Group 15 elements — comparison of properties and allotropes of phosphorus

Question

Compare the properties of Group 15 elements (N, P, As, Sb, Bi). Describe the allotropes of phosphorus and explain why white phosphorus is more reactive than red phosphorus.

(NCERT Class 12, Chapter 7)

Solution — Step by Step

Property	Trend down the group
Atomic radius	Increases (N < P < As < Sb < Bi)
Ionization energy	Decreases
Electronegativity	Decreases (N is most electronegative)
Metallic character	Increases (N, P = non-metals; As, Sb = metalloids; Bi = metal)
Oxidation states	+5, +3, -3 all possible; stability of +3 increases down group (inert pair effect)
Bond with itself	N forms $\text{N}\equiv\text{N}$ triple bond; P forms single bonds ( $\text{P}_4$ )

Nitrogen is unique because of its small size and ability to form strong $p\pi$ - $p\pi$ multiple bonds. Heavier elements prefer single bonds and $d\pi$ - $p\pi$ bonding.

White phosphorus ( $\text{P}_4$ ): Tetrahedral molecules with P-P-P bond angle of 60° (highly strained). Waxy, translucent, glows in dark (chemiluminescence). Very reactive, catches fire in air. Stored under water. Toxic.

Red phosphorus: Polymeric chains formed by breaking one P-P bond in each $\text{P}_4$ unit and linking tetrahedra together. Less strained, less reactive, non-toxic, does not glow. Stable in air.

Black phosphorus: Layered structure (like graphite). Most stable allotrope. Semiconductor. Formed under high pressure.

The $\text{P}_4$ tetrahedron has P-P-P bond angles of 60° — much less than the ideal 109.5° for $sp^3$ hybridisation. This extreme angle strain makes the bonds weak and the molecule unstable. The strained bonds break easily, making white phosphorus highly reactive.

Red phosphorus has a polymeric chain structure where the strain is relieved (bond angles closer to 100°). The extended network also requires breaking more bonds to initiate reaction, making it kinetically stable.

Why This Works

Allotropy in phosphorus is driven by the tension between the element’s preference for single bonds (unlike nitrogen’s triple bond) and the geometric constraints of different arrangements. The $\text{P}_4$ tetrahedron is the simplest cluster but pays a heavy price in strain energy. Polymerisation into red or black forms relieves this strain at the cost of entropy.

The reactivity difference has practical implications: white phosphorus is used in incendiary devices and smoke screens, while red phosphorus is safe enough to use on matchbox striking surfaces.

Alternative Method

You can also compare allotropes using thermodynamic stability. The order is: black P (most stable) > red P > white P. White phosphorus spontaneously converts to red on heating above 250°C or on exposure to sunlight. This conversion is irreversible under normal conditions.

For NEET, the most commonly asked facts about Group 15: (1) why $\text{N}_2$ is a gas but $\text{P}_4$ is a solid (N forms triple bond, P forms weaker single bonds needing cluster formation), (2) why BiH $_3$ is the least stable hydride (weakest Bi-H bond), (3) inert pair effect explains why Bi prefers +3 over +5.

Common Mistake

Students often state “nitrogen cannot form $\text{N}_4$ like phosphorus forms $\text{P}_4$ because nitrogen is smaller.” The real reason is that nitrogen forms very strong $p\pi$ - $p\pi$ triple bonds ( $\text{N}\equiv\text{N}$ , 946 kJ/mol) that are much more stable than three single N-N bonds (3 $\times$ 163 = 489 kJ/mol). For phosphorus, the triple bond is weak (less effective $p\pi$ overlap for larger atoms), so single bonds in $\text{P}_4$ are preferred.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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