The concept of resonance is a useful method for illustrating the chemical makeup of a compound when that compound is not representative of a single Lewis structure but is, rather, a hybrid of multiple Lewis structures. The presence of multiple resonance structures helps explain why certain molecules are more chemically stable than one might expect. Benzene, adenosine triphosphate, or ATP, and adenosine diphosphate, or ADP, are examples of chemicals that exhibit resonance stabilization.
Resonance occurs when the electrons in a molecule are able to move around to offer the molecule added stability. This electron movement is called electron delocalization. This makes a single Lewis structure an inaccurate representation of the molecule because certain electrons are not fully committed to a particular bond or to the valence shell of a particular atom.
When drawing resonance structures for a molecule, all of the atoms must remain in the same position; only bonds and lone pairs of electrons can change position. The formal charges on the different atoms may change, but the overall charge of the molecule should remain the same from structure to structure.
Resonance stabilization is part of the reason why ATP readily undergoes hydrolysis to ADP, releasing useful energy in the process. When ATP hydrolyzes, a phosphate molecule and an ADP molecule are released. Both have more possible resonance structures than ATP, meaning they are lower in energy and more chemically stable.