Phosphorus

Phosphorus is the element that makes plants grow, matches strike and DNA hold together. It was discovered in one of history's most bizarre experiments, by a man trying to make gold from urine, and its most unstable form glows in the dark.

  • Atomic Number1515 protons, 15 electrons
  • Atomic Mass30.97376200 uAbout 31× heavier than hydrogen
  • State at Room TempSolidsolid, several allotropes
  • Density1.82 g/cm³Denser than water in most forms
  • Melting / Boiling44.2°C / 280.5°CWhite phosphorus melts at just 44°C
  • Discovered1669Hennig Brand, 1669

How does phosphorus compare to its neighbours in mass?

Phosphorus sits between silicon and sulfur, close to the centre of the lighter half of the table.

Atomic Mass Comparison
Silicon28 u
Phosphorus31 u
Sulfur32 u
Chlorine35.5 u
Iron56 u

Phosphorus at 31 u sits just one step above silicon (28 u) and just below sulfur (32 u). These three elements form a run of Period 3 non-metals with very different properties despite similar masses.

What is phosphorus?

Phosphorus is a non-metal in Group 15, directly below nitrogen. It exists in several different forms called allotropes. White phosphorus is a waxy, semi-transparent solid that glows faintly in air through a slow chemical reaction with oxygen, ignites spontaneously at approx. 34°C and is extremely poisonous. Red phosphorus is much more stable and is what you find on the striking strip of a matchbox. Black phosphorus is the most stable form and looks rather like graphite.

Phosphorus gets its name from the ancient Greek word phosphoros, meaning light-bearer or morning star: the ancient name for the planet Venus before sunrise. It was chosen because white phosphorus glows faintly in air. The symbol P comes from the Latin phosphorus. Lavoisier was the first to recognise it as a true element in 1777.

Fact Phosphorus was the first element to be discovered in modern times, and its discoverer was not a trained scientist. In 1669, the German merchant Hennig Brand boiled down around 60 buckets of concentrated urine, trying to make gold. Instead he obtained a white substance that glowed eerily in the dark and burst into flames in air.

Where you find phosphorus

On Earth

Phosphorus is found mainly in phosphate rock, a sedimentary rock formed from the bones and shells of ancient marine organisms. Unlike most elements, phosphorus has no significant gaseous form, it stays locked in the ground or dissolved in the sea.

  • Phosphate rock. Most of the world's phosphorus is mined as calcium phosphate. Morocco holds approx. 70% of the world's economically useful reserves, a geopolitical advantage as important as oil.
  • Bones and teeth. The mineral hydroxyapatite (a calcium phosphate) makes up most of your bones and tooth enamel. About 85% of the phosphorus in your body is in your skeleton.
  • DNA and ATP. Phosphate groups form the backbone of DNA and RNA, linking the genetic letters together. ATP: the molecule that powers every cell, also contains phosphate groups.

How we use phosphorus

  • Fertilisers. About 80% of all phosphorus mined goes into fertilisers. Plants cannot grow without phosphorus, it is essential for roots, flowers and seeds. Without phosphate fertilisers, modern agriculture could not feed the world's population.
  • Matches. The striking strip on a matchbox contains red phosphorus. When you strike a match, friction converts a tiny amount of red phosphorus to white phosphorus, which ignites the match head.
  • Detergents. Phosphate compounds help detergents work by softening water. Many countries have now restricted phosphates in washing detergents because they cause algae blooms in rivers and lakes.
  • Flame retardants. Phosphorus compounds are widely used in plastics, textiles and furniture to slow down or prevent burning.
Did you know? Phosphorus is a finite resource that cannot be replaced. Unlike carbon or nitrogen, which cycle through the atmosphere, phosphorus has no atmospheric form and moves very slowly through the environment. Once phosphate fertiliser washes off fields into rivers and seas, it is essentially lost to agriculture. Many scientists warn that "peak phosphorus", where economically recoverable reserves run short, could arrive this century.

How it was discovered

Phosphorus was discovered in 1669 by Hennig Brand, a German merchant-alchemist from Hamburg. Searching for the legendary philosopher's stone that could turn metals to gold, Brand obtained enormous quantities of human urine and evaporated and heated them through a series of processes. What he eventually obtained was a white, waxy solid that glowed in the dark and caught fire spontaneously. Brand tried to keep his discovery secret but sold the process to other alchemists. Phosphorus became one of the most talked-about substances of the 17th century.

Deeper dive: phosphorus in biology and the phosphorus cycle

Phosphorus is essential for all life. DNA and RNA have a phosphate-sugar backbone that holds the genetic sequence together. ATP (adenosine triphosphate) is the universal energy currency of cells, when you break one of ATP's high-energy phosphate bonds, energy is released to power muscle contractions, nerve impulses and all the chemical reactions in your body. Without phosphorus, there would be no metabolism, no genetics and no life as we know it.

The phosphorus cycle is much slower than the carbon or nitrogen cycles because phosphorus has no gaseous form. It moves from rocks (via weathering), through soil and water, into plants, through the food chain, and eventually back to sediment on the sea floor, a journey that can take millions of years. Humans have dramatically accelerated part of this cycle by mining phosphate rock and spreading it on fields as fertiliser, then allowing excess phosphate to wash into waterways.

White phosphorus is one of the most hazardous materials known. It ignites spontaneously in air at around 34°C, burns at 1,300°C, and sticks to anything it touches. It was used as a horrific incendiary weapon in both World Wars. Even tiny amounts that penetrate skin continue to burn through tissue until fully consumed or deprived of oxygen.

Phosphorus is the element that fuels both plant growth and the genetic code of life. Moving one step along the row to 16 protons brings us to sulfur, an element humans have known since biblical times as brimstone.