Animated Solution for Chemistry - s and p-Block Elements: Which one of the following is formed (mainly) when red phosphorus is heated in a sealed tube at $803\text{ K}$ ?

Phosphorus is a highly versatile element that exists in several distinct structural forms, known as allotropes. The most commonly studied allotropes in chemistry are white phosphorus, red phosphorus, and black phosphorus. Each of these forms has unique physical and chemical properties, largely dictated by how the phosphorus atoms are bonded to one another.

White phosphorus is highly reactive and consists of discrete $P_4$ tetrahedral molecules. Red phosphorus is a polymeric chain of $P_4$ tetrahedra, making it much more stable and less reactive than the white form. Black phosphorus, on the other hand, is the most thermodynamically stable allotrope and has a layered, flaky structure similar to graphite.

Black phosphorus itself exists in two distinct crystalline forms: $\alpha$-black phosphorus and $\beta$-black phosphorus. The key to mastering this topic for competitive exams like JEE is remembering the specific starting materials and conditions required to synthesize each form.

When red phosphorus is heated in a sealed tube at a specific temperature of $803\text{ K}$, it undergoes a structural rearrangement to form $\alpha$-black phosphorus. This form is opaque, does not conduct electricity, and can be sublimed in a vacuum.

In contrast, $\beta$-black phosphorus is synthesized by heating white phosphorus at $473\text{ K}$ under extremely high pressure (around $12000\text{ atmospheres}$). $\beta$-black phosphorus is a moderate conductor of electricity and has a corrugated sheet structure.

Returning to our specific problem, the question explicitly asks for the product formed when red phosphorus is heated in a sealed tube at $803\text{ K}$. Based on our understanding of the phase transitions of phosphorus, this specific set of conditions exclusively yields $\alpha$-black phosphorus.

Therefore, the correct answer is option (d).