Phosphorus—an element important to all living organisms—is the second element in Group 15 of the periodic table. The elements in this group are sometimes referred to as the nitrogen family of elements. Phosphorus has an atomic number of 15, an atomic mass of 30.97376, and a chemical symbol of P.
Phosphorus forms the basis of a large number of compounds, by far the most environmentally important of which are phosphates. All plants and animals need phosphates for growth and function, and in many natural waters the production of algae and higher plants is limited by the low natural levels of phosphorus. As the amount of available phosphorus in an aquatic environment increases, plant and algal growth can increase dramatically leading to eutrophication.
Phosphorus exists in three allotropic forms, named after their colors: white (or yellow) phosphorus; red phosphorus; and black (or violet) phosphorus. White phosphorus is a waxy, transparent solid with a melting point of about 111°F (44.1°C), a boiling point of about 536°F (280°C), and a density of 1.09 ounces per cubic inch (1.88 grams per cubic centimeter). If kept in a vacuum, white phosphorus sublimes if exposed to light. White phosphorus is also phosphorescent, giving off a beautiful greenish white glow. It does not dissolve in water, although it does dissolve in many organic liquids, such as benzene, chloroform, and carbon disulfide. White phosphorus sometimes appears slightly yellowish because it contains traces of red phosphorus.
Red phosphorus is a red powder produced by heating white phosphorus in the presence of a catalyst. Red phosphorus does not melt when heated, but sublimes at a temperature of about 781°F (416°C). Its density is 1.35 ounces per cubic inch (2.34 grams per cubic centimeter). It does not dissolve in most liquids.
Black phosphorus looks like graphite. It can be made by exposing the white allotrope of phosphorus to high pressures. The black allotrope has a density of 2.06 to 2.21 ounces per cubic inch (3.56 to 3.83 grams per cubic centimeter). One of its unusual properties is that it conducts an electric current (a common property of metals) even though it is a nonmetal.
White phosphorus is the most active form of the element, with a tendency to catch fire spontaneously at room temperature. For this reason, white phosphorus is usually stored under water in chemical laboratories as a safety precaution. All allotropes of phosphorus also combine with the halogens and with metals to form compounds known as phosphides: 3Mg + 2P → Mg3P2 (magnesium phosphide).
Occurrence and Extraction
The abundance of phosphorus in the Earth's crust is estimated to be 0.12%, making it the 11th most common element. Its estimated crustal abundance is 1.68 x 10-2 ounces per pound (1.05 x 103 milligrams per kilogram). It usually occurs as a phosphate, such as calcium phosphate [Ca3(PO4)2], the major component of phosphate rock. The United States is the largest producer of phosphate rock in the world, with about 13,000,000 metric tons being mined each year. That amount is about a third of the world's total phosphate rock. Nearly 90% of phosphate rock comes from two states: North Carolina and Florida. Other producers of phosphate rock include Morocco, China, Russia, Tunisia, Jordan, and Israel. Phosphorus's estimated oceanic abundance is 8 x 10-6 ounces per gallon (6 x 10-2 milligrams per liter).
Discovery and Naming
Phosphorus was discovered in 1669 by the German physician and alchemist Hennig Brand (c. 1630–c. 1692). Brand is famous in the history of chemistry as being the last of the alchemists. Brand was convinced that the key to changing base metals into gold could be found in human urine. In the process of heating and purifying urine for his experiments, he obtained a white wax substance that glowed in the dark. The element was named for this phenomenon, the process of phosphorescence. In fact, phosphorus comes from the Greek word phosphoros meaning light bearing. It was given this name because many of its compounds are phosphorescent; that is, they store light and emit it later.
The dangerous properties of elemental phosphorus were discovered early in Brand's research. The story is told that one of his servants left some phosphorus on top of Brand's bed. Later that night, the bed covers burst into flame when the phosphorus caught fire spontaneously.
Elemental phosphorus has relatively few uses because of its tendency to ignite spontaneously. However, many of its compounds are widely used. More than 90% of all phosphate rock mined in the United States is converted to synthetic fertilizer. Phosphorus is one of three macronutrients needed by growing plants, the other two being nitrogen and potassium. The production of synthetic fertilizers to meet the needs of farmers in the United States and around the world is now by far the greatest application for compounds of phosphorus.
Another familiar, although much less important, use of phosphorus is the manufacture of wood and paper safety matches. Phosphorus pentasulfide (P2S5) and phosphorus sesquisulfide (P4S3) are used to coat the tip of the match, providing a material that ignites easily when scratched. Another compound of phosphorus with many uses is phosphorus oxychloride (POCl3), which is used in the manufacture of gasoline additives, in the production of certain kinds of plastics, as a fire retardant agent, and in the manufacture of transistors for electronic devices.
Phosphorus is essential to the health of plants and animals. One of the most important compounds in living cells is adenosine triphosphate (ATP), an energy-carrying molecule that makes possible many of the chemical reactions that take place in cells. Phosphorus is also critical to the development of strong bones and teeth in the form of the compound fluorapatite. Finally, phosphorus is a critical component of nucleic acids that carry genetic information in cells and that provide the instructions which direct the activities of those cells.
On the other hand, elemental phosphorus is highly toxic. Swallowing even a speck of white phosphorus can cause diarrhea with loss of blood; damage to the liver, stomach, intestines, and circulatory system; and coma. A piece of white phosphorus no larger than 50 to 100 milligrams (about 0.0035 ounce) can even cause death. Interestingly, red phosphorus seems to be much less toxic than white phosphorus.
In the past, one of the major contributors to phosphorus pollution was household detergents containing phosphates. These substances have now been banned from these products. Other contributors to phosphorus pollution are sewage treatment plants and runoff from cattle feedlots. (Animal feces contain significant amounts of phosphorus.) Erosion of farmland treated with phosphorus fertilizers or animal manure also contributes to eutrophication and water pollution.