Hydrogen Bonding

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Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. It results from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as a N, O, or F atom and another very electronegative atom. Hydrogen bond strengths range from 4 kJ to 50 kJ per mole of hydrogen bonds.

In molecules containing N-H, O-H or F-H bonds, the large difference in electronegativity between the H atom and the N, O or F atom leads to a highly polar covalent bond (i.e., a bond dipole). The electronegativities are listed below.

element	electronegativity value
H	2.1
N	3.0
O	3.5
F	4.1

Because of the difference in electronegativity, the H atom bears a large partial positive charge and the N, O or F atom bears a large partial negative charge.

A H atom in one molecule is electrostatically attracted to the N, O, or F atom in another molecule.

= O

= N

= H

Hydrogen bonding between two water (H2O) molecules. Note that the O atom in one molecule is attracted to a H atom in the second molecule.

Hydrogen bonding between a water molecule and an ammonia (NH3) molecule. Note that the N atom in the NH3 molecule is attracted to a H atom in the H2O molecule.

Physical Consequences of Hydrogen Bonding