Hydrogen bonding

A hydrogen bond forms when a hydrogen atom that is bonded to a highly electronegative atom (fluorine, oxygen or nitrogen) is attracted to another such atom nearby. The electronegative atom pulls the bonding electrons away, leaving the hydrogen with a positive charge (δ+), which is then attracted to the negative end (δ–) of a neighbouring molecule: H^δ+–F^δ– ··· H^δ+–F^δ–.

A hydrogen bond is drawn as a dotted line. It is much weaker than a covalent bond, but stronger than the ordinary van der Waals forces between molecules.

There are two kinds:

• Intermolecular hydrogen bonds join different molecules, as in HF and H₂O. They hold the molecules together more tightly, so the substance has a higher boiling point than expected: without hydrogen bonds, water would be a gas at room temperature. In ice, hydrogen bonds hold the molecules in an open cage-like pattern, so ice is less dense than liquid water and floats.
• Intramolecular hydrogen bonds form within one molecule, between two parts of it. In o-nitrophenol, the hydrogen of the –OH group is attracted to an oxygen of the neighbouring –NO₂ group.

The strength of hydrogen bonding follows the electronegativity of the atom involved: F > O > N.