Phosphorus (P)

Phosphorus was the first element discovered by a named person, and the method was grotesque: in 1669 Hennig Brand boiled down roughly 5,500 litres of human urine looking for the philosopher's stone and got a waxy solid that glowed in the dark. White phosphorus ignites in air at about 30 °C and is stored under water; red phosphorus, the same element rearranged, is stable enough to print on a matchbox. Phosphorus has no substitute in biology — it is the P in ATP and in the backbone of DNA — and unlike nitrogen it cannot be pulled from the air, so the world depends on mining a finite number of phosphate rock deposits.

Properties

Atomic number

15

Symbol

P

Standard atomic weight

30.974 u

Category

Nonmetal

Group

15

Period

3

Block

p-block

Electron configuration

[Ne] 3s² 3p³

Electrons per shell

2, 8, 5

Melting point

317.3 K — 44.2 °C — 111.5 °F

Boiling point

553.65 K — 280.5 °C — 536.9 °F

Density

1.82 g/cm³

Electronegativity

2.19 (Pauling)

Oxidation states

+5, +3, -3

Discovered

1669 by Hennig Brand

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p³

Noble gas shorthand

[Ne] 3s² 3p³

Shells

K: 2 · L: 8 · M: 5

What it is used for

• Phosphate fertiliser, which is most of world production
• The striking strip on a matchbox, as red phosphorus
• Phosphoric acid in cola drinks and rust converters
• Flame retardants in furniture foam and electronics
• Doping silicon to make n-type semiconductors

Hazards

• White phosphorus ignites spontaneously in air and causes deep, slow-healing burns
• White phosphorus is acutely toxic by ingestion at doses under 100 mg
• Phosphate runoff drives algal blooms and dead zones in rivers and coastal water

The melting and boiling points given are for white phosphorus. Red phosphorus does not melt at ordinary pressure — it sublimes around 690 K — and the allotropes differ so much that quoting one figure for "phosphorus" is always an approximation.

Neighbours in the table

Group 15

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Phosphorus sits in group 15 with nitrogen, arsenic, antimony, bismuth, moscovium.