Hydrogen Bonding

This is a type of intermolecular force that is significantly stronger than other types of intermolecular force (such as van der Waal's). It is about 10 times weaker than a covalent bond, and can occur with in a molecule (such as in DNA).

Hydrogen bonding only occurs in very specific cases when a hydrogen atom is covalently bonded to one of three electronegative elements oxygen, fluorine or nitrogen.

The hydrogen atom lies between 2 highly electronegative atoms. It has a covalent bond to one, and a hydrogen bond to the other. This is caused by the high polarity of the covalent bond, the small hydrogen atom is strongly attracted to the electronegative atom and can get in close due to its small size.

Hydrogen bonding accounts for the following phenomena:

• open structure of ice

• high boiling points of HF, NH₃ and water.

• the pairing of base pairs in DNA.

Your syllabus specifies that you need to understand how this accounts for some unique properties of water. Click here for this page (or the next button).