Question

Which of the following statements is/are true?

• A. Among the group 15 elements, Nitrogen has the lowest tendency for catenation
• B. Among the group 15 elements, Phosphorus has the highest tendency for catenation
• C. Both Nitrogen and Phosphorus do not exhibit catenation at all
• D. Both Nitrogen and Phosphorus have very similar trends in terms of ability of catenate

Answer 1

Answer: (A), (B)

The correct options are
A Among the group 15 elements, Nitrogen has the lowest tendency for catenation
B Among the group 15 elements, Phosphorus has the highest tendency for catenation

Thermodynamic stability gives a beautiful explanation as to why Nitrogen exists as $N_2$ molecule while the others exist as $P_4$, $A{s}_4$, etc.
Nitrogen atom is small in size and has the tendency to form $p\pi -p\pi$ multiple bonds. In fact, the Nitrogen – Nitrogen triple bond in $N_2$ molecule has very high thermodynamic stability. It has a dissociation energy of $942kJ.mo{l}^{-1}$.
Just consider the N - N in the case of hydrazine $N_2{H}_4$, the other hydride of Nitrogen. The shape and geometry of each Nitrogen atom is very similar to the N atom in the $N{H}_3$ molecule. Logically, Nitrogen being a small atom, should imply a strong $\sigma$ overlap in the N - N. But the reality is just the opposite. Due to the increased repulsions between the lone pair(s) and the bond, the N - N is weak. This is why the extent of catenation for Nitrogen is much lower than the other group-15 elements.
Contrarily, Phosphorus is a larger molecule and does not form stable multiple bonds between its own kind except at very high temperatures. $P_{11}^{3-}$ and other puckered layer structures that have extended P – P have been known for a long time. Unlike the unstable N – N bond, single bonds between atoms of Phosphorus thrive with stability. Even the other elements show greater catenation than Nitrogen.
To sum it up, P has the highest tendency to catenate while N has the lowest (among group 15 elements). Also, as we go down the group (15), the stability of the Element – element single bond increases while the stability of element – element multiple bond decreases.