Question

What is placed between a molecule's resonance structures to indicate resonance?

Answer 1

A double-ended arrow.

There are a few different arrow symbols you should keep track of.

Double-ended arrows indicate resonance, such as for the acetate anion.

One-ended arrows indicate a forward reaction, or a reaction that is substantially skewed towards the products. In the case of the above example, `K_(eq)` is over `4xx10^4`, which is substantially skewed towards the products.

Double-ended half-arrows indicate equilibrium. Some people like to make one half-arrow shorter than the other to emphasize that one reaction direction is favored more than the other, if they know that is the case. In this case, `K_(eq)` is about `1.58`, which is somewhat skewed towards the products.