In chemistry, a hydrogen bond is primarily an electrostatic force of attraction between a hydrogen atom which is covalently bonded to a more electronegative "donor" atom or group , and another electronegative atom bearing a lone pair of electrons—the hydrogen bond acceptor . It is the strongest of Keesom forces. Such an interacting system is generally denoted .mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}Dn−H···Ac, where the solid line denotes a polar covalent bond, and the dotted or dashed line indicates the hydrogen bond. The most frequent donor and acceptor atoms are the period 2 elements nitrogen , oxygen , and fluorine .