Rules for Writing Resonance Structures in Chemistry

Drawing resonance structures in organic chemistry is a very hard concept to master. Resonance structures are written for molecules that are not represented by a single Kekulé structure alone. These structures exist as a resonance hybrid, which means that the molecule at its nature state exists as an average of all resonance structures. Resonance structures differ in the placement of pi bonds and nonbonding valence electrons. Connections between the atoms and atom placement remains the same. Many organic chemistry students struggle writing resonance structures. Below are some rules that may help students write resonance structures, as discussed in Organic Chemistry.

1. Individual forms help one determine the resonance hybrid. This is not the molecule's real form.

As explained before, the molecules exist in a resonance hybrid. The molecules do not switch between forms shown on paper, but are an average of all resonance structures shown.

2. Resonance structures use curved arrow notation.

The curved arrow does not show the movement of atoms, but shows the movement of electrons. Electrons move in the path of element to bond or bond to element. Electrons should also be moved from electron dense to electron deficient regions.

3. The different resonance structures of molecule are not always equivalent.

The resonance structure that resembles the resonance hybrid the best is the stable form of the structure. The most stable structure is the structure with no charges and all closed shells.

4. Resonance must follow the same rules used as drawing Lewis structures.

Rules such as the octet rule and rules of valency also apply when writing resonance structures. The only time one can break the octet rule is when it is allowed by the d-orbitals. If these rules are not followed, the structure is not a valid resonance structure.

5. Individual resonance structures are not as stable as the resonance hybrid.

As a rule of thumb, the more resonance structures a molecule has, the molecule will be more stable.

Resources

McMurry, John. Organic Chemistry. Belmont, CA: Thomson-Brooks/Cole, 2004. Print.