Question

Explain why in spite of having nearly the same electronegativity, why Nitrogen can form hydrogen bonds while Chlorine cannot.

Answer 1

Hint: Nitrogen and Chlorine belong to the 2nd and 3rd periods of the periodic table respectively. If we know that their electronegativities are fairly similar, then think of what properties would differ between the elements of the 2nd and 3rd period.

Complete step by step solution:
Before we begin to answer this question, let us first try and understand what Hydrogen bonding really means.
“Hydrogen bonding is a special type of attraction where dipole-dipole forces act between molecules. It results from the attractive force when a hydrogen atom is covalently bonded to a very electronegative atom such as N, O, or F atom which have high electronegativity. Strengths of hydrogen bond range from 4 kJ to 50 kJ per mole.
The large difference in electronegativity between the H atom and the N, O or F atom leads to a highly polar covalent bond in molecules which contain N-H, O-H or F-H bonds. The electronegativities are listed below.

Element	Electronegativity
H	2.1
N	3.0
O	3.5
F	4.1

- With this in mind, and with the knowledge that the electronegativity of Chlorine is 3.16 (higher than that of Nitrogen), let us now analyse why it still cannot undergo Hydrogen bonding in its compounds.
- In spite of nearly the same electronegativity, nitrogen forms hydrogen bonding while chlorine does not, the reason behind this is the small size of nitrogen atom as compared to the chlorine atom. The small size makes electron density per volume higher which results in greater dipole-dipole interaction.
- Thus we can conclude that due to less electron density, chlorine cannot form hydrogen bonds like nitrogen.

Note: The example of HCl is not to be confused with hydrogen bonding as it does not meet the criteria of hydrogen bonding which is “H should be bonded to lone pair of electrons on the above mentioned highly electronegative elements but in this case it is bonded to Cl.”