A Visual Aid for Teaching the Resonance Concept Francis Delvlgne Athenee Royal, 3 9 Faubourg d'Arival. 6760 Virton, Belgium
The concept of resonance is not easily imderstood, especially by young students. I would like to present, therefore, a visual aid based on a technique somewhat similar to that described by Richardson.' I use it with students as young as 16-17 years old who are familiar with Lewis structures and representations of "electron clouds" by dot clouds. To make the resonance concept understandable, the representation is used as follows: Each electron pair is represented by a dot cloud, a given number of dots corresponding to an electron pair (see Figs. 1-3). If one considers resonance between two structures that contribute equally to the representation of the molecule or ion, one draws the corre-
sponding dot-cloud structures on separate transparencies; the best representation is obtained by overlaying the two transparent sheets on an overhead projector (Figs. 1 and 2). If one structure makes a smaller contribution to the representation of the real molecule or ion, the number of dots used for an electron pair in that structure is lessened, more or less, according to the weight attributed to that structure in the representation (Fig. 3). It is of course possible to consider more than two resonance structures if necessary (Fig. 3). The spatial distribution of dots is obviously the main feature to consider. The purpose of this method is not to give an actual view of
' Richardson, W. S. J. Chem. Educ 1886, 63,518-519.
Figure 1. SWuchue of NO2-. Each eleCrmn pair is represewed by a cloud of about r25 dots; no(+) are used innead of dots lo represent electrons in pi bonds. Higher dd density (indicated by anow) appears when two electron mtsare "seen" as if JuDerimDDsedbecause of the tetrahedral disoosition. ial shows one resonance smrcture and (01 the aver,ap of lhe two similar strucIdler The figure clsariy shows that electrons dlstrloution is ~nsymrnetricalin (a) and symmebical in (b)
.
.
Figure 2. Sbucture of benzene. Only pi electron palrsare represented, each by a cloudof about 950dMs. (a) showsone Kekul6 structureand (b) the overlapof two strunures. Symmeby Is obvious in (b).
Volume 86 Number 8 June 1989
481
..........:..:.... .. .....
...
... ..
,
... . ..
. . . . . . . . . . . .. . .. .. . .. ..
. .
.
:
Figure 3. Structure of a reacting benzene molecule Ionic r e s o m c e ssuchlres have more weigM than Kekulb smcturaJ (for lnstanm. I chwss 611 for the WelgM ratio). (a) shows one Ionic struckre, wkh 950 dots used for each electron par. (01 shows the overlap of ma f o ~ resonance r structures: the KeKulb ssumnes are the same as in Figure 2. but only 160 dots are used for each pi bond
electrons in molecules, so I do not use precise shapes or densities for electron-cloud representations, preferring simplicity and clarity. It is even possible to represent only those electron pairs that take part in resonance, such as pi bonds in benzene (Figs. 2 and 3). The drawines are hand-made on a tracing-. oaoer . -out over paper with squares (such as graph paper) to help counting the dots. These must be set at random to avoid amoirl. effect when transparencies are overlaid. The following method reduces the amount of work: I lay over a paper drawing a transparency made from it; a photocopy gives a figure 4 t h twice the initial number of dots. The dot densities must he high enough to give a satisfactory representation of a single
462
Journal of Chemical Education
resonance structure, hut if they are too high the overlaid sheets will give a rather dark picture. The geometry has to he right, otherwise a problem will appear when overlaying transparencies. To avoid such a problem I make two transparencies from the same paper drawing. The use of dot clouds has a fundamental advantage: Students get a descriptive, concrete experience of a formal representation and the method puts a concrete image in their memory to which they can refer each time they use the abstract concept of resonance. The importance of concrete experience has been discussed hy Hemon.= Herron, J. D. J. Chem. Educ. 1978,55, 165-170.
