A hydrogen atom (H) that is covalently attached to a more electronegative "donor" atom or group (Dn) and another electronegative atom containing a lone pair of electrons—the hydrogen bond acceptor—are attracted to one another via an attraction that is mostly electrostatic (Ac). The second-row elements fluorine, oxygen, and nitrogen are the most common donor and acceptor atoms (F). The two types of hydrogen bonding are intramolecular and intermolecular, respectively (occurring among parts of the same molecule). The energy of a hydrogen bond can range from 1 to 40 kcal/mol and is influenced by the geometry, environment, and nature of the particular donor and acceptor atoms. This makes them weaker than fully covalent and slightly stronger than a van der Waals interaction.