Editing Intermolecular force (section)

== Hydrogen bonding ==
{{Main|Hydrogen bond}}

A ''hydrogen bond'' refers to the attraction between a [[hydrogen]] atom that is covalently bonded to an element with high [[electronegativity]], usually [[nitrogen]], [[oxygen]], or [[fluorine]], and another highly electronegative atom.<ref name=GoldBook-H02899>{{GoldBookRef |file= H02899 |title= hydrogen bond}}</ref> The hydrogen bond is often described as a strong electrostatic interaction. However, it also has some features of covalent bonding: it is directional, stronger than a [[van der Waals force]] interaction, produces interatomic distances shorter than the sum of their [[van der Waals radius|van der Waals radii]], and usually involves a limited number of interaction partners, which can be interpreted as a kind of [[Valence (chemistry)|valence]]. The number of hydrogen bonds formed between molecules is equal to the number of active pairs. The molecule which donates its hydrogen is termed the donor molecule, while the molecule containing lone pair participating in H bonding is termed the acceptor molecule. The number of active pairs is equal to the common number between number of hydrogens the donor has and the number of lone pairs the acceptor has.

[[Image:Hydrogen-bonding-in-water-2D.svg|thumb|Hydrogen bonding in water]]

Though both are not depicted in the diagram, water molecules have four active bonds. The oxygen atom’s two lone pairs interact with a hydrogen each, forming two additional hydrogen bonds, and the second hydrogen atom also interacts with a neighbouring oxygen. Intermolecular hydrogen bonding is responsible for the high boiling point of [[water]] (100&nbsp;°C) compared to the other [[Hydrogen chalcogenide|group 16 hydrides]], which have little capability to hydrogen bond. Intramolecular hydrogen bonding is partly responsible for the [[secondary structure|secondary]], [[tertiary structure|tertiary]], and [[quaternary structure]]s of [[protein]]s and [[nucleic acid]]s. It also plays an important role in the structure of [[polymers]], both synthetic and natural.<ref>{{Citation | vauthors = Lindh U | contribution = Biological functions of the elements | veditors = Selinus O | title = Essentials of Medical Geology | pages = 129–177 | publisher = Springer | place = Dordrecht | year = 2013 | edition = Revised | isbn = 978-94-007-4374-8 | doi = 10.1007/978-94-007-4375-5_7 }}</ref>