Phosphorus (Molecule of the Month for November 2005)
White, Red, Black Phosphorus, WP
Phosphorus was discovered in 1669 by Brand, who prepared it from urine. Brand attempted to distill salts by evaporating urine, and in the process produced a white material that glowed in the dark and burned brilliantly. Since that time, phosphorescence has been used to describe substances that shine in the dark without burning. It was named from the Greek words meaning 'light' and 'bring' and the word phosphoros used ancient name for the planet Venus when appearing before sunrise and the Latin for 'morning star'.
Phosphorus exists in several allotropic forms: white (or yellow), red, and black (or violet). The common form is white phosphorus, a waxy white solid; when pure it is colorless and transparent. It is insoluble in water, but soluble in carbon disulfide and has two modifications: alpha and beta with a transition temperature at -3.8C. The most common are red and white phosphorus, both of which consist of networks of tetrahedrally arranged groups of four phosphorus atoms. The tetrahedra of white phosphorus form separate groups; the tetrahedra of red phosphorus are linked into chains.
Never found free in nature, it is widely distributed in combination with minerals. Phosphate rock, which contains the mineral apatite, an impure tri-calcium phosphate, is an important source of the element. Large deposits are found in Russia, in Morocco, and in Florida, Tennessee, Utah, Idaho, and elsewhere. Phosphorus is an essential ingredient of all cell protoplasm, nervous tissue, and bones.
White phosphorus spontaneously catches fire burning in air to produce pentoxide. It is very poisonous, 50 mg constituting an approximate fatal dose. White phosphorus should be kept under water, as it is dangerously reactive in air, and it should be handled with forceps, as contact with the skin may cause severe burns. White phosphorus is used in military applications as incendiary bombs, for smoke-screening as smoke pots and smoke bombs, and in tracer ammunition. Sadly it is also used as a chemical weapon, the firing of phosphorus bombs at people causes horrific burns. The Geneva Conventions ban the use of white phosphorus as an incendiary weapon against civilian populations and in air attacks against military forces in civilian areas. Israel has for the first time admitted it used controversial phosphorus bombs during fighting against Hezbollah in Lebanon in July and August 2006. US troops used white phosphorus as a weapon in last year's offensive in the Iraqi city of Falluja, the US has said. "It was used as an incendiary weapon against enemy combatants," spokesman Lt Col Barry Venable told the BBC - though not against civilians, he said. The US had earlier said the substance - which can cause burning of the flesh - had been used only for illumination. Col Venable denied that white phosphorous constituted a banned chemical weapon. White phosphorus is an incendiary weapon, not a chemical weapon Col Barry Venable Pentagon spokesman US military interview. Washington is not a signatory to an international treaty restricting the use of the substance against civilians
When exposed to sunlight or when heated in its own vapor to 250C, it is converted to the red variety, which does not phosphoresce in air as does the white variety. This form does not ignite spontaneously and is not as dangerous as white phosphorus. It should, however, be handled with care as it does convert to the white form at some temperatures and it emits highly toxic fumes of the oxides of phosphorus when heated. The red modification is fairly stable, sublimes with a vapor pressure of 1 atm at 17C, and is used in the manufacture of safety matches, pyrotechnics, pesticides, incendiary shells, smoke bombs, tracer bullets. Phosphorus is also important in the production of steels, phosphor bronze
Formal Chemical Name (IUPAC)
Update by Karl Harrison
(Molecule of the Month for November 2005 )