The Chemistry oh Phosphorous

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The Chemistry of Phosphorus

Among the non-metals, on the right hand side of the periodic table, with Atomic Number 15, is the element Phosphorus. The word ‘phosphorus’ is derived from the Greek word ‘phosphoros’ meaning ‘bringer of light’ and its discovery was completely accidental. It has the electronic configuration 2,8,5 and has 5 electrons in its outer most shell, hence it being in group 5 of the periodic table, under Nitrogen.                                                                                                In 1669, German alchemist Hennig Brand was attempting to create the ‘philosopher’s stone’, which was a supposedly magical substance that would turn metals into gold, by evaporating urine. One day, after boiling urine into a paste, heating the paste to a high temperature and passing the vapours through water, where he hoped to find gold, was a mysterious white waxy substance that seemed to glow in the dark, and burst into flames when in contact with air. Brand had discovered phosphorus, well was the first to record this discovery. In fact, several other chemists could have discovered it at a similar time, and we now know that the substance he found was actually ammonium sodium hydrogen phosphate: (NH4)NaHPO4. This method produced only about 60g of phosphorus and used 1100L of urine, so was not very efficient, and it was only later on that investigators found alternative methods of obtaining it. One of its current processes of production is by heating one of its compounds ‘phosphate’, with silica and coke in a furnace.                                 

Phosphorus exists in 3 main allotropic forms: white, red and black. Allotropes are forms of an element with different physical and chemical properties. White phosphorus in its pure form, is a transparent waxy solid, however it is sometimes yellow due to traces of red phosphorus impurities that exist within it. White phosphorus is extremely volatile in air and so spontaneously bursts into flames (pyrophoric – it self ignites in air), forming Phosphorus Pentoxide: P4O5  , which is the reason for it being stored underwater at room temperature; it’s insolubility in water allows this to happen.  Phosphorus is so reactive because it has weak intermolecular forces hence making it easy to bond with other atoms. The reaction of white phosphorus with air is exothermic; this causes it to melt under its own heat until it self-ignites. Due to its high flammability in air, it is commonly used by the military in incendiary weapons (weapons used to create fires, or damage sensitive equipment) such as missiles and bombs, but also as a smoke screening agent. Once it ignites, it oxidises producing by-products of intense heat and thick smoke, and it also reacts with the air to form phosphoric acid, which is one of the main reason for its usage as a weapon. Once ignited, it will then continue to burn until deprived of oxygen or the reagent is used up. Its structure consists of 4 atoms covalently bonded in a tetrahedral arrangement. This arrangement means the molecules are weakly bonded, so are easy to break therefore it has high reactivity. Other properties include its boiling point of 280⁰c and melting point of 44⁰c, its distinct garlic odour, and its high toxicity. Exposure to the vapours of white phosphorus cause a disease called ‘phossy-jaw’.         This is where vapours attacked the bones in the jaw causing them to decompose; it also proved to cause brain damage.                                        Another property of white phosphorus is its glow-in-the-dark traits. When it undergoes oxidation due to exposure to air, light is produced, hence in the dark white phosphorus continues to glow. This property is called phosphorescence.                                                                Red phosphorus is a red powder and can be formed by heating white phosphorus to 280⁰c with a catalyst present. Without a catalyst, temperatures of up to 416⁰c are needed. After cooling, the red phosphorus may still contain traces of the white allotrope, so it is boiled with caustic soda  to remove any residue left over and finally dried in a vacuum. Its structure is a covalent crystal lattice, consisting of infinite chains of phosphorus atoms; it has stronger intermolecular bonds, making is less reactive than white phosphorus. Unlike white phosphorus, it does not ignite at temperatures under 280⁰c and has a melting point of 590°c, and so is safer to handle. Red phosphorus is also used by the military, however because it is less reactive and does not ignite spontaneously, its usage is limited to smoke-screening and less for incendiary weapons. However this is not its main use. Originally matches were made by coating the match head in a mixture containing white phosphorus; the matches could be ignited on any rough surface, however many people were killed due to the poisonous nature of white phosphorus and so it was banned in several countries including France and Switzerland.   Cue the use of the red allotrope in the invention of the ‘safety match’. Modern ‘safety matches’ are now used worldwide. These matches are ‘safer’ because the reagents are separated and also because red phosphorus is less reactive and less toxic than white phosphorus. The match head now contains potassium chlorate, whilst the striking surface contains red phosphorus (instead of white). As the match is ‘struck’, friction heat causes some of this red phosphorus to convert to white phosphorus, which ignites instantly.                         The 3rd and final allotrope is black phosphorus. It is created by heating white phosphorus but also applying pressures of up to 12,000 atmospheres. The structure of it means that every atom is bonded to 3 other atoms, making it the most stable allotrope and least reactive. The structure and properties of black phosphorus are similar to those of graphite; both are aligned in layers of atoms and conduct electricity. Strangely, black phosphorus is one of very few non-metals that can conduct electricity.  It can conduct electricity because it’s arranged in layers, between the layers some electrons have not bonded, so are free to move and carry charge. Commercially, black phosphorus is not commonly used, however one of its very few uses is in the coating of screws. It is used to coat screws because it is very un-reactive under normal conditions, so it protects from corrosion.

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The abundance of phosphorus in the Earth’s crust is 0.12%, making it the 11th most common element. Its most common occurrence in nature is as a phosphate, which is a compound that contains phosphorus, oxygen, and one other element, for example: Calcium Phosphate Ca3(PO4). Normally, phosphate consists of 1 phosphorus atom, in the centre of 4 oxygen atoms. Phosphorus itself does not occur naturally in the environment. The most common source of phosphorus is from extraction from phosphate rock: phosphorite, which contains the mineral apatite, an impure tri-calcium phosphate. 86% of the world’s phosphate rock source comes from North Carolina, Florida. ...

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