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Allotropes of Phosphorus

VSAT 2022

Allotropes of Phosphorus - Properties, Structure, Comparison, and FAQs

Last updated date: 27th Jan 2023
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Phosphorus, a crucial element utilised in Chemistry, was discovered for the first time in the 17th era and was crucial in Lavoisier's formulation of the word "element," which helped to usher in the current period of Chemistry. The most unstable crystal modification, white phosphorus element, is where it was initially found. Presently, a wide range of allotropes with empirical support are understood.

The ability of certain chemical components to reside in a variety of forms while maintaining the similar physical nature is described as allotropy or allotropism, and these forms are referred to as allotropes of the element. Numerous allotropic forms of phosphorus are available. Quantum-chemical techniques were employed to evaluate the chemical stability of over 50 crystalline examples of phosphorus allotropes that were anticipated. This makes the phosphate periodic table the largest structural entity. The main allotropic forms of phosphorus are presented in this article.


What is Phosphorus?

One of the most peculiar elements on the periodic table is phosphorus. It was found to be the thirteenth element in the current periodic table. Phosphorus has several recognized allotropes, and the names given to these various types are inconsistent and confusing.

The following are a few examples of phosphorus allotropes:

All three phosphorus synthesis, structure, properties and uses of phosphorus are discussed below.


White Phosphorus

Phosphorus is present in this frequent allotrope. White phosphorus, which is found in structures composed of four atoms in a tetrahedral configuration. White phosphorus is produced from phosphate rocks rather than being found spontaneously.


Synthesis and Structure of White Phosphorous

Phosphate rock is heated industrially in the context of carbon and silica in an electric furnace. Phosphoric acid can be utilised to extract the released phosphorus element as a vapour. Uses of white phosphorus are weapons. White phosphorus has a ring-like configuration. The white phosphorus bond has a 60-degree angle. Three separate phosphorus atoms form covalent bonds using each phosphorus atom. These particles are attracted to one another by weak Van Der Waals forces.


Physical and Chemical Characteristics of White Phosphorus

  • The solid form of white phosphorus is waxy and transparent. It requires careful care because it is incredibly fragile.

  • It cannot dissolve in water. But it breaks into carbon tetrachloride or carbon disulphide.

  • It is dangerous and very reactive.

  • The solid is soft. It is practically colourless when it is first manufactured, further, it turns a light-yellow colour. It is regularly alluded to as yellow phosphorus as a response.

  • It is so soft.

  • The molecular weight of white phosphorus is 30.97 g/mol.

  • At about 35°C, white phosphorus spontaneously ignites in the air. This temperature is slightly greater than the usual room temperature. It is preserved in water for this purpose. It burns and releases phosphorus pentoxide.

  • White phosphorus passes through an oxidation process once it is exposed to damp air. A light emission that sparkles arises from this interaction. As a result, it sparkles carelessly.

Red Phosphorus

Uses of red phosphorus are excellent fire resistant, particularly in thermoplastics and thermosets. The establishment of polyphosphoric acid is responsible for the flame retardant outcome. In combination with the organic polymers, these acids form a char that stops the flames from spreading.

Synthesis and Structure of Red Phosphorus

While white phosphorus is exposed to heat at 573K in an inert environment for several days, red phosphorus is formed. Red phosphorus is a polymeric compound. The P4 molecule has a tetrahedral shape, with every phosphorus atom joined to 3 additional phosphorus atoms by a covalent connection. It is significantly less active than white phosphorus due to its polymeric character.

Physical and Chemical Characteristics of Red Phosphorus

  • The colour of red phosphorus is iron grey. It is a crystal-like substance that shines brightly.

  • It is not harmful and has no unpleasant smell. Both carbon tetrachloride and water do not break down red phosphorus.

  • It doesn't disintegrate into boiling white phosphorus that resembles caustic soda. In effect, it breaks down in alcoholic potash.

  • In normal circumstances, it is stable and does not ignite the atmosphere.

  • On the other hand, when we heat it to about 400°C, it burns.

  • Red phosphorus solely interacts with oxygen at a temperature of 565 K, where it produces phosphorus pentoxide.

  • Sulphides are created when red phosphorus and sulphur combine.

Black Phosphorus - Synthesis, Structure, and Properties

Only when heated at 416 degrees Celsius in a closed chamber and at the proper temperature does red phosphorus change into black phosphorus. Every phosphorus atom in black phosphorus is covalently connected to 3 of its neighbours and has a crystalline character. The black phosphorus' bond angle is 99 degrees and its bond length is 218 Pm.

There are 3 crystalline forms and one amorphous kind. β-black phosphorus is non-conductive, and α-black phosphorus is a rather good conductor of electricity. Black phosphorus shines with a shiny black lustre.

Physical and Chemical Properties of Black Phosphorus

Following are some of the physical properties of Black Phosphorus :

  • Black Phosphorus is black in color.

  • Melting point of black phosphorus is 416 degree celsius.

  • The specific gravity of black phosphorus is 2.69.

  • The exact mass of black phosphorus is 30.973762 g/mol.

  • The density of black phosphorus is 2.34 g/cm3

  • The solubility of black phosphorus in water (H2O) is 0.3 g/l.

  • The molecular weight of Black phosphorus is 30.97.

  • Black phosphorus exists in both crystalline and amorphous forms.

Chemical Properties of Black Phosphorus:

  • Following are some of the chemical properties of Black Phosphorus :

  • Black phosphorus is the most stable allotrope of all the allotrope of phosphorus.

  • Black phosphorus is the least reactive allotrope of all the allotrope of phosphorus

Difference Between Black Phosphorus, White Phosphorus and Red Phosphorus

Sr.No

Basis

Black Phosphorus

White Phosphorus

Red Phosphorus

1.

Colour

Black

yellowish

Red

2.

Stability

Most stable among all

Least stable among all

Moderate stable among all

3.

Reactivity

Least Reactive among all

Most Reactive among all

Moderate Reactive among all

Key Features

  • Chemiluminescence is displayed by white phosphorus, while it does not occur in red phosphorus.

  • The primary source of phosphorus is phosphates, which are compounds that possess the (PO43-) phosphate symbol. Phosphates are present in ATP, DNA, RNA, and phospholipids, which are all necessary parts of organisms.

  • Ancient phosphate symbol sources included bone ash and human urine. Human urine was indeed the earliest recorded supply of the basic phosphate symbol.

  • The formation of black phosphorus occurs once the white form is heated at 473 K at high pressure.

Conclusion

Therefore, it can be concluded that among the several allotropic forms of phosphorus, white, red, and black are the most prevalent types. In theory, phosphorus can take on a wide variety of structural configurations; the variety of potential allotropes is comparable to that of carbon. Because of this variation, the electrical frameworks also change greatly, giving each unit a distinct colour. Hence, the synthesis, structure, and properties of those 3 prevalent allotropic forms of phosphorus were concentrated mainly in this article.

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FAQs on Allotropes of Phosphorus

1. What are the Main Allotropes of Phosphorus and Which One is the Most Reactive Form Among Them?

Phosphorus exists in three allotropic forms; these are white phosphorus, red phosphorus and black phosphorus. White phosphorus is the most reactive form of phosphorus. Due to angular strain in its structure, it is less stable. Therefore, more reactive.

2. Why Do We Store White Phosphorus in the Water?

White phosphorus is the most reactive form of phosphorus. It reacts with the oxygen present in the air and catches the fire. Therefore, it is stored in the water to protect it from the air.

3. Is White Phosphorus Stable or Not? Explain with Reason.

No, white phosphorus is not stable. It exists as a P4 molecule in a tetrahedral ring form. In this ring structure, the bond angle is 60°  so the distance between the bonds is very less, due to which the bond-bond repulsion takes place. This repulsion leads to the angular strain in the molecule. Therefore, the molecule becomes highly unstable. Due to this high instability, the molecule becomes very reactive.

4. Why white Phosphorus is stored underwater? 

White phosphorus reacts with the oxygen present in the air and catches fire as a result.  Due to this reason, white phosphorus is stored under water. 

5.  Which is the most reactive allotrope of Phosphorus ?

White Phosphorus is the most reactive phos[hpus among all the allotropes of phosphorus. 

6. What are the three prevalent types of phosphorus?

White, red, and yellow are the three prevalent types of phosphorus.

7. What are other names of white phosphorus?

Tetra Phosphorus and Yellow Phosphorus are the other names for white phosphorus. 

8. Which type of allotrope of phosphorus generates garlic smell?

White phosphorus is one which smells distinctly of garlic.

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