Explaining Resonance-A
Colorful Approach
A common misunderstanding held by many chemistry students is that resonance structures exist as unique entities that oscillate back and forth. It is challeneine students understand that the "actual" structure is not - to heln. beeinnine " any one of the resonance structures, but a hybrid of all. s i m d y put, we do not have an easy way of showing what a resonance structure looks like, 80 we use several fictitious structures to describe the one actual structure. This concept is then further confused by the fact that sometimes the resonance species contribute differently to the hybrid. We have found two ways to help explain the resonance phenomenon in both general and organic classes-a color analogy and modified resonance arrows. Many students believe mistakenly, that if you could somehow "take a picture" of a resonance molecule for just a moment, i t would show one of the resonance structures. The color analogy works well in explaining resonance. Green is a combination of blue and yellow. Assume that we don't have a way of describing green except in terms of primary colors. We don't say that a green wall is blue one second and e ell ow the next. No matter how fast you take a picture of it. a meen tree is still meen. If one structure (color) contributes more, we can say it looks like a blueish green, not blue f& tGo moments and then yellow for one. Take for example the three possible resonance structures for NzO:
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We can see from formal charges that the first two structures represent hypothetical species that would be more stable than the third, and thus contribute mare to the actual structure. 0 +I -1 -1 +1 0 -2 +I +I Explaining this in terms of the three primary colors, blue, yellow, and red, we would say that Nz0 has all three colors in it. Since two structures (say blue and red) contribute more than the third (yellow), the NzO molecule would "look" violet. This is not red two moments then blue two moments then yellow one moment, but violet. When it is necessary in class to show which resonance structures contribute most to the resonance molecule, we have found a slightly modified resonance arrow to effectively and efficiently convey all the necessary information. When one structure &tributes more, add a tip to the arrow pointing toward that species. Just as we do not imply precise quantitative meaning to the @OH OH length of equilibrium arrows ( -1, neither do we intend II i quantitative interpretation as to the number of tips on the CH,CCH3
