Allotropes of Phosphorus

Phosphorus, an important element, was discovered in the 17th century and played a crucial role in the development of the concept of "element" in Chemistry.

• The initial discovery of phosphorus was in its most unstable form called white phosphorus. 
• Since then, we have discovered that phosphorus can exist in different forms known as allotropes.
• Allotropy refers to the ability of certain chemical elements to exist in different forms while maintaining their similar physical properties. 
• These different forms of the element are called allotropes. 
• Phosphorus has multiple allotropes, and scientists have used quantum-chemical techniques to study and evaluate over 50 different crystalline structures of phosphorus allotropes.

These studies have contributed to our understanding of the stability and properties of different phosphorus allotropes, making phosphorus one of the most extensively studied elements on the periodic table. 

Read More: Difference Between Allotropes and Isomers

Key Terms: Allotropes, Crystalline, Element, Furnace, Phosphorus, White Phosphorus, Black Phosphorus, Red Phosphorus

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What is an Allotrope?

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Allotropy is a fancy word for when an element can exist in different forms or shapes. 

• These different forms of the same element are called allotropes. 
• Many elements can have different allotropes, and each allotrope has its unique physical properties. 
• However, their chemical properties are similar.

Read More: Law of multiple proportions

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Allotropes of Phosphorus

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The following are a few examples of phosphorus allotropes:

White Phosphorus

White phosphorus is a type of phosphorus that looks like wax and is see-through. 

• It is fragile and requires careful handling.
• It doesn't dissolve in water but can dissolve in substances like carbon disulphide or carbon tetrachloride. 
• When heated with a strong alkaline solution called caustic soda in the presence of air, it breaks down and forms sodium hypophosphite and phosphine.

Synthesis and Structure of White Phosphorous

In simple terms, phosphate rock is heated in an electric furnace along with carbon and silica. 

• This process releases phosphorus as a vapor, which can be extracted using phosphoric acid. 
• White phosphorus, which has a circular shape, is commonly used in weapons. 
• The white phosphorus molecules are made up of three phosphorus atoms bonded together at a 60-degree angle. 
• These molecules are attracted to each other by weak forces called Van Der Waals forces.

White Phosphorous

In the picture above, you can see the shape of white phosphorus. 

• It looks like a pyramid with four sides. 
• Each phosphorus particle is connected to three other phosphorus particles through strong bonds. 
• Additionally, there are weak forces that hold these particles together. 
• It's important to know that white phosphorus is highly reactive and dangerous.

Physical and Chemical Characteristics of White Phosphorus 

White phosphorus, in its solid form, looks like wax and is see-through. It is very delicate and needs to be handled with care. It doesn't dissolve in water, but it can break down when in contact with certain substances.

• The solid form of white phosphorus is soft. 
• When it is first made, it is almost colorless, but it gradually turns light yellow. This is why it is sometimes called yellow phosphorus.
• White phosphorus has a low melting point, so it can easily catch fire when exposed to air at temperatures slightly higher than room temperature. 
• To prevent this, it is usually stored in water. 
• When it burns, it produces phosphorus pentoxide.
• When white phosphorus comes into contact with moist air, it undergoes an oxidation process. 
• This reaction creates a bright and sparkling light, making it glow without much effort.
• The molecular weight of white phosphorus is 30.97 g/mol & the melting point is 44°C.
• It is dangerous and very reactive.

Red Phosphorus

Red phosphorus is a great material to prevent fires, especially in things like plastic that can melt easily when they get hot.

• When red phosphorus combines with certain chemicals, it creates a special acid that helps stop flames. 
• This acid, along with other materials, forms a protective layer that keeps the fire from spreading further.

Synthesis and Structure of Red Phosphorus

When white phosphorus is heated to a specific temperature in a special environment for a long time, it changes into red phosphorus. 

• Red phosphorus is made up of many connected units and has a different structure compared to white phosphorus. 
• It is less reactive than white phosphorus because of its unique structure.

Red Phosphorus

Physical and Chemical Characteristics of Red Phosphorus 

Here are few properties of Red Phosphorus:

• Red phosphorus looks like a shiny, crystal-like substance and has a color similar to iron grey. 
• It doesn't cause harm and doesn't have a bad smell. It doesn't break down when exposed to carbon tetrachloride or water.
• When red phosphorus is heated, it doesn't turn into boiling white phosphorus, which is a strong chemical. 
• Instead, it breaks down when in contact with alcoholic potash.
• Normally, red phosphorus is stable and doesn't catch fire in the air. However, if we heat it to around 400°C, it will burn.
• When red phosphorus combines with oxygen at a temperature of 565 K, it forms phosphorus pentoxide.
• If red phosphorus combines with sulphur, it creates a substance called sulphides.

Black Phosphorus

There are two types of black phosphorus called α-black and β-black. When we heat red phosphorus in a closed tube at a temperature of 803K, it changes into the α-black form. This α-black form can turn into gas and disappear in the air. 

• It has either rhombohedral crystals or an opaque monoclinic shape.
• It doesn't get affected by oxygen in the air. 
• On the other hand, when we heat the white form of phosphorus under high pressure at a temperature of 473K, it turns into the β-black form. 
• This β-black form doesn't catch fire in the air until it reaches a temperature of 673K.
• The structure of the black form of this substance is shown below.

Black Phosphorous

Physical and Chemical Properties of Black Phosphorus 

Here are few properties of Black Phosphorus:

• Black phosphorus has a 416 degree celsius melting point. 
• The black phosphorus has a specific gravity of 2.69.
• Black phosphorus has a precise mass of 30.973762 g/mol.
• Black phosphorus has a density of 2.34 g/cm³.
• Black phosphorus dissolves 0.3 g/l in water (H₂O). 
• Black phosphorus has a molecular mass of 30.97. 
• There are crystalline and amorphous forms of black phosphorus.

Read More: Potassium Hypochlorite Formula

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Difference Between Black Phosphorus, White Phosphorus, and Red Phosphorus 

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The basic differences are mentioned in the table below. 

Also Read:

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Things to Remember

• White phosphorus exhibits chemiluminescence, but red phosphorus does not.
• Phosphates, which are substances with the phosphate symbol (PO₄^3-), are the main sources of phosphorus. 
• Phosphates are found in ATP, DNA, RNA, and phospholipids, all of which are critical components of living things. 
• Both bone ash and human urine were used as early phosphate symbol sources. 
• The oldest known source of the fundamental phosphate symbol was human urine. 
• The white form of phosphorus is heated to 473 K under great pressure, where it turns into black phosphorus.

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