When highly electronegative elements form a covalent bond with the hydrogen atom, the electrons constituting the covalent bond are shifted towards the more electronegative atom. This results in a partial positive charge getting developed on the hydrogen atom which helps in the bond formation with the electronegative atoms of the other molecules. This particular bond is called the hydrogen bond and it is comparatively weaker than the covalent bond. We can see an example here; in HF molecule there is a hydrogen bond between hydrogen atom of one molecule and the fluorine atom of another molecule.
– – – H?+– F?- – – – H?+– F?-– – – H?+– F?-
In this case, the hydrogen bond acts as a bridge between two atoms, where one atom is held by a covalent bond and the other atom is held by a hydrogen bond. In the structure above, the hydrogen bond is depicted by the dotted line (—) and the covalent bond is shown by solid line.
So it can be said that a hydrogen bond is just an attractive force which binds the hydrogen atom of one molecule to the electronegative atoms (F, O or N) of another molecule.
Cause of hydrogen bond formation
Hydrogen atom is bonded with a highly electronegative element, and therefore the shared pair of electrons move away from the hydrogen atom towards the electronegative atom. Hydrogen atom becomes electropositive with respect to the electronegative element. This results in the development of positive charge over hydrogen atom and partial negative charge over the electronegative element. This further leads to the formation of a polar molecule with electrostatic force of attraction. The magnitude of H-bond depends on the physical state of the compounds. It reaches a maximum value in solid state and minimum in gaseous state.
Types of Hydrogen bonding
There are two types of H-bonds, and it is classified as the following:
- Intermolecular hydrogen bonding – This type of bond formation occurs between the different molecules of same or different compounds. For example- hydrogen bonding in water and alcohol.
- Intramolecular hydrogen bonding – This type of bond formation occurs when the hydrogen atom lies in between the two electronegative elements present in the same molecule.
|Compound||Molar Mass||Normal Boiling Point|
|H2O||18 g/mol||373 K|
|HF||20 g/mol||292.5 K|
|NH3||17 g/mol||239.8 K|
|H2S||34 g/mol||212.9 K|
|HCl||36.4 g/mol||197.9 K|
|PH3||34 g/mol||185.2 K|
We have seen the hydrogen bonds and the reason for its formation and its types. Hydrogen bond has a very powerful impact on the properties and structure of many compounds and is the reason for the anomalous expansion of water. For more details on this topic, download Byju’s Learning App.