Teaching hydrogen bridges: it is not FON anymore!

Teaching hydrogen bridges (H-bridges) in introductory chemistry courses is either oversimplified or dogmatically taught; the heuristic that ‘only fluorine, oxygen and nitrogen (FON) form H-bridges’ is commonly used, for example. This rule limits choices, does not match reality, and does not clarify the difference between H-bridge donors (HBD) and H-bridge acceptors (HBA) nor intermolecular versus intramolecular cases. Hydrogen fluoride is a significant HBD but a poor HBA. Oxygen is not always an HBD or HBA. Ammonia is a significant HBA but a poor HBD. There are many examples of H-bridges that do not contain these three elements. All examples of H-bridges should be shown in a way to provide symbolic and molecular structures. Is it necessary to ‘explain’ H-bridges in introductory classes? We conclude not, but it might be necessary to increase the total amount of time to present the material so that students do not rely on superficial rules.