Phosphorus is a vital element used in Chemistry, and it is mainly obtained through phosphates, which are molecules containing the phosphate ion \({\rm{P}}{{\rm{O}}_4}^{3 – }.\) Phosphates are found in ATP, DNA, RNA, and phospholipids, all essential components of cells. Human urine was the earliest source of elemental Phosphorus, and bone ash was an important early phosphate source. Elemental Phosphorus exists in several allotropic forms in that the most common forms are white Phosphorus, red Phosphorus and black Phosphorus. Students can find relevant details related to allotropes of phosphorus in this article.

What are Allotropes of Phosphorus?

Allotropes are structural modifications of an element in which the atoms of the element are bonded in a different way. Phosphorus exhibits allotropy. There are several allotropic forms of phosphorus, including white phosphorus, red phosphorus, scarlet phosphorus, metallic or \(\alpha \)-black phosphorus, \(\beta \)-black phosphorus, and violet phosphorus. However, white, red, and black phosphorus are the most common allotropic forms.

White Phosphorus

The most common form of phosphorus is white or yellow phosphorus. It is made by heating phosphate rock, silica, and coke in an electric furnace at \(1770\,{\rm{k}}.\)

Structure of White Phosphorus

It exists as \({{\rm{P}}_4}\) units. The four \({\rm{s}}{{\rm{p}}^3} -\) hybridized phosphorus atoms are positioned at the four corners of a regular tetrahedron with \(\angle {\rm{PPP}} = 60^\circ .\) Each phosphorus atom is covalently linked to three other \({\rm{P}}\)-atoms, completing the octet. On exposure to light, white phosphorus turns yellow. Therefore, it is called yellow phosphorus.

Properties of White Phosphorus

The physical and chemical properties of white phosphorus are explained below:

Physical Properties of White Phosphorus

1. It is a soft solid. When freshly prepared, it is nearly colourless but acquires a pale yellow colour on standing. As a result, it is frequently referred to as yellow phosphorus. It is so soft that you can cut it with a knife. It has a distinct garlic-like odour.
2. It is naturally poisonous. Phossy Jaw is a disorder in which the jawbones deteriorate in people who work with phosphorus.
3. It is soluble in carbon disulphide and oils but not in water.
4. The various \({{\text{P}}_4}\) molecules of white phosphorus are held together only by weak van der Waals forces of attraction and hence the melting point \(\left( {317\,{\rm{k}}} \right)\) and boiling point \(\left( {553\,{\rm{k}}} \right)\) of white phosphorus are quite low.

Chemical Properties of White Phosphorus

1. The action of air: The angle strain in white phosphorus is very high. Therefore, it is very reactive. It readily catches fire in the air with a greenish-yellow flame which is visible in the dark. This phenomenon is called chemiluminescence or phosphorescence.
2. Burning: When heated in the air, white phosphorus becomes extremely reactive and catches fire. It produces dense white phosphorus pentoxide gases when burned.
3. Reaction with metals: It reacts with several metals such as \({\rm{Na}},\,{\rm{K}},\,{\rm{Mg}},\,{\rm{Ca}},\,{\rm{Ag}},\) etc., forming their respective phosphides.
4. For example,
5. \({{\rm{P}}_4} + 6{\rm{Mg}} \to \mathop {2{\rm{M}}{{\rm{g}}_3}{{\rm{P}}_2}}\limits_{{\rm{Magnesium}}\,{\rm{phosphide}}}\)
6. \({{\rm{P}}_4} + 6{\rm{Ca}} \to \mathop {2{\rm{C}}{{\rm{a}}_3}{{\rm{P}}_2}}\limits_{{\rm{Calcium}}\,{\rm{phosphide}}} \)
7. 4. Reaction with non-metals like halogens and sulphur.
8. \({{\rm{P}}_4} + 6{\rm{C}}{{\rm{l}}_2} \to 4{\rm{PC}}{{\rm{l}}_3}\)
9. \({{\rm{P}}_4} + {10_{\rm{S}}} \to \mathop {\mathop {2{{\rm{P}}_2}\;{{\rm{S}}_5}}\limits_{{\rm{Phosphorus}}{\mkern 1mu} } }\limits_{{\rm{Pentasulphide}}}\)
10. 5. Reaction with caustic soda: It produces phosphine when heated in an inert atmosphere with an aqueous solution of caustic soda.
11. \({{\rm{P}}_4} + 3{\rm{NaOH}} + 3{{\rm{H}}_2}{\rm{O}} \to \mathop {3{\rm{Na}}{{\rm{H}}_2}{\rm{P}}{{\rm{O}}_2}}\limits_{{\rm{Sodium}}\,{\rm{hypo}}\,{\rm{phosphite}}}  + \mathop {{\rm{PH}}3}\limits_{{\rm{Phosphine}}}\)
12. 6. Reducing properties: It is a powerful reducing agent. It reduces sulphuric acid to sulphur dioxide and nitric acid to nitrogen dioxide. \({{\rm{P}}_4} + 10{{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to 4{{\rm{H}}_3}{\rm{P}}{{\rm{O}}_4} + 10{\rm{S}}{{\rm{O}}_2} + 4{{\rm{H}}_2}{\rm{O}}\)
13. \({{\rm{P}}_4} + 20{\rm{HN}}{{\rm{O}}_3} \to 4{{\rm{H}}_3}{\rm{P}}{{\rm{O}}_4} + 20{\rm{N}}{{\rm{O}}_2} + 4{{\rm{H}}_2}{\rm{O}}\)

Red Phosphorus

It is a violet-red powder made up of small crystals and some amorphous phosphorus. It is made by heating white phosphorus in an inert atmosphere for several hours at \(525\,{\rm{k}}.\)

Preparation of Red Phosphorus

Yellow phosphorus is collected in an egg-shaped cast iron retort with two thermometer jackets and a tall pipe with a safety valve and feeder at the top. Iodine is also added in modest amounts. The conversion of white phosphorus to red phosphorus is accelerated by iodine. An inert gas, such as coal gas or carbon dioxide, replaces the air in the retort. After heating for the necessary time, the retort was allowed to cool. The red Phosphorus hard lump is obtained. To dissolve any remaining white phosphorus, it is pulverised and cooked in a caustic soda solution. The red phosphorus does not dissolve. It has been completely washed and dried.

The red phosphorus molecule has a polymeric structure. It consists of chains of \({{\rm{P}}_4}\) tetrahedra linked together. Red phosphorus is less reactive than white phosphorus because of its polymeric nature.

Properties of Red Phosphorus

Some important properties of Red Phosphorus are as follows:

1. It is a hard, crystalline, odourless solid with an iron-grey lustre.
2. It has no poisonous properties.
3. It is insoluble in water and organic solvents like \({\rm{C}}{{\rm{S}}_2},\) alcohol, and ether.
4. It is less reactive than white phosphorus since it is polymeric.
5. At room temperature, it is a relatively stable phosphorus allotrope. It has a much higher ignition temperature \(\left( {543\,{\rm{k}}} \right)\) than white phosphorus \(\left( {303\,{\rm{k}}} \right).\) As a result, it does not easily catch fire.
6. It has a higher density \(\left( {2.16\,{\rm{g}}\,{\rm{c}}{{\rm{m}}^{ – 3}}} \right)\) than white phosphorus \(\left( {1.84\,{\rm{g}}\,{\rm{c}}{{\rm{m}}^{ – 3}}} \right)\) and is a poor electrical conductor.
7. When heated, it sublimes, producing vapours similar to those produced by white phosphorus. When these vapours condensate, white phosphorus is formed. This provides a straightforward method for converting red phosphorus to white phosphorus.
8. Because it is less reactive than white phosphorus, it only forms salts with halogens, sulphur, and alkali metals when heated.

9. It produces phosphorus pentoxide when burned in oxygen at \(565\,{\rm{K}}.\)

10. It is unaffected by caustic alkalies. This feature is used to distinguish between red and white phosphorus.

Black Phosphorus

Black Phosphorus is explained in detail in the section below:

Metallic Or \(\alpha \)-Black phosphorus

This type of black phosphorus is made by dissolving red phosphorus in the molten lead for a long time at \(400\,^\circ {\rm{C}}\) in a sealed tube. Black phosphorus crystals form when the mixture cools. Treatment with weak nitric acid dissolves the lead. It is an extremely stable phosphorus allotrope that doesn’t oxidise in the air until it is heated very strongly. It does not conduct electricity.

\(\beta \)- Black Phosphorus

It is obtained by heating white phosphorus at \(473\,{\text{k}}\) under very high pressure of about \(4000 – 12000\,{\text{atm}}.\) It may also be obtained by heating white phosphorus at \(220 – 370\,^\circ {\rm{C}}\) for \(7 – 8\) days in the pressure of mercury which acts as a catalyst. This is the only form of phosphorus whose structure is definitely known.

Structure of Black Phosphorus

It is crystalline in nature and consists of corrugated sheets, each phosphorus atom being covalently bonded to the three neighbouring phosphorus atoms. The \({\text{P}} – {\text{P}}\) angles are \({99^ \circ }.\) The \({\text{P}} – {\text{P}}\) distance between the two nearest phosphorus atoms is \(2.18\,{{\text{A}}^ \circ }.\)

General Properties of Black Phosphorus

The general properties of black phosphorus are given below:

1. It exists in three crystalline and one amorphous form.
2. \(\alpha \)-black phosphorus is a pretty good conductor of electricity, but β-black phosphorus is a non-conductor.
3. \(\beta \)-black phosphorus possesses a black metallic lustre.
4. It is the least reactive and most stable form of phosphorus.

Scarlet Phosphorus

This allotrope is obtained as an amorphous scarlet powder by boiling a \(10\% \) phosphorus solution in phosphorus tribromide for about \(10\) hours. Pure scarlet phosphorus can be made by heating phosphorus tribromide with mercury at \(2400\,^\circ {\rm{C}}.\)

\(2{\rm{PB}}{{\rm{r}}_3} + 3{\rm{Hg}} \to 3{\rm{HgB}}{{\rm{r}}_2} + \mathop {2{\rm{P}}}\limits_{{\rm{Scarlet}}{\mkern 1mu} {\rm{Phosphorus}}} \)

Scarlet phosphorus resembles red phosphorus in its physical properties and white phosphorus in its chemical properties.

Violet Phosphorus

Violet phosphorus is obtained by heating white phosphorus with a trace of sodium at \(230\,^\circ {\rm{C}}\) under high pressure. It is crystalline in structure.

Comparison of Reactivity of Allotropic Form of Phosphorus

The three allotropic forms of Phosphorus differ widely in their chemical reactivity. White Phosphorus is the most reactive, while black Phosphorus is the least reactive. Therefore, white Phosphorus is stored underwater to protect it from the air, while red and black Phosphorus is stable in air.

Uses of Allotropes of Phosphorus

1. Red phosphorus is used in the manufacture of matches due to its non-poisonous nature.
2. Red phosphorus is used in making phosphorus bronze which is hard, tenacious and not corroded by water.
3. It is also employed in the preparation of compounds like phosphorus chlorides, phosphoric acids and hypophosphites used in industry and in medicine.
4. White phosphorus is used as a rat poison.
5. Yellow phosphorus is used in the manufacture of tracer bullets, incendiary bombs and for producing smoke screens.

Summary

There are several allotropic forms of phosphorus, including white, red and black. White phosphorus is used in the manufacture of matches due to its non-poisonous nature. White Phosphorus is used as a rat poison and Yellow Phosphorus is used to manufacture tracer bullets, incendiary bombs, and produce smoke screens.

FAQs on Allotropes of Phosphorus

Frequently asked questions related to allotropes of phosphorus is listed as follows:

Q.1. How many allotropic forms of phosphorus are there?
Ans: There are several allotropic forms of phosphorus, including white phosphorus, red phosphorus, scarlet phosphorus, metallic or \(\alpha \)-black phosphorus, \(\beta \)-black phosphorus, and violet phosphorus. However, white, red, and black phosphorus are the most common allotropic forms.

Q.2. What is the most stable allotrope of phosphorus?
Ans: Black phosphorus is the most stable allotropic form of phosphorus.

Q.3. How many types of phosphorus are there?
Ans: There are several forms of phosphorus, including white phosphorus, red phosphorus, scarlet phosphorus, metallic or \(\alpha \)-black phosphorus, \(\beta \)-black phosphorus, and violet phosphorus. However, white, red, and black phosphorus are the most common allotropic forms.

Q.4. Which is more stable white or red phosphorus?
Ans: Red phosphorus is more stable than white phosphorus.

Q.5. What are the three most common allotropes of phosphorus?
Ans: Following are the three allotropes of phosphorus;
1. White phosphorus
2. Red phosphorus
3. Black phosphorus