Phosphorus
Phosphorus is a chemical element with symbol P and atomic number 15. As an element, phosphorus exists in two major forms—white phosphorus and red phosphorus—but due to its high reactivity, phosphorus is never found as a free element on Earth. Instead phosphorus-containing minerals are almost always present in their maximally oxidised state, as inorganic phosphate rocks.
Identity
- Atomic mass
- 30.974 u
- Category
- Reactive non-metals
- Group
- 15
- Period
- 3
- Block
- p
- Oxidation States
- −3, +3, +5
- Phase (25 °C)
- Solid
- Appearance
- colourless, waxy white, yellow, scarlet, red, violet, black
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p3
- Short configuration
- [Ne] 3s2 3p3
- Electrons per shell
- 2, 8, 5
- Electronegativity (Pauling)
- 2.19
- Electron affinity
- 72.0 kJ/mol
- 1st ionization energy
- 1.01e+3 kJ/mol
- Ionization energies
- 1012 · 1907 · 2914 · 4964 · 6274… kJ/mol
Physical properties
- Melting point
- —
- Boiling point
- —
- Density
- 1.823 g/cm³
- Molar heat capacity
- 23.8 J/(mol·K)
Chemical behavior of phosphorus
Phosphorus has five valence electrons ([Ne] 3s2 3p3) and is the chameleon nonmetal of period 3: it exists as -3 in phosphides and phosphine, as +3 in halides and as +5 in phosphates, its stable biological form. Its middling electronegativity (2.19) gives polar covalent bonds. Allotropy rules its reactivity: white phosphorus (strained tetrahedral P4) burns alone in air, while polymeric red is manageable.
Reactivity
P₄ + 5 O₂ → P₄O₁₀
White phosphorus burns spontaneously in air with yellow flame and white smoke of pentoxide. This combustion underlies incendiary bombs and pure phosphoric acid synthesis.
P₄ + 6 Cl₂ → 4 PCl₃
With chlorine it gives phosphorus trichloride, which with more chlorine becomes pentachloride. These halides are key intermediates for pesticides and plastics.
Ca₃P₂ + 6 H₂O → 3 Ca(OH)₂ + 2 PH₃
Phosphides hydrolyze releasing phosphine (PH3), a foul, flammable and very toxic gas. It explains will-o'-the-wisps and the danger of fumigated grain silos.
P₄O₁₀ + 6 H₂O → 4 H₃PO₄
Pentoxide is a brutal desiccant that tears water even from other molecules to give phosphoric acid. Industry makes it this way for fertilizers and cola drinks.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Phosphoric acid | H₃PO₄ | Fertilizers, cola drinks and detergents |
| Adenosine triphosphate (ATP) | C₁₀H₁₆N₅O₁₃P₃ | Energy currency of all cells |
| Phosphine | PH₃ | Grain fumigant and chip dopant |
| Phosphorus pentoxide | P₄O₁₀ | Extreme desiccant and acid precursor |
| Calcium phosphate | Ca₃(PO₄)₂ | Bones, teeth and fertilizers |
Electron shell distribution
View in the atom builderIsotopes
All natural phosphorus is phosphorus-31 (31P), with sixteen neutrons: it is monoisotopic and stable. Phosphorus-32 (32P), with a 14-day half-life, is used as a radioactive tracer in molecular biology and in brachytherapy against tumors.
Occurrence and production
It is the eleventh crust element (about 0.1%) but essential for life: it appears as phosphates in Apatite, phosphorite and bones. It never exists free for its reactivity. It is obtained by reducing calcium phosphate with coke and silica in an electric furnace (white phosphorus) or as acid by the wet route.
History and origin of the name
Hennig Brand discovered it in 1669 distilling urine while seeking the philosopher's stone: he got a substance glowing in the dark and called it phosphorus (light-bearer). It was the first element found by modern method. Its glow fascinated Boyle and Leibniz; its incendiary white form marked 20th-century wars.
Applications and relevance of phosphorus
Uses of the element
Red phosphorus strikes safety matches and white lights tracers and smoke bombs. High-purity phosphorus dopes Semiconductor and makes LEDs. Its chemiluminescence still marks buoys and signs.
Uses of its compounds
Phosphates fertilize half the world's crops; ATP moves every muscle; phosphoric acid sparkles in colas.
Health, safety and environment
Biological role
It is vital: DNA carries phosphates in its backbone, ATP transports cell energy, and bones are calcium phosphate. Deficiency weakens growth; excess in diet competes with calcium.
Safety
White phosphorus is dreaded: it ignites in air, sticks burning to skin and phosphine kills by inhalation. Red phosphorus is of low toxicity.
Environment
Fertilizer and detergent phosphorus eutrophies rivers and seas, and phosphorite reserves cluster in few countries and may run short.
Frequently asked questions about Phosphorus
Why does phosphorus glow in the dark?
White phosphorus oxidizes slowly in air and that oxidation emits a cold greenish light, without heat: it is chemiluminescence, not magic. Brand saw it in distilled urine and thought he had captured light. Cemetery will-o'-the-wisps are burning phosphine the same way. Red phosphorus never glows.
Why is white phosphorus such a feared weapon?
Because it ignites on contact with air, sticks to skin while burning and keeps burning even underwater, causing very deep wounds, besides poisoning. Its thick white smoke blinds and burns lungs. Humanitarian groups seek to ban it as an incendiary weapon in populated areas.
Do cola drinks contain phosphorus?
Yes, phosphoric acid: it gives their characteristic acid snap and preserves. One can carries about 50 mg, harmless occasionally but worrying in daily abuse, for excess displaces calcium from bones. Nephrologists watch it in kidney patients.
Can the planet run out of phosphorus?
It is not destroyed, but its mines are: Morocco and the Sahara hold most phosphorite and some studies fear a phosphorus peak this century. Without it there are no fertilizers or harvests for 8 billion. The answer is recycling it from manure and treatment plants.
What has phosphorus to do with urine?
Its discovery: Brand boiled litres of urine seeking gold and got glowing phosphorus, for we excrete phosphates. For a century urine was its only source. Today the cycle closes in reverse: we want to recover phosphorus from wastewater to fertilize fields.
Would there be no life without phosphorus?
Literally: DNA carries phosphates in its backbone, ATP transports the energy of every cell, and bones and teeth are calcium phosphate. No other element does all three at once. So astronomers seek phosphorus when hunting life on distant worlds: no genes, energy or skeletons without it.
What is the eutrophication caused by phosphates?
The deadly feast of algae: phosphorus from fertilizers and detergents fertilizes rivers and lagoons, algae multiply unchecked, die and their decay exhausts oxygen, killing fish. Hence phosphates were banned from detergents. Over-fertilizing impoverishes: excess nutrient kills the whole ecosystem.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.