Resonance (chemistry)

In chemistry, resonance describes how certain molecules or polyatomic ions are bonded. It is also known as mesomerism. Scientists use this concept to explain bonding that a single Lewis structure cannot accurately show. A Lewis structure is a drawing that shows atoms following the octet rule. These drawings use bonds of positive integer order to connect atoms. While many molecules are happy with one drawing, some have electrons that are delocalized. Delocalized electrons are spread out over several atoms rather than being stuck between just two.

To represent these molecules, chemists use several contributing structures. These are also called resonance structures or canonical structures. No single contributing structure is the actual molecule. Instead, the true molecule is a resonance hybrid. The hybrid is an average of all the theoretical contributing structures. It is the most accurate way to describe the real chemical species. The contributing structures only differ in how electrons are formally assigned to atoms. They must always have the same number of valence electrons and the same spin multiplicity.

We can see how this works by looking at the nitrite anion, NO2⁻. In a single Lewis structure, one N–O bond might be a single bond while the other is a double bond. However, experiments show that both N–O bonds in nitrite are actually equal in length. They measure at 125 pm. This length is intermediate between a typical single bond of 145 pm and a double bond of 115 pm. The true bond order for each bond is 1.5. This happens because the nitrite anion is a resonance hybrid of two major contributing forms.

Resonance provides a great deal of stability to a molecule. This stability comes from electron delocalization. When electrons are spread out over a larger area, it decreases electron-electron repulsion. This process lowers the potential energy of the system. The difference in energy between the actual hybrid and the lowest-energy contributing structure is called resonance energy. It is also known as delocalization energy. While resonance energy is not a directly measurable physical quantity, it is a vital concept for understanding molecular behavior.

It is important to distinguish resonance from isomerism. Isomers are different chemical species with the same formula but different arrangements of nuclei. Resonance contributors are not different species. They are just different ways to draw the same single molecule. A common mistake is thinking electrons are rapidly shifting back and forth between positions. This is not true. The molecule has one well-defined geometry and one steady distribution of electrons. Think of a narwhal. You might describe it using the features of a unicorn and a leviathan. The narwhal is not constantly switching between being a unicorn and a whale. The mythical creatures are just tools to describe the real animal.

The history of resonance involves several important scientists. In 1899, Johannes Thiele proposed the "Partial Valence Hypothesis." He wanted to explain why benzene was so stable. He suggested that benzene had "partial valences" and called these systems "conjugated." Later, in 1926, Werner Heisenberg introduced resonance into quantum mechanics. He used a comparison to classical harmonic oscillators to discuss the helium atom. In 1928, Linus Pauling used this mechanism to explain partial valence. He published several key papers between 1931 and 1933 to develop the theory further.

Chemists use specific symbols to represent resonance in diagrams. They use a double-headed arrow ( ↔ ) to separate contributing structures. This arrow is different from an equilibrium arrow ( ⇌ ), which shows a chemical change. Sometimes, all the structures are placed inside large square brackets to show they represent one ion. Another way to draw a hybrid is to use dashed lines or curves. These lines represent partial pi bonds. In aromatic rings like benzene, chemists often draw a solid circle to show the delocalized pi-electrons.