Question

When can resonance structures be written for a molecule?

Answer 1

You can write resonance structures for any compound or ion that contains one or more double or triple bonds.

Explanation:

Whenever you can draw two or more structures for a substance that differ only in the location of the electrons, the actual structure is none of them.

Rather, the actual structure is a resonance hybrid of them all.

For example, we can write three equivalent structures for formaldehyde, differing only in the position of the electrons.

`"H"_2"C=O" ⟷ "H"_2stackrelcolor(blue)(+)("C")-stackrelcolor(blue)("-")("O") ⟷ "H"_2stackrelcolor(blue)("-")("C")- stackrelcolor(blue)(+)("O")`

Admittedly, the charged structures are minor contributors, but they are contributors.

Resonance is more important when you can draw equivalent structures for a substance.

For example, you can draw three equivalent structures for the carbonate ion, `"CO"_3^"-"`:

www.chem.ucla.edu

A well-known example of resonance is that of benzene.

upload.wikimedia.org

It has two equivalent structures that differ only in the position of the electrons.