A strong dipole attraction between H bonded to N, O or F and a lone pair on another N, O or F.
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A hydrogen bond is a particularly strong dipole-dipole interaction that occurs when a hydrogen atom bonded to nitrogen, oxygen or fluorine is attracted to a lone pair on another nitrogen, oxygen or fluorine atom. The bonded hydrogen carries a large partial positive charge because these electronegative atoms draw electron density away from it, and its small size allows close approach. Hydrogen bonds typically have energies of about 10 to 40 kJ/mol, stronger than other dipole interactions but weaker than covalent bonds. They explain the high boiling point of water, the structure of ice and the base pairing in DNA. Hydrogen bonding is central to biological structure and molecular recognition.
Key ideas: Conditions: H bonded to N, O or F (donor) and a lone pair on N, O or F (acceptor). Count the sites: each water can form up to 4 bonds, hydrogen fluoride about 2, ammonia fewer because of one lone pair and three hydrogens. Typical energy 20 kJ/mol for O-H...O. Boiling points of the hydrides H2O, HF and NH3 are anomalously high compared with their group analogues.
Common mistakes and connections: Students describe hydrogen bonds as covalent bonds to hydrogen, or apply the term to any H such as C-H. Another error is saying hydrogen bonds are the bonds broken when water is split into hydrogen and oxygen. Links to DNA, protein folding, boiling point trends, solubility of alcohols and water properties.
Hydrides with N-H, O-H or F-H bonds (NH3, H2O, HF) boil far above those of heavier analogues because of hydrogen bonding. The interactive lesson shows this as a live animation or chart.
The two strands of the DNA double helix are held together by hydrogen bonds between complementary base pairs.
A strong dipole attraction between H bonded to N, O or F and a lone pair on another N, O or F.
Hydrogen Bonding is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Bonding & Molecular Structure first: Intermolecular Forces, London Dispersion Forces, Dipole-Dipole Forces.
The next topics in Bonding & Molecular Structure are Ion-Dipole Forces and Dipole-Induced Dipole Forces.
Yes. The interactive lesson includes a self-check question on Hydrogen Bonding, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Hydrogen Bonding is topic 73 of 104 in Bonding & Molecular Structure.