One form of interaction between molecules are hydrogen bonds, which are a special case of dipole forces. When a hydrogen atom is bonded to a small, electronegative atom such as oxygen, nitrogen or fluorine, the highly electronegative atom draws off most of the electron density around the hydrogen, leaving a partially bare proton. When this proton closely approaches the small, electronegative atom on another molecule, a strong dipole force is created and holds the molecules together.

Hydrogen bonding is critical for life in many ways. Solid water H₂O is hydrogen bonded, which explains its very high melting and boiling point as compared to similar molecules such as H₂S and H₂Se.

In addition, biological molecules such as many proteins are held together with hydrogen bonds, which help determine their shape. 

Example: Which of the following molecules are hydrogen bonded? NH₃, H₂O, HF, HCl, PH₃, dimethyl ether (CH₃OCH₃).

_Solution: Hydrogen bonding occurs between when a molecule contains hydrogen bonded to a small electronegative atom such as N,O, or F. Of the above, NH₃, H₂O and HF all meet the criteria and contain hydrogen bonds. HCl and PH₃ contain hydrogen but not a small electronegative atom, and dimethyl ether contains both hydrogen and oxygen, but the two are not bonded.

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