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

Phosphorus has 15 electrons, one for every proton, so the atom carries no charge. Below are the same 15 electrons written two ways.

Simple form: electrons per shell

shell 12shell 28shell 35

Counting outward from the nucleus: 2, 8, 5. The outer shell is picked out.

Orbital form: electrons per subshell

1s² 2s² 2p⁶ 3s² 3p³

Short form: [Ne] 3s² 3p³, where [Ne] stands for the 10 electrons neon already has.

3s²↑↓

3p³↑↑↑

The boxes hold the electrons beyond [Ne]. Each box is one orbital and takes two electrons at most, spinning opposite ways. They spread out singly before they pair up.

How to read the two

The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 2 here is 2s² 2p⁶, and 2 + 6 is 8.

Phosphorus sits in period 3 and has 3 shells. The period number counts the shells.

What that means for bonding

The outer shell holds 5 electrons. Those are the ones phosphorus can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 15 means 5.

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.