Allotropic Forms of Phosphorous

ChemistryThe p-Block Elements

Introduction

The allotropes of phosphorus are found in different allotropic forms. The main allotropes of phosphorus include the white phosphorus, red phosphorus and black phosphorus. In addition to this, there is also the existence of violet phosphorus. The black phosphorus is black in colour while the white phosphorus bears yellowish colour and red phosphorus has red colour.

Allotropes of Phosphorus

Allotropy is called the science of occurrence of an element in multiple or more than one physical shape. Here allotropes refer to different types of physical shapes of the same element. It is seen that allotropes is the property of some chemical elements to exist in two or more different forms.

In nature, there is the presence of various allotropic forms of phosphorus. The important forms include −

• White phosphorus

• Black phosphorus

• Red phosphorus

White Phosphorus

White phosphorus is also called tetra phosphorus and yellow phosphorus. It cannot be fetched naturally but it can be simulated from phosphate rocks. It has some features and they are as follows:

Physical properties

Some physical properties of white phosphorus are described below

• It looks like a waxy solid because it is translucent.

• It contains a garlic-like odour.

• White phosphorus is known as a polar compound and the reason for this is that it is soluble in carbon dioxide

• The molecular weight is 30.97g/mol.

• It is corrosive and highly toxic in nature.

Chemical properties

The chemical properties of this phosphorus include

• It results in metal phosphide when reacted with a metal.

• It reacts with the oxygen present in the air and catches fire, and due to this reason it is stored underwater

• It is seen that when this phosphorus is heated at 573K in an inert atmosphere for a few days, it creates red phosphorus.

Structure

The white phosphorus is made of four atoms and all the atoms are connected in a covalent bond. The structure of the white phosphorous looks like a ring and it creates an angle bond of 60°.

Red Phosphorus

Red phosphorus is another kind of allotrope of phosphorus and it can be got by heating the white phosphorus. It can be gotten through heating at a high temperature of around 573 K in an inert atmosphere for several days. It consists of some properties and has a specific structure and they are described below.

Physical properties

Some physical properties of red phosphorus as follows

• It isodourless phosphorus.

• Red phosphorus is non-toxic in nature.

• It bears the melting point is 860 K

• The molecular weight is 30.97 g/ mol.

• It does not glow in the dark.

• The name suggests that it is deep red in colour.

Chemical properties

Some chemical properties of red phosphorus include

• This phosphorus reacts with oxygen at 565 K and results in phosphorus pentoxide.

• It also reacts with sulfur and results in sulphides.

Structure

It is seen that red phosphorus contains the structure the same as black phosphorus and it is polymeric in nature.

Black Phosphorus

Black phosphorus is caused by heating red phosphorus at 416°C. The red phosphorus is converted into black phosphorus when it is heated in a sealed tube and at the right temperature. It includes some characteristics and they are as follows −

Physical properties − Some physical properties of black phosphorus are

• It is black in colour.

• The melting point is 416°C.

• The specific gravity is 2.69.

• Molecular weight is 30.97.

• It exists in both crystalline and amorphous forms.

Figure 1: Physical properties of black phosphorus

Chemical properties − the chemical properties are

• It is called the most stable allotrope among the allotrope of phosphorus.

• It is the least reactive allotrope.

Structure − The structure of this phosphorus is zig-zag lines of P - P bonds. It mainly looks like a honeycomb and that is the reason it is called a honeycomb structure. The bond angles form 99° and here the bond length is 218 Pm.

Difference between Black, White, and Red phosphorus

Serial No.	Basis	Black phosphorus	White phosphorus	Red phosphorus
1	Colour	Black	Yellowish	Red
2	Stability	Most stable	Least stable	Moderate stable
3	Reactivity	Least reactive	Most reactive	Moderate reactive

Table 1: Difference between black, white, and red phosphorus

Uses of phosphorus

• Phosphorus is most important for forming lives and it forms a basic constituent in the animal and plant matter.

• It is found present in blood, bones and the brain of all the animals and also, in living cells.

• Some of the compounds are applied in the industries and the most essential of these chemicals are orthophosphoric acid and phosphatic composts.

Conclusion

This tutorial shows different types of allotropes of phosphorus and they bear different characteristics. All the red, black, and white phosphorus contain both physical and chemical properties, and a structure. They have specific melting points and specific colours having the molecular weight. It states that the molecular weight of black phosphorus, white phosphorus, and red phosphorus is 30.97 g/ mol.

FAQs

Q1. Can white phosphorus be stored in the water?

Ans. The white phosphorus is considered to be the most reactive form of phosphorus. It mainly reacts with oxygen present in the air and it catches fire. That is the reason it is stored in the water for protecting it from the air.

Q2. Which is the most reactive allotrope of phosphorus?

Ans. It is seen that white phosphorus is the most reactive allotrope of phosphorus. Although it is most reactive; however, it is the least stable among all the allotrope of phosphorus.

Q3. Which phosphorus is applied in matchstick?

Ans. The red phosphorus is applied in a little amount in the matches. Here the potassium chlorate is ignited by the heat from this and thus the match gets flamed.