What does resonance in molecular structures indicate?

Resonance in molecular structures indicates that there are multiple valid Lewis structures for the molecule. When a molecule exhibits resonance, it means that no single Lewis structure can fully describe the electronic arrangement of the molecule. Instead, multiple structures, called resonance structures, contribute to the overall hybrid structure that represents the actual arrangement of electrons in the molecule.

These resonance structures differ only in the placement of electrons, particularly in pi bonds or lone pairs, while the positions of the atoms remain unchanged. The true electronic structure of the molecule is a blend or hybrid of these resonance structures, which allows for delocalization of electrons. This delocalization contributes to the stability of the molecule as it can distribute electron density over a larger area, lowering the overall energy of the system.

In contrast, the other options do not accurately reflect resonance. While it is true that some molecules can be unstable, resonance does not inherently indicate that a molecule is unstable or that it cannot stabilize. Additionally, saying that only one valid Lewis structure exists contradicts the essence of resonance, which is all about the existence of multiple valid structures. Therefore, the identification of resonance in structures is critical for understanding molecular behavior and stability.