Phosphorus (IPAEng: ˈfɒsfərəs) is the chemical element that has the symbol P and atomic number 15. The name comes from the (meaning "light") and lang: φόρος (meaning "bearer"). A multivalent nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks.

Due to its high reactivity, phosphorus is never found as a free element in nature on Earth. The first form of phosphorus to be discovered (white phosphorus, discovered in 1669) emits a faint glow upon exposure to oxygen — hence its name given from Greek mythology, lang: Φωσφόρος meaning "light-bearer" (Latin Lucifer), referring to the "Morning Star", the planet Venus.

Phosphorus is a component of DNA, RNA, ATP, and also the phospholipids which form all cell membranes. It is thus an essential element for all living cells. The most important commercial use of phosphorus-based chemicals is the production of fertilizers.

Phosphorus compounds are also widely used in explosives, nerve agents, friction matches, fireworks, pesticides, toothpaste and detergents.

Glow from white phosphorus

Phosphorus was discovered by German alchemist Hennig Brand in 1669. Working in Hamburg, Brand attempted to distill some kind of "life essence" from his urine, and in the process produced a white material that glowed in the dark. The phosphorus had in fact been produced from inorganic phosphate, which is a significant component of dissolved urine solids. White phosphorus is highly reactive and gives-off a faint greenish glow upon uniting with oxygen. The glow observed by Brand was actually caused by the very slow burning of the phosphorus, but as he saw no flame nor felt any heat he did not recognize it as burning.

It was known from early times that the glow would persist for a time in a stoppered jar but then cease. Robert Boyle in the 1680s ascribed it to "debilitation" of the air; in fact, it is oxygen being consumed. By the 18th century, it was known that in pure oxygen, phosphorus does not glow at all; there is only a range of partial pressure at which it does. Heat can be applied to drive the reaction at higher pressures.

In 1974, the glow was explained by R. J. van Zee and A. U. Khan.Emsley, John (2000). The Shocking History of Phosphorus. London: Macmillan. ISBN 0-330-39005-8 A reaction with oxygen takes place at the surface of the solid (or liquid) phosphorus, forming the short-lived molecules HPO and P₂O₂ that both emit visible light. The reaction is slow and only very little of the intermediates is required to produce the luminescence, hence the extended time the glow continues in a stoppered jar.

Although the term phosphorescence is derived from phosphorus, the reaction which gives phosphorus its glow is properly called chemiluminescence (glowing due to a cold chemical reaction), not phosphorescence (re-emitting light that previously fell onto a substance and excited it).