The Formation of Hydrogen Bonds

August 19, 2023
David Sunnyside

Hydrogen bonding is a very weak force of attraction between molecules that causes many surprising properties. It explains why pond-skater insects can walk on the surface of water, why you can place about 40 drops of water on a single small coin (try it!), why ice floats when we'd expect it to be more dense than water and sink, and why so many organic compounds have high boiling points compared with their chemical composition.

To form a hydrogen bond, the hydrogen atom in question must be attached to one of the highly electronegative elements (usually oxygen, nitrogen or fluorine). These electron-donating atoms attract the electron cloud around the hydrogen nucleus and decentralize it, leaving the hydrogen atom with a partial positive electrical charge. This large positive charge density is electrostatically attracted to the lone pair of electrons on the oppositely charged, more electronegative, heteroatom in the adjacent molecule, known as the hydrogen-bond acceptor.

This interaction is much weaker than an ionic bond or covalent bond, but stronger than the relatively weak van der Waals forces. It can occur between atoms in different molecules or between parts of the same molecule. The bonds have bond enthalpy values of 15-35 kJ/mol.

Intermolecular hydrogen bonds are the dominant type of H-bonding. These bonds increase a compound's boiling and melting points, as well as its solubility in water. They are the reason why NH3, H2O, and HF molecules boil at lower temperatures than the corresponding hydrides of Group 6 elements. The strength of this kind of bond increases with the number of water molecules that participate in it. This is because water molecules have their own polar groups, resulting in the formation of more permanent dipoles and higher permanent multipoles, which contribute to the attractive forces between them.

David Sunnyside
Co-founder of Urban Splatter • Digital Marketer • Engineer • Meditator
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