Phosphorus (P) — Atomic Number 15

Phosphorus is a chemical element with the symbol P and atomic number 15. It is a highly reactive non-metal existing in several allotropes. It is essential for life — found in DNA, RNA, ATP, and bones. The name comes from Greek phosphoros (“light-bringer”) — it glows in the dark.

At a Glance

Discovery

First element discovered in modern times with a recorded discoverer. In 1669, German alchemist Hennig Brand boiled down urine and heated the residue with sand, obtaining a white waxy substance that glowed in the dark.

Allotropes

White Phosphorus (P₄): Waxy, toxic, glows greenish, ignites spontaneously in air above ~30 °C. Stored under water. Lethal dose ~50 mg.

Red Phosphorus: Stable, non-toxic, dark red powder. Used on matchbox striking surfaces — friction converts a tiny amount to white P, which ignites.

Chemical Properties

1. Combustion: $4\text{P}+5{\text{O}}_2\to {\text{P}}_4{\text{O}}_{10}$
2. P₄O₁₀ is acidic: ${\text{P}}_4{\text{O}}_{10}+6{\text{H}}_2\text{O}\to 4{\text{H}}_3{\text{PO}}_4$ (phosphoric acid)
3. With chlorine: $2\text{P}+3{\text{Cl}}_2\to 2{\text{PCl}}_3$

Compounds & Uses

• Fertilisers (phosphates)
• Matches (red phosphorus)
• Flame retardants

Biological Role

Phosphorus is essential for DNA/RNA (phosphate backbone), ATP (energy transfer), and bones (as calcium phosphate).

IGCSE Essentials

1. Two allotropes: white P (toxic, ignites in air) vs red P (stable, matchboxes)
2. P₄O₁₀ is acidic — forms phosphoric acid with water
3. Stored under water (white P)

Sources

• Phosphorus — Wikipedia
• Phosphorus — Ptable.com