Phosphorus preparation and properties, Group 15 P Block Elements
→ Phosphorus was first discovered by Brand in 1669. Scheele isolate it from bone ash. In 1777 Lavoisier proved that it is an element. It glow in dark (greekword, phos-light and phero-carry) so it is named as phosphorus.
→ Phosphorus is the second element of nitrogen family. Its atomic number is 15. Its electronic configuration is 12 2s2 2p6 3s2 3p3. It is a very reactive non-metal hence it does not exist in free state. Some important minerals of phosphorus are as follows :
→ Phosphorus is found in our teeth, bones, muscles, brain and nerve tissues. It is also present in milk, fish, beans, eggs etc.
→ When phosphorite ie, Ca3(PO4)2 is heated with coke and silica in an electric furnace upto 1770 K, then phosphorus is isolated.
→ The vapours obtained from above process are condensed then white phosphorus is isolated.
Allotropic form of Phosphorus :
Phosphorus show the property of allotropy. The main allotropes of phosphorus are as follows:
→ White or Yellow Phosphorus : It is the common allotrope of phosphorus. It is obtained by heating phosphorite rock with coke and sand in an electric are at 1770 K temperature.
→ On exposure to air, it becomes yellow, therefore it is called yellow phosphorus. It exist as P, molecules. In P molecule, each phosphorus atom is linked to three other P-atoms by covalent bonds. Each phosphorus atom in PA molecule have one lone pair of electrons. The P-P-P bond angle in P, molecule is 60°.
Some important characteristics of white phosphorus are as follows:
- It is transparent, white and soft waxy solid.
- It can be cut easily with knife.
- It has a garlic odour.
- It is insoluble in water but soluble in organic solvents like CS, alcohol, benzene etc.
- It’s vapours are highly poisonous. The persons working in phosphorus industry, develop a disease known as Phossy jaw. In this diesase jaw bones of persons decay slowly.
- Ita melting point is 317 K while its boiling point is 553 K because of presence of very weak Vander Waal’s force of attraction.
In contact with air, it undergoes slow combustion and glow in dark. This phenomenon is known as phosphorescence. Hence phosphorus shows phosphorescence.
- In pure state it is white in colour but on standing it converts slowly into yellow colour.
- Its ignition temperature is very low so it readily catches fire or show combustion with air and produce dense white fumes.
P4 + 5O2 → P4O10
- Due to above reason white phosphorus is stored under water.
- Upon heating with aqueous NaOH in inert atmosphere it evolves phosphine gas.
→ Red Phosphorus : Red phosphorus can be obtained by heating white or yellow phosphorus in an inert atmosphere of CO, at 573 K for several days.
→ Red phosphorus has polymeric structure in which P, tetrahedral units are linked together as shown in Fig. 7.11
Some important characteristic of red phosphorus are as follows:
- It is dark red coloured solid.
- It is brittle and odourless.
- It is non-poisonous in nature.
- It is insoluble in water as well as in carbon disulphide.
- It is denser than white phosphorus. Its specific gravity is 2.1.
- It does not show phosphorescence.
- It is quite stable and have quite high (543 K) ignition temperature. Thus, it does not catch fire easily.
- It is not chemically active hence it burns on heating above 533 K temperature.
- It does not react with caustic soda so with the help of this property it can easily be separated with white phosphorus.
→ Black Phosphorus : Black phosphorus can be prepared by heating white phosphorus at 473 K temperature under very high pressure in an inert atmosphere.
→ It has double layered structure having crystalline lattice. In black phosphorus each atom is forming three strong bonds with its neighbours. Here the bond angle is 99. It is highly polymeric. Its structure is shown in Fig. 7.12
→ Some important characteristics of black phosphorus are as follows:
- It is found in two allotropic form:
- α-Black phosphorus
- β-Black phosphorus
- It has black metallic lustre.
- Its melting point is 860 K.
- Its specific gravity is 2.69.
- It is chemically inert and it does not oxidise in air.
- It does not ignite in air upto 673 K temperature.
- It is a moderate conductor of heat and electricity.
→ Thus, we can say that in three allotropic forms of phosphorus, white phosphorus is very reactive while black phosphorus is the least reactive. All the properties of three allotropic forms of phosphorus has been summarised in table 7.8. Table
7.8 : Differences between white, red and black phosphorus
|Properties||White||Red phosphorus||Black phosphorus|
|Physical state||phosphorus Soft waxy solid||Brittle powder||Crystalline|
|Colour||White but turns yellow on exposure to air||Dark red||Black|
|Ignition temperature||307 K||543 K||673 K|
|Melting point||317 K||Sublimes in absence of air at 543.||860 K|
|Stability||Less stable||More stable||Most stable|
|Reaction with NaOH||Evolve phosphine||No reaction||–|
Uses of Phosphorus :
- Red phosphorus is generally used in match box industry.
- White phosphorus is used in making rat poison.
- It is used in making smoke screen.
- Phosphorus is used in making Bronze alloy, Bronze is an alloy of Cu, Sn and P.
- It is also used in making fire works.
- Radioactive P-32 is used in the treatment of lukemia (blood cancer) and other blood related diseases.