Phosphorus exhibits allotropy (i.e., it has multiple forms in the same physical state); the physical constants given above are for the common white phosphorus. White phosphorus is an extremely poisonous, yellow to white, waxy, solid substance, nearly insoluble in water but very soluble in carbon disulfide. When exposed to air it ignites spontaneously, burning to form white fumes of phosphorus pentoxide, P₂O₅. Because of its toxicity and pyrophoric nature, phosphorus is stored underwater. Contact with the skin may cause burns. White phosphorus is phosphorescent (i.e., glows without emitting heat).

When white phosphorus is heated to about 250°C in the absence of air, it changes into the more stable red phosphorus. This form appears as dull, reddish-brown cubic crystals or amorphous powder. Its specific gravity is 2.34. The red form is less dangerous than the white form, but should be handled with caution. It is insoluble in carbon disulfide and most other solvents. It does not ignite unless heated to about 200°C, does not phosphoresce, and is not poisonous. Another form of phosphorus is black phosphorus, a crystalline electrically conductive material similar to graphite in appearance. It was first prepared by P. W. Bridgman by heating white phosphorus to 200°C under a pressure of 12,000 atmospheres. Its specific gravity is 2.70.

Because of its chemical activity phosphorus does not occur uncombined in nature but is widely distributed in many minerals. A major source is apatite, an impure calcium phosphate mineral found in phosphate rocks. In the United States major deposits are found in Florida, Tennessee, Montana, and Idaho. White phosphorus is prepared commercially from phosphate rock in an electric furnace or blast furnace. The principal use of phosphorus is in compounds; for this reason, most of the phosphorus produced in furnaces is burned to make phosphorus pentoxide, a white powdery substance. While the pentoxide is used as a drying agent and chemical reagent, it is chiefly converted to phosphoric acid, H₃PO₄, also called orthophosphoric acid, by reaction with water. Another important source of phosphoric acid is from phosphate rocks by treatment with sulfuric acid; this is the so-called wet-acid process.

Phosphorus is present in plants and animals. There is over 1 lb (454 grams) of phosphorus in the human body. It is a component of adenosine triphosphate (ATP), a fundamental energy source in living things. It is found in complex organic compounds in the blood, muscles, and nerves, and in calcium phosphate, the principal material in bones and teeth. Phosphorus compounds are essential in the diet. Organic phosphates, ferric phosphate, and tricalcium phosphate are added to foods. Dicalcium phosphate is added to animal feeds.

White phosphorus is used as a deoxidizing agent in the preparation of steel and phosphor bronze. It is also used in rat poisons and to make smoke screens (by burning) for warfare. Red phosphorus is used in making matches. The major use of phosphorus compounds is in fertilizers, especially in a mixture called superphosphate, obtained from phosphate minerals by sulfuric acid treatment; and in nitrophosphates. Phosphorus compounds are also used commercially in detergents, water softeners, pharmaceuticals, dentifrices, and in many other less important uses. Toxic nerve gases such as sarin contain phosphorus.

Phosphoric acid is primarily used in the production of phosphate compounds. It is also used in pickling metals, in sugar refining, and in soft drinks. Phosphorus forms a number of compounds with the halogens, e.g., the trichloride, PCl₃, and the pentachloride, PCl₅, both used as reagents. It also forms an oxychloride, POCl₃. It reacts with sulfur to form a pentasulfide, P₂S₅, and a thiochloride, PSCl₃, used in insecticides and oil additives. Phosphine, PH₃, is a poisonous gas. Besides the pentoxide, phosphorus forms several other oxides; there are several acids other than the orthophosphoric acid noted above. Phosphorus also combines with various other nonmetals and with some metals.