drawing my attention to this point.
R. J. Gillespie Department of Chemistry McMaster University Hamilton, ON L8S 4M1 Canada
Literature Cited 1. Gillespie, R. J.; Spencer, J. N.; Moog, R. S. J. Chem. Educ. 1996, 73, 617 and 627. 2. Melrose, M. P.; Scerri, E. R. J. Chem. Educ. 1996, 73, 498. 3. Scerri, E. R. J. Phy. Chem. 1993, 97, 5786. 4. Rouvray, D. Chem. Intell. 1996, 2(3), 39.
J. N. Spencer and R. S. Moog Department of Chemistry Franklin and Marshall College Lancaster, PA 17604-3003
Eric Scerri California Institute of Technology, 228-77, Pasadena, CA 91125
The authors reply: We appreciate the comments of Scerri on our recent articles. He is correct that our discussion on p 621 is incorrect. The data in Table 3 are correct, but our next-to-thelast sentence in the paragraph on scandium is convoluted and in error; however this has no impact on the main ideas discussed. Scerri comments that we state that Allen’s scale is the only one that involves directly measurable quantities. We do not state this anywhere in the article. We do state that none of the electronegativity scales in current use can give an exact value for the electronegativity of an atom in a molecule because electronegativity is by nature a qualitative concept.
480
Correction In the article “A History of the Structural Theory of Benzene—The Aromatic Sextet Rule and Hückel’s Rule” by Shigeaki Kikuchi (February 1997 issue, page 194), we regret that eq 1 was not correctly represented. It should have appeared as follows:
Journal of Chemical Education • Vol. 74 No. 5 May 1997
HC
CH
HC
CH N H
aromatic
H2C H2
CH
H2C
H2C
CH
H2
CH2
H2C
N H
CH2 N H
nonaromatic
(1)
