Phosphorus

Discovered by an alchemist boiling urine, it glows in the dark in one form, catches fire in warm air, and holds your DNA together.

At a glance

Symbol: P
Atomic number: 15
Average atomic mass: 30.97
Group: 15
Period: 3
Family: nonmetal
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p³, or [Ne] 3s² 3p³
Valence electrons: 5
Electronegativity: 2.19
Common ions: P³⁻ in a few phosphides; mostly +5, as in the phosphate ion PO₄³⁻

On the table

Period 3, group 15. The other nonmetals are tinted, and every cell opens its own article.

Electrons and bonding

Five valence electrons. Phosphorus mostly shares electrons in covalent bonds. White phosphorus is made of P₄ molecules, four atoms at the corners of a pyramid, whose strained bonds make it very reactive.

Properties

State at 25 °C: solid
What it looks like: white phosphorus is a waxy, pale yellow solid that glows in air; red phosphorus is a dark red powder
Melting point: 44.2 °C
Boiling point: 280.5 °C
Density: 1.82 g/cm³
Figures above: for white phosphorus, the most reactive form

Where it is found

Never free in nature. It is mined as phosphate rock, mostly in Morocco, China and the United States, and it is in every living cell: the backbone of DNA, the ATP that carries energy, and the calcium phosphate of bones and teeth.

Discovery and name

Hennig Brand, a Hamburg alchemist looking for the philosopher's stone, boiled down large amounts of urine in 1669 and was left with a white substance that glowed in the dark. He named it from the Greek phosphoros, light-bearer. It was the first element whose discoverer is known by name.

What it is used for

Nearly all mined phosphate becomes fertilizer, since crops need phosphorus and most soils run short of it. It also goes into animal feed, detergents, steel and food: phosphoric acid gives colas their tang. Red phosphorus on the side of a matchbox lights safety matches.

In compounds

Ca₃(PO₄)₂calcium phosphate, in boneH₃PO₄phosphoric acidP₄O₁₀phosphorus pentoxide, a drying agentPCl₃phosphorus trichloride(NH₄)₃PO₄ammonium phosphate, a fertilizerPH₃phosphine, a toxic gas

Handle with care

White phosphorus catches fire on its own in air at about 30 °C and causes deep burns, so it is stored under water. It is also highly toxic. Red phosphorus is far safer.