Phosphorus

Phosphorus is vital for life, mainly through phosphates in DNA, RNA, ATP, and cell membranes. Early sources included human urine and bone ash, while phosphate mines often contain fossils. Phosphorus is crucial for fertilisers and is found in minerals such as phosphate. It exists as highly reactive white and red phosphorus and is never found free. The Earth’s crust contains about one gram of phosphorus per kilogram.

1.0Phosphorus: Important Details

Phosphorus, with an atomic number of 15, is in the p-block of Group 15 and Period 3 of the periodic table. 

• It is a solid with a relative atomic mass of 30.974 at room temperature. 
• The electronic configuration of phosphorus is [Ne] 3s² 3p³, indicating five valence electrons. 
• This element exhibits a melting point of 44.15 °C (111.47 °F, 317.3 K) and a boiling point of 280.5 °C (536.9 °F, 553.7 K). 
• White phosphorus, a common allotrope, has a 1.823 g/cm³ density. 
• Phosphorus has a stable and naturally occurring key isotope, ³¹P.

Occurrence

Phosphorus compounds are widespread and commonly found in phosphate rocks, minerals, bones, and teeth. Key phosphate minerals include calcium phosphate (Ca₃(PO₄)₂), apatite (Ca₅(PO₄)₃OH), fluoroapatite (Ca₅(PO₄)₃F), and chloroapatite (Ca₅(PO₄)₃Cl). Elemental phosphorus was first obtained by H. Brand, and its name, derived from Greek words meaning "light" and "I bear," reflects the phosphorescence of white phosphorus due to slow oxidation.

Allotropic Forms

The table below clearly and concisely compares the allotropic forms of phosphorus, highlighting their fundamental properties, chemical behaviour, preparation methods, and appearance.

2.0Compounds of Phosphorus

1. Phosphorus(V) Compounds

• Phosphate (PO₄³⁻): Tetrahedral anion, the conjugate base of phosphoric acid (H₃PO₄), used in fertilisers.
• Phosphoric Acid: Triprotic, forming three conjugate bases stepwise.
• Polyphosphates: Chains/rings with P-O-P bonds used in ATP and sodium tripolyphosphate (STPP).
• Phosphorus Pentoxide (P₄O₁₀): Anhydride of phosphoric acid reacts vigorously with water.
• Phosphate Salts: Form polymeric salts with metal cations, insoluble with 2⁺ or 3⁺ charge, found as common minerals.
• PCl₅ and PF₅: PF₅ is a colourless gas with trigonal bipyramidal geometry; PCl₅ is a solid with ionic form (PCl₄⁺ PCl₆⁻), also trigonal bipyramidal when molten/vaporised. PBr₅ is unstable; PI₅ is unknown.

2. Phosphorus(III) Compounds

• Trihalides: PF₃ (gaseous), PCl₃ and PBr₃ (yellowish liquids), and PI₃ (solid), all hydrolyse to phosphorous acid. PCl₃ is made by chlorinating white phosphorus. PF₃ is toxic as it binds to haemoglobin.
• Phosphorus(III) Oxide (P₄O₆): Also called tetraphosphorus hexoxide, it is the anhydride of P(OH)₃.

3. Phosphorus(I) and Phosphorus(II) Compounds

• P-P Bonds: Common in catenated derivatives of phosphine and organophosphines. 

Phosphides and Phosphines

• Phosphides are formed from the reaction of metals with red phosphorus. Alkali and alkaline earth metals form ionic phosphides, which react with water to form phosphine. Metal-rich phosphides are hard, refractory compounds, while phosphorus-rich phosphides are less stable and can include semiconductors. Schreibersite is a naturally occurring metal-rich phosphide found in meteorites.
• Organic phosphorus compounds containing phosphorus-carbon (P-C) bonds are vital in various biological and industrial processes. These compounds are integral to many biochemical processes and industrial applications, showcasing the versatility of phosphorus in organic chemistry. Some key types include Phosphines, Phosphonates and Phosphinates Phosphates, Phosphoramidites and Phosphonium Salts.

Oxoacids of Phosphorus

• Phosphorus oxoacids are extensive, often commercially important, and sometimes structurally complex. They all have acidic protons bound to oxygen atoms; some also have non-acidic protons bound directly to phosphorus. Some oxoacids contain phosphorus-phosphorus bonds. While many phosphorus oxoacids exist, only nine are commercially significant, with phosphorous, hypophosphorous, and phosphoric acid being essential.

Learn more: Oxoacids of Phosphorus

3.0Reactivity of Phosphorus 

Phosphorus primarily forms covalent bonds with five valence electrons (valence shell configuration 3s²3p³). Elemental phosphorus can be produced from any phosphate rock. The process involves:

Reaction of Crushed Phosphate Rocks and Sand: 

2Ca₃(PO₄)₂  + 6SiO₂  →  P₄O₁₀  + 6CaSiO_3​

Reduction by Carbon: 

P₄O₁₀ + 10C → P₄ + 10CO

White phosphorus forms waxy solids of molecular crystals consisting of P₄\text{P}_4P4​ molecules. These undergo spontaneous combustion in the air: 

P₄  + 5O₂ → P₄O₁₀

The P_4​ structure is a distorted tetrahedron, explained by phosphorus’s electronic configuration. Sharing three electrons with other P atoms results in six P−P bonds, leaving a lone pair.

Reactions with Oxygen:

• With Insufficient Oxygen:

 P₄ + 3O₂ → P₄O₆

In P₄O_6, Oxygen atoms are inserted into each P−P bond.

• With Excess Oxygen: 

P₄ + 5O₂ → P₄O₁₀

An additional oxygen atom attaches directly to phosphorus.

Oxides of Phosphorus:

•  P₄O₆  and P₄O₁₀   Features:

P₄O₁₀   dissolves in water to form phosphoric acid: 

P₄O₁₀ + 6H₂O → 4H₃PO₄

4.0Phosphorus cycle

The phosphorous cycle involves the movement and recycling of phosphorus through rocks, water, soil, sediments, and organisms. This biogeochemical cycle includes several steps influenced by rock type, air, and water. Phosphorus is crucial for cell development and energy storage in the form of ATP (Adenosine Triphosphate). A deficiency of phosphorus in the soil can lead to poor crop yields.                  

5.0Uses of Phosphorus

Phosphorus exists in four common forms: white, black, red, and violet. It plays a crucial role in living organisms. Phosphorus is a key component of animal and plant matter found in bones, the brain, blood, and living cells. In industry, important phosphorus compounds include orthophosphoric acid and phosphatic fertilisers.

• Fireworks: Phosphorus is used to create vibrant colours and effects.
• Fertilisers: Essential for plant growth, phosphorus is a crucial component.
• Baking Powder: It acts as a leavening agent.
• Steel Manufacturing: Phosphorus improves the properties of steel.
• Fine China and Special Glasses: Phosphates are used in their production for better quality and durability.

6.0Health Effects of Phosphorus

Phosphorus, commonly found as phosphates in the environment, is essential for human health, contributing to DNA and energy delivery. It is also present in plants and added to soils through phosphate-rich manure, detergents, and foods like cheese and processed meats.

Excessive phosphate levels can lead to health issues such as kidney damage and osteoporosis, while phosphate shortages pose health risks. White phosphorus, a highly toxic form of phosphorus, is dangerous and can be fatal if ingested or inhaled.