The electrostatic force of attraction between the hydrogen atom of one molecule and more electronegative atom of the same or another molecule, is called hydrogen bond. When hydrogen atom gets bonded to atoms of highly electronegative elements such as fluorine, oxygen or nitrogen,the formed molecule is polar.Hence the hydrogen atom forms a weak bond with the electronegative atom of the other molecule. This weak bond is called 'hydrogen bond'.
The electron pair shared between the two atoms lies far away from the hydrogen atom. As a result, hydrogen atom (H) becomes highly electropositive with respect to the other atom (A). Since the electrons are displaced towards A, it acquires partial negative charge (d-) while hydrogen atom gets partial positive charge (d+). In other words, the bond H-A becomes polar and may be represented as Hd+- Ad-. The electrostatic force of attraction between positively charged hydrogen atom of one molecule and negatively charged atom of the neighbouring molecule results in the formation of hydrogen bond. This may be represented as:
The hydrogen atom acts as a bridge between two atoms, holding one atom by a covalent bond and the other atom by a hydrogen bond. The hydrogen bond is represented by dotted line (....) while the covalent bond is represented by solid line (
).
For example, in hydrogen fluoride HF, the hydrogen atom, while remaining bonded to its fluorine atom, forms another weak bond with fluorine atom of the neighbouring molecule. As a result of hydrogen bonding, HF exists as a cluster of hydrogen fluoride molecules and is represented as (HF)n.
The hydrogen bond may be shown as:
HF ...............HF ...............HF .......…..
Strength of hydrogen bond
Hydrogen bond is much weaker than a covalent bond. The strength of hydrogen bond ranges from 10-40 kJ mol-1 while that of a normal covalent bond is 400 kJ mol-1. Thus, a hydrogen bond is about one-tenth of the strength of a covalent bond. The bond length of a hydrogen bond is larger than that of a covalent bond. For example, in case of HF molecule, the covalent bond between H and F is 109 pm, while the bond length of hydrogen bond between F and H is 155 pm.
The following conditions are necessary for the formation of hydrogen bond:High electronegativity of atom bonded to hydrogen
The molecule should contain an atom of high electronegativity such as F, O or N bonded to hydrogen atom. The common examples are H2O and NH3.
Small size of the electronegative atom
The size of the electronegative atom should be small. The smaller the size of electronegative atom, the greater is the attraction for the bonded electron pair. This causes greater polarity in the bond between H and the electro- negative atom, resulting in stronger hydrogen bond.
For example, in NH3 and HCl, N and Cl have the same electronegativity (3.0) and both N and Cl have Hydrogen atoms, only NH3 shows hydrogen bonding as N is smaller in size than Cl.The main points of difference between hydrogen bond and covalent bond are:
