Hydrogen Bond
Hydrogen bond is the force binding the hydrogen atom of one molecule with highly electronegative atom (F, O, or N) of another molecule:
It is a weak bond. It is of two type:
1) Intermolecular hydrogen bonding.
2) Intramolecular hydrogen bonding.
Intermolecular Hydrogen Bonding:
It is formed between two different molecule of the same substance, e.g. between the molecules of hydrogen fluoride. It results into association of molecule and increases melting point, boiling point etc.
Intramolecular Hydrogen Bonding:
It is formed between hydrogen atom and the highly electronegative atom present in the same molecule. It prevents association and does not affect physical properties.
Important feature of H-bonding:
1) Hydrogen bonding decreases the volatility and increases the viscosity and surface tension of a substance.
2) H-bond is much stronger than van der Waals forces but weaker than a covalent bond and an ionic bond.
3) Because of hydrogen bonding water has higher boiling point than that of hydrogen sulfide.
4) Ice has lower density than water due to the formation of open cage-like tetrahedral structure because of formation of hydrogen bonds in water molecules.
5) The marked difference between the melting point and boiling point of alcohols is also due to H-bonding.