Atomic number 15

Phosphorus

P
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Shells Orbitals 3s 3p

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15 electrons in 3 shells: 2, 8, 5.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Nonmetal
Period: 3
Group: 15
Block: p
Standard atomic weight: 30.974(1) Da
Electron configuration: [Ne] 3s² 3p³
Phase at 25 °C: solid
Density: 1.82 g/cm³
Electronegativity (Pauling): 2.19
Discovered: 1669, Hennig Brand

Neighbours in the table

Si S

White phosphorus is made of P₄ molecules, each a regular tetrahedron with an atom at every corner, every atom joined to the other three by a single bond. OpenStax finds that unit in every state. Each phosphorus atom brings fifteen electrons, arranged 2, 8 and 5 by shell and written [Ne] 3s² 3p³. A group 15 atom is three electrons short of a full valence shell, so in the free element it forms three covalent bonds.

The white form is a waxy solid; OpenStax puts its melting point at 44.2 °C and Los Alamos at 44.15 °C. It catches fire in air, so it is kept under water. Heated without air, it turns red: OpenStax gives 270–300 °C, while Los Alamos gives 250 °C in its own vapour, or exposure to sunlight. In red phosphorus, OpenStax says, the P₄ tetrahedra appear to be joined by P–P single bonds into a three-dimensional network.

Red phosphorus is denser, melts near 600 °C, reacts far less readily and is, in OpenStax’s words, essentially nontoxic. Nor does it glow in air like the white form. Los Alamos lists four or more allotropes (white, also called yellow; red; and black), while OpenStax names three common ones. Black phosphorus, the least reactive, has no significant commercial use.

Where it is found

Phosphorus never occurs as the free element. OpenStax points out that it is alone in its group in this. The element is spread through minerals instead, about 1,050 milligrams per kilogram of Earth’s crust, and 0.06 milligrams per litre of seawater. Its ore is phosphate rock, which carries apatite, an impure calcium phosphate.

Most of that rock, the US Geological Survey says, is sedimentary, laid down as marine phosphorites. The biggest sedimentary deposits are in China, the United States, the Middle East and northern Africa; Brazil, Canada, Finland, South Africa and Russia have igneous ones. Morocco holds 50 billion of the world’s estimated 74 billion tons of reserves. The Survey sees no shortage coming.

Freeing the element takes a furnace. Calcium phosphate from the rock is heated with sand and coke; the products are liquid calcium silicate, carbon monoxide gas and P₄ vapour, which distils out of the furnace. The vapour is condensed to a solid, or burned to P₄O₁₀. White phosphorus is an industrial product; the safety file in PubChem notes that it does not occur naturally at all.

Why it matters

Hennig Brand prepared phosphorus from urine in 1669. CIAAW calls him a German merchant, Jefferson Lab a physician and alchemist. The Jefferson Lab text has him boiling, filtering and processing up to 60 buckets of urine. The name followed the glow in the dark that its first observers noticed. Greek phosphoros means light-bearing, and Los Alamos adds that it was an old name for Venus before sunrise.

For farming, the element matters as phosphate. Of the rock American mines produced in 2024, over 95 per cent became phosphoric acid, then ammonium phosphate fertilisers and animal feed supplements. The rest went to elemental phosphorus, turned into industrial compounds, chiefly the herbicide glyphosate. The Geological Survey states that agriculture has no substitute for phosphorus. Fertilisers used worldwide in 2024 contained an estimated 47.5 million tons of P₂O₅, the Survey estimates.

Red phosphorus goes into safety matches, and the compound P₄S₃ into the heads of strike-anywhere matches. Phosphor bronze and ferrophosphorus are alloys that need the element. Bone ash, a calcium phosphate, is used for fine china and for the monocalcium phosphate in baking powder. The radioactive isotope phosphorus-32 lets researchers follow processes inside plants. The IAEA’s atomic mass table gives its half-life as 14.269 days.

Properties

Properties of phosphorus Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	15
Standard atomic weight	30.974(1) Da	Da	IUPAC/CIAAW 2024
Melting point	44.2 °C	°C	PubChem: 317.3 K
Boiling point	280.50 °C	°C	PubChem: 553.65 K
Density	1.82 g/cm³	g/cm³
Electronegativity (Pauling)	2.19
Electron configuration	1s² 2s² 2p⁶ 3s² 3p³
Phase at 25 °C	solid

Both figures are for white phosphorus (Los Alamos National Laboratory).

Safety

Elemental phosphorus is hazardous, and how much depends on the form. White phosphorus is extremely toxic and catches fire in air a little above room temperature, more readily when the air is moist; long exposure damages the jaw. Red phosphorus is far less toxic, but the EU classifies it as a flammable solid that harms aquatic life over the long term. The PubChem Laboratory Chemical Safety Summary and the NIOSH card for white phosphorus are the sheets to consult.

Questions

Why is white phosphorus stored under water?

White phosphorus catches fire on contact with air, and OpenStax and the Jefferson Lab text in PubChem's element record both give that as the reason it is kept under water. Los Alamos adds that the solid does not dissolve in water. PubChem's safety data put its ignition at about 30 °C in moist air and higher in dry air. Red phosphorus is much less reactive, and Los Alamos notes that it does not ignite spontaneously in air.

Does phosphorus form compounds that nitrogen cannot?

Yes. Both elements sit in group 15, but OpenStax notes that nitrogen forms only NF₃ with fluorine, while phosphorus forms both PF₃ and PF₅. Nitrogen pentahalides do not exist, OpenStax explains, because a nitrogen atom cannot form more than four bonds. Phosphorus also forms compounds with phosphorus–phosphorus bonds, such as P₄S₃, which OpenStax places in the heads of strike-anywhere matches. Solid phosphorus pentachloride turns out to be ionic, built from PCl₄⁺ and PCl₆⁻ ions.

Where does phosphate fertiliser come from?

From phosphate rock, mined at about 240 million tons worldwide in 2024, according to the US Geological Survey. China, Morocco, the United States and Russia were the largest producers, in that order. American rock came from Florida, Idaho, North Carolina and Utah, and the Survey lists no recycling of it. Almost all of the rock mined in the United States is turned into phosphoric acid before it becomes fertiliser or animal feed.

Why does the body need phosphorus?

Calcium and phosphorus are the most common minerals in the body, and OpenStax's Anatomy and Physiology says both are stored in the skeleton, where they harden bone. Its nutrition table adds metabolism and ATP production to bone formation, names meat and milk as sources and calls a deficiency rare. Los Alamos also finds phosphorus in nervous tissue and in the protoplasm of every cell.

Compounds containing phosphorus

H₃PO₄ Phosphoric acid 8 atoms, 7 bonds.
C₃H₈NO₅P Glyphosate 18 atoms, 17 bonds.
C₁₀H₁₆N₅O₁₃P₃ Adenosine triphosphate 47 atoms, 49 bonds.