Phosphorus in Periodic Table
Phosphorus (P), nonmetallic chemical element of the nitrogen, arsenic, antimony, and bismuth family or Group 15 (VA) of the periodic table are collectively called pnictogen or pnictides from Greek word choking. It is a colourless, semitransparent, soft, waxy solid at room temperature that glows in the dark condition. The elements of Group 15 like nitrogen and phosphorus are essential constituents of the living body but other heavier member like arsenic is extremely toxic.
Solid phosphorus has several allotropic forms like white, red, violet, and black. In the liquid and gaseous state below 800 °C, it constitutes a tetrahedral P4 molecule but above 800 °C, it forms a P2 molecule. Some physical and chemical properties like electronic configuration, oxidation number or state, density, melting, and the boiling point are given below the table.
|Properties of Phosphorus|
|Electronic configuration||1s2 2s2 2p6 3s2 2p3|
|Melting point||white – 44.15 °C|
red – 590 °C
|Boiling Point||white – 280.5 °C|
|Density||white – 1.823 g/cm3|
red – 2.34 g/cm3
violet – 2.36 g/cm3
black – 2.39 g/cm3
|Oxidation State||-3, -2, -1, 0, +3, +4, +4, +5|
Occurrence and Production
Phosphorus is a highly abundant chemical element in crustal rocks (1120 ppm). It is not found free in nature but widely distributed in many minerals like fluorapatite, 3Ca3(PO4)2, CaF2. Over 200 phosphate minerals are distributed in different countries of the world. Morocco, United States, Australia, China, and Russia are the major depositor of phosphorus minerals. It also occurs in plants and animals where it is an essential biological constituent of life.
Phosphorus has 23 isotopes with mass numbers ranging from 25P to 47P. 31P is the only stable isotope of the element. 32P (beta ray emitter, t½ = 14 days) is the most important radioactive isotopes obtained by (n,p) reaction on 32S or (n, γ) nuclear reaction on 31P in reactors. The radioactive decay of isotopes is extensively used in tracer studies.
Phosphorus is obtained by reducing phosphate rocks with carbon (coke) in presence of sand at 1400 to 1500 °C in an electric furnace. The overall reaction may be idealized as 2Ca3(PO4)2 + 6SiO2 + 10C → 6CaSiO3 + 10CO + P4. The vapour is condensed under water molecules to form white phosphorus.
Properties and Allotropes
Solid phosphorus has several allotropic forms like white, red, violet, and black. White phosphorus is the most common form with a melting point of 44°C and a boiling point of 287 °C. It becomes yellow on exposure to light. It is insoluble in water but highly soluble in carbon disulfide, benzene, liquid sulfur dioxide, liquid ammonia, PCl3, and POCl3. Concentrated nitric acid slowly oxidizes it to form phosphoric acid. White phosphorus is highly poisonous and contacts on skin or inhalation which may cause harmful metabolic disorders.
Red phosphorus is obtained by getting white-P in absence of air at 270 to 300 °C. It is an amorphous solid, melting under pressure at 592 °C. The non-poisonous red phosphorus is much denser than white-P. Its reactivity is much less and unattacked by aqueous alkali metals.
Black phosphorus is the least reactive allotrope of the element form by heating (∼ 200 °C) white-P under very high pressure. An orthorhombic form is obtained at 1200 atmosphere pressure, still higher pressure forms rhombohedral and cubic crystal lattice. It does not ignite in the air even at 400 °C and thermodynamically the most stable form of the element.
The chemistry of phosphorus differs in the same way from that of nitrogen as silicon differs from carbon due to the presence of empty d-orbital. The electronic configuration suggests that phosphorus is closer to the next noble gas than the preceding noble gas. The formation of +5 cation by losing five outer electrons is just impossible. A huge amount of energy necessary for the purpose. The fact suggests that sum of five ionization energy cannot be compensated by gain lattice energy by ionic bonding. Therefore most of the compounds in the +5 state of phosphorus are ionic in nature.
Phosphorus formed volatile hydride like phosphine (PH3) by covalent bonding with sp3 hybridization. The hydride can be obtained by hydrolysis of metal phosphide. The thermal stability of the Group-15 hydrides decreases rapidly with the increasing atomic weight of the parent element. The hydrides are less stable than the Group-14 hydrides.
Due to the presence of vacant d-orbital, phosphorus forms trihalides along with pentahalides but the nitrogen has no d-orbitals for such explanation. All the covalent tetrahalides (PX3) have tetrahedral (sp3) structures with a lone pair of electrons residing in the fourth orbital of the atom. The pentahalides contain a trigonal bipyramidal structure with sp3d2 hybridization and 10 valence shell electrons. PF5 is a very strong Lewis acid, forming complex compounds with amines, ethers. The valence shell of phosphorus also expands in octahedral complexes like [PCl6]- that contain 12 electrons with sp3d2 hybridization.
Phosphorus forms the acidic oxides in the +3 and +5 oxidation states but nitrogen forms the acidic oxides with oxidation states +1, +2, +3, +4, and +5. Most of the nitrogen oxides are monomers but phosphorus oxides are based on the P4 skeleton.
Phosphorous oxoacids are extensively used and commercially important compounds. It is a structurally complicated chemical. They are acidic protons bound to oxygen atoms, some have nonacidic protons that are bonded directly to phosphorus and some contain phosphorus – phosphorus chemical bonding. Only nine are commercially important, and three of them are hypophosphorous acid, phosphorous acid, and phosphoric acid. Phosphoric acid (H3PO4) contains a tetrahedral skeleton with sp3 hybridization.
|Acids||Properties of oxoacids|
|Phosphonic acid (H3PO2)||It is a monobasic reducing agent known only in the solution.|
|Phosphorus acid (H3PO3)||It is a dibasic acid crystalline solid.|
|Hypophosphoric acid (H4P2O6)||It is a tetrabasic acid, although the known salts are Na2H2P2O6.|
|Phosphoric acid (H3PO4)||It is a tribasic acid with a very low melting point and a variety of salts of phosphoric acid is known.|
Uses of Phosphorus
Phosphorus is the essential constituent of plants or animals and is used mainly for the preparation of concentrated phosphoric acid and phosphate fertilizer. It is converted to P4S10 and P4S3 that are used in the production of organo-phosphorus compounds and matches. Ferrophosphorus obtained as a byproduct in phosphorus preparation is used in high-density concrete, in radiation shields for nuclear reactors, and in making special steel and cast iron.
Phosphates enter the plant and animal kingdom through food chains and play a vital role in the formation of the nucleic acids, metabolic functions via adenosine triphosphate (ATP), and in the formation of bone, teeth, etc. A huge amount of phosphorus is used in different types of detergent preparation.