Chemistry is a difficult subject for many, many students. The nomenclature, the math, the diagrams, all of them combine to make the subject quite demanding. Most students seem to do ok with a topic that is well-presented and clearly defined. However, resonance structures continue to trip up many beginning students, and so this article lists several tips for chemistry educators / teachers to help their students adapt to this tricky subject.
Resonance structures are a way of understanding the positioning of electrons. This is a vital topic for chemists, as understanding where electrons are located within the molecule is the key to understanding the molecules chemical behavior. You can’t really say anything about chemical reactivity or chemical properties without knowing what parts of the molecule are electron-rich (containing a large density of electrons) and which are electron-poor. Normally, chemists depict molecules on paper as atoms connected by straight lines, which represent the bonds that hold the molecule together. One straight line represents one pair of electrons, so two electrons for each straight line. However, electrons are not quite so well behaved as to lie down nicely in a straight line and never go anywhere else. Electrons are probably best envisioned as a cloud of negative charge, and this cloud can be “smeared” across several atoms, giving all of those atoms a certain negative charge character; this cannot be well represented by single lines. Here are some tips to help your students grasp this concept.
First of all, even though chemistry is a “hard” science depending on facts and observations, make sure your students understand that the tools we have to depict molecules – pen and paper, for example – are not the best. We like to think of something either being here, or being there; for electrons, they can be spread out over several atoms at once. Writing correct resonance structures means the students absolutely have to be comfortable with this type of academic freedom, to be able to use the bond lines wherever they have to in order to draw the molecule. Students have to be encouraged to write whatever molecular structure they like, providing that they follow the (very strict) rules about correct Lewis structure formation for whatever they develop. Break your students of the idea that because they may not have the same structure as someone else, that their idea is wrong; it very well could be that both ideas are right.
This brings us to the next tip: make sure that your students understand that what they write down as resonance structures are only suggestive of the electron pattern within an actual molecule. If the molecule has two resonance structures (if the electrons can be placed in two different patterns, using the straight line methodology), it doesn’t mean that the molecule actually exists in two different forms, or that the two forms are somehow switching back and forth between each other. This mistake is not helped by the choice of “resonance” for this phenomenon, as that word suggests that the molecules “resonate” and somehow switch the electron orientation. Teach your students repeatedly that the actual molecule is a hybrid of the two resonance structures. As humans, we have to write multiple resonance structures for one molecule, as our tools for writing are not sophisticated enough to draw the actual molecule in one go. The molecule does not “resonate” – it’s a mixture of all the qualities from all the resonance structures.
Give your students the analogy of a mule: it’s not a donkey, and it’s not a horse, it’s somewhere inbetween. A mule does not switch back and forth between being a donkey and a horse. It’s a mule. Similiarly, a molecule does not switch around. It is what it is: a compilation of all possible electron arrangements (resonance structures) coexisting at the same time.
Chemistry students will never understand chemical reactivity and behavior if they don’t understand the behavior of electrons. Being able to correctly and accurately depict electron placement within a molecule (via resonance structures) is therefore a fundamental, keystone skill. As a chemical educator, you can help your students avoid the numerous logic pitfalls that exist, and this will greatly boost their understanding and appreciation of chemistry.