Allotropes of Phosphorus

Allotropes of phosphorus primarily start as P₄ molecules, and there are about 12 known allotropes of phosphorus, each with unique properties and structures.

1.0Allotropic Forms of Phosphorus

Phosphorus allotropes are based initially on the P₄ molecule, and approximately 12 known allotropes of phosphorus exist. Phosphorus exists in various allotropic forms, the most important being black, red, and white phosphorus.

• White Phosphorus, also known as yellow phosphorus, is the most reactive allotrope. It appears as a waxy solid or plastic-like material and is highly flammable.
• Red Phosphorus: Much less reactive than white phosphorus, red phosphorus is a key component in matchstick heads. Careful heating can convert it into white phosphorus.
• Black Phosphorus: This allotrope is the least reactive and has a layered structure

2.0White phosphorus 

White phosphorus consists of discrete tetrahedral P₄ molecules, while red phosphorus has a more complex network-like bonding structure. White phosphorus is highly reactive and will unpromptedly ignite in the air, necessitating storage underwater to prevent natural combustion. In contrast, red phosphorus does not spontaneously ignite in the air.

In white phosphorus, the four sp³ hybridised phosphorus atoms are positioned at the corners of a regular tetrahedron with P-P-P bond angles of 60°, and all bonds are covalent.

White phosphorus is prepared by heating phosphate rock, silica, and coke in an electric furnace at 1770 K.

Physical and Chemical Characteristics of White Phosphorus

1. Physical Characteristics:

• Appearance: Waxy, transparent solid.
• Colour: Initially colourless, it turns light yellow over time, often called yellow phosphorus.
• Texture: Soft and fragile.
• Solubility: Insoluble in water but dissolves in carbon tetrachloride and carbon disulphide.
• Molecular Weight: 30.97 g/mol.

2. Chemical Properties of White Phosphorus

White phosphorus requires careful handling due to its high reactivity and toxic nature. Its tendency to ignite spontaneously and glow in the dark highlights its unique and hazardous properties.

• Reactivity: Due to the angular strain in the P₄ molecule (with a bond angle of 60°), white phosphorus is thermodynamically less stable and more reactive than other solid allotropes.
• Combustion: When exposed to air, white phosphorus readily catches fire, producing dense white fumes of phosphorus pentoxide (P₄O₁₀).

P4 + 5O₂ → P_4​O_10​

• Reaction with NaOH: In an inert atmosphere of CO₂, it dissolves in boiling sodium hydroxide (NaOH) to yield phosphine (PH₃).

P₄​ + 3NaOH + 3H₂​O → PH₃​ + 3NaH₂​PO_2​

• Partial Oxidation: In an insufficient supply of oxygen, white phosphorus produces phosphorus trioxide (P₄O₆).

P₄ + 3O₂ → P₄O₆

• Reaction with Metals: White phosphorus reacts readily with several metals to form phosphides.

6Li + P₄ → 2Li₃P

• Reaction with Non-metals: It reacts with non-metals like halogens and sulfur, which have higher electronegativity than phosphorus.

P₄ + 6Cl₂ → 4PCl₃

P_{4 ​}+ 10S → 2P₂​S_5​

• Reducing Agent: P₄ is a powerful reducing agent. It can minimise sulfuric acid to sulfur dioxide while producing phosphoric acid.

P₄ + 5H₂SO₄ → 2H₃PO₄ + 5SO₂ + 2H₂O

3.0Red Phosphorus 

Red phosphorus is polymeric, consisting of chains of P₄ tetrahedra linked together, forming a giant molecule. This structure is more stable and less reactive than white phosphorus.  Red phosphorus is produced by heating white phosphorus at 573 K in an inert atmosphere over several days.

Red phosphorus is more stable and less reactive than white phosphorus because of its network-like structure, which forms chains of linked P₄ tetrahedra. The increased stability is attributed to the extended network of linked atoms, which reduces the angular strain present in the P₄ molecules of white phosphorus.

Physical and Chemical Properties of Red Phosphorus

1. Physical Properties:

• Appearance: Iron-grey lustrous crystalline solid.
• Odour: Odourless.
• Melting Point: 860 K.
• Solubility: Insoluble in water and carbon disulphide.
• Glow: Does not glow in the dark.

2. Chemical Properties:

• Reactivity: Red phosphorus is stable under normal conditions and does not ignite in air. It reacts with oxygen at 565 K to form phosphorus pentoxide (P₄O₁₀). 

P₄+5O₂→P₄O₁₀

• Reaction with NaOH: It does not react with boiling NaOH but disintegrates in alcoholic potash.
• Conversion to Black Phosphorus: Can be converted to black phosphorus under high pressure in an inert atmosphere.
• Reactivity with Other Substances: Less reactive than white phosphorus. It only forms salts with halogens, sulfur, and alkali metals when heated.
• P₄(s) + 6Cl₂(s) → 4PCl₃(s)
• 8P₄(s) +3S₈(s) → 4P4S₃ (s)
• P₄₍s) + 12Na(s) → 4Na₃P(s)

4.0Black Phosphorus

Black phosphorus exists in two forms: α-black and β-black.

• α-Black Phosphorus: Produced by heating red phosphorus in a sealed tube at 803 K. It can be sublimed in air and typically has rhombohedral or opaque monoclinic crystal structures.
• β-Black Phosphorus: Produced by heating white phosphorus under high pressure (around 12,000 atm) at 473 K.

Properties of Black Phosphorus

1. General Properties:

• Stability: Black phosphorus is the most stable of all phosphorus allotropes.
• Reactivity: Both α-black and β-black do not oxidise in air. They are non-toxic, non-flammable, and chemically inert to very high temperatures.
• Conductivity:
• • β-Black Phosphorus: Conducts electricity due to its structure.
  • α-Black Phosphorus: Does not conduct electricity.

2. Structural Properties:

• α-Black Phosphorus: Appears as opaque, monoclinic or rhombohedral crystals.
• β-Black Phosphorus: Has a corrugated sheet-like structure resembling graphite, forming flaky crystals.

3. Thermal Properties:

• β-Black Phosphorus: Does not burn in the air up to 673 K.

5.0Violet Phosphorus

Violet phosphorus is obtained by heating and crystallising red phosphorus under specific conditions. It forms pentagonal "tubes" in its structure.

Properties of Violet Phosphorus

• Ignition: Do not ignite in the air until heated to 300°C.
• Solubility: Insoluble in all solvents.
• Reactivity: Not attacked by alkali, reacts slowly with halogens.
• Oxidation: Oxidized to phosphoric acid by nitric acid: 

$P4(s)+20HNO3(aq)\to 4H3PO4(aq)+20NO2(g)+4H2O(l){\text{P}}_4(s)+20{\text{HNO}}_3(aq)\to 4{\text{H}}_3{\text{PO}}_4(aq)+20{\text{NO}}_2(g)+4{\text{H}}_2\text{O}(l)P4(s)+20HNO3(aq)\to 4H3PO4(aq)+20NO2(g)+4H2O(l)$

• Sublimation: Sublimes in an inert atmosphere (e.g., nitrogen) to form white phosphorus. Rapid condensation of vapour yields violet phosphorus.
• Structure: Polymer with high molecular mass; decomposes into P₂ molecules on heating, which then dimerise to form P₄.

6.0Comparison Of The Different Allotropes Of Phosphorus

7.0Uses of Phosphorus Allotropes

1. White Phosphorus:

• Chemical Fertilizers: These are used in the production of fertilisers.
• Cleaning Agents: Included in cleaning substances.
• Pesticides: Employed in pest control.
• Fireworks and Explosives: Used in firecrackers, explosives, and pyrotechnics, especially in warfare.
• Rat Poison: Utilized in manufacturing rat poison.
• Synthesis: Important for producing phosphorus chlorides, phosphoric acids, and hypophosphites used in industry and medicine.

2. Red Phosphorus:

• Matchsticks: The primary component of the striking surface in matchboxes, igniting the match when struck.
• Additives: Used with iron as additives in gasoline and lubricating oil.
• Flame Retardant: Employed in many thermoplastics and thermosetting plastics to prevent fire.
• Smoke Devices: Mixed with magnesium and a binder to create smoke devices for smokescreens.

3. Black Phosphorus:

• Biomedical Applications: Nanosheets of black phosphorus are used in nanomedicine for photothermal and photodynamic therapy.