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.

• 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

• At 25^oC, nitrosyl fluoride (ONF) is a gas whereas water is a liquid. Why?
• ONF and water have about the same shape.
• ONF has a higher molecular weight (49 amu) than water (18 amu).
• Conclusion: London dispersion forces not responsible for the difference between these two compounds.
• ONF and water have the same dipole moment.
• Conclusion: dipole-dipole forces not responsible for the difference between these two compounds.
• ONF cannot form hydrogen bonds; water can form hydrogen bonds.

		= O = F = N = H
The structure of ONF.	The structure of H2O.
Microscopic view of ONF at 25oC.	Microscopic view of H2O at 25oC.