Nitrogen and Phosphorus are both non-metallic elements in Group 15. While Phosphorus can form compou

Question

Nitrogen and Phosphorus are both non-metallic elements in Group 15. While Phosphorus can form compounds like $\mathrm{PCl}_5$ and $\mathrm{PCl}_6^{-}$, Nitrogen's highest observed covalency is different. Additionally, the $\mathrm{N}-\mathrm{N}$ single bond is significantly weaker than the P- P single bond. What is the maximum covalency of Nitrogen, and what is the primary reason for the weaker N- N bond compared to $\mathrm{P}-\mathrm{P}$ ?

Professor PC Thomas Classes · Answer

Nitrogen is an element of Group 15 with atomic number 7 . Its electronic configuration is $1 s^2 2 s^2 2 p^s$. In its valence shell ( $\mathrm{n}=2$ ), it has only s and p orbitals. It can form three covalent bonds by sharing its three unpaired p-electrons. Additionally, it can use its lone pair of electrons in the 2s orbital to form one coordinate bond. Therefore, the maximum covalency of Nitrogen is 4 (e.g., in $\mathrm{NH}_4^{+}$ion). Nitrogen cannot expand its octet beyond 4 because of the absence of vacant d-orbitals in its valence shell. Phosphorus, on the other hand, is also in Group 15 but has vacant 3d-orbitals in its valence shell. This allows phosphorus to expand its octet and exhibit a covalency of 5 (e.g., in $\mathrm{PCl}_5$ ) or even 6 (e.g., in $\mathrm{PCl}_6^{-}$).

Regarding the bond strength, the $\mathrm{N}-\mathrm{N}$ single bond is significantly weaker than the $\mathrm{P}-\mathrm{P}$ single bond. This is primarily due to the small atomic size of Nitrogen. When two small nitrogen atoms are bonded together, the non-bonding lone pairs of electrons on each nitrogen atom are in close proximity. This leads to significant interelectronic repulsion between these lone pairs, which destabilizes the $\mathrm{N}-\mathrm{N}$ single bond and makes it weaker. For phosphorus, due to its larger atomic size, the lone pairs are further apart, and the inter-electronic repulsion is much less, resulting in a stronger $\mathbf{P}-\mathbf{P}$ single bond.
Thus, option C correctly states the maximum covalency of Nitrogen and the primary reason for the weaker $\mathbf{N}-\mathbf{N}$ bond.