A hydrogen bond is a weak attraction between molecules, formed when a hydrogen atom bonded to fluorine, oxygen or nitrogen is attracted to a lone pair on a neighbouring molecule.
A hydrogen bond is a weak attraction between molecules, formed when a hydrogen atom that is bonded to fluorine, oxygen or nitrogen is attracted to a lone pair on a neighbouring molecule. Malay: ikatan hidrogen · Chinese: 氢键.
Fluorine, oxygen and nitrogen are strongly electronegative, so when hydrogen bonds to one of them the shared electrons are pulled away from the hydrogen. This leaves the hydrogen with a small positive charge and the F, O or N with a small negative charge and a lone pair. The slightly positive hydrogen of one molecule is then attracted to the lone pair of a neighbouring molecule, and this attraction is the hydrogen bond. It is far weaker than the covalent bonds inside the molecule but stronger than most other intermolecular forces, so it needs extra energy to break.
Example. Water molecules are held to one another by hydrogen bonds between the positive hydrogen of one molecule and a lone pair on the oxygen of the next. These bonds are the reason water has an unusually high boiling point for such a small molecule, and why ice is less dense than liquid water.
Confusion to avoid. A hydrogen bond is a force between molecules, not a bond inside a molecule. Do not confuse it with the covalent O–H or N–H bond, which is many times stronger. It also only forms when hydrogen is bonded directly to F, O or N, not to carbon.
The hydrogen bond is the strongest of the intermolecular forces you meet in this chapter, and it explains the raised melting and boiling points that appear when properties are compared in SPM Chemistry.
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