Hydrogen bonding is the strongest of the intra- or intermolecular interactions we'll study. It is weaker than an ionic or covalent bond, while being stronger than other non-bonding forces such as van der Waals or London dipersion forces. It is actually an extreme form of dipole-dipole interaction. Because hydrogen is poorly electronegative, strongly electronegative atoms like oxygen, nitrogen and halogens form very strong dipole-dipole attractions with the partially positive hydrogen. This important non-bonding attraction is a major reason behind the high boiling points of alcohols and other H-bond containing molecules as well as the structure of biological molecules such as proteins.