Allotropy: Definition, Examples and Questions

Allotropy can be defined as the phenomenon in which an element can exist in two or more forms. While allotropes do retain identical physical states and usual chemical properties, they do not necessarily retain the same chemical activity or molecular structure. Several p-block elements showcase allotropic nature. This is because allotropy is the property that majorly non-metals have and p-block elements or group 15 elements contain non-metals in the chart. Some elements exhibiting allotropic behavior are Carbon, Phosphorus and Sulphur. The word "Allotropy" is derived from the Greek word allotropia that means the ability to change.

Key Terms: Allotropy, Allotropic, Sulphur, Oxygen, Carbon, Phosphorous.

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Properties of Allotropy

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The properties of allotropy are as follows -

• Allotropes may showcase different physical and chemical properties as they are different structural types of the same element
• Sometimes, the allotropes may always not be in the same state.
• The stability of different allotropes varies based on certain conditions.
• The changes in structure and forms of allotropes are caused by the forces of light, temperature as well as pressure. They also influence the structural arrangements of other elements. 

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Examples of Allotropy

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There are various examples of elements that exhibit allotropic nature, especially the non-metals or the p-block elements. Some of them are as given below-

• Oxygen

Oxygen has two common allotropes, namely, O₂ (diatomic oxygen) and O₃ (Ozone). While diatomic oxygen is a reactive and paramagnetic allotrope of oxygen, ozone is a pungent-smelling allotrope of oxygen. 

In its pure form, ozone can be found as a pale blue gas, violet-black solid and dark-blue liquid. Ozone is quite unstable thermodynamically and immediately decomposes into oxygen giving off heat. 

• Carbon

The allotropes of Carbon element generally occur in two forms: Crystalline form and Amorphous form. 

Allotropes of Carbon

Under Crystalline, there are three allotropes of carbon, namely, graphite, diamond and fullerene. In Amorphous, there is coal, lamp black and charcoal. The amorphous form is also known as micro-crystalline form. 

1. Diamond: It is a transparent, colourless and hardest allotrope of carbon that shines in the dark. It does not conduct electricity and when heated strongly, produces only carbon dioxide. Used for class cutters and strong drills due to its hard and rigid nature, the atoms of diamond are bonded with 4 carbon covalent bonds. 
2. Graphite: It is an electricity conducting allotrope of carbon which is greyish in colour and opaque in nature. Graphite is soft and slippery to touch. 
3. Fullerene: Fullerene or Buckminsterfullerene is regarded as the purest form of carbon allotrope. The atoms of carbon are arranged in the shape of a football and contain 60 carbon atoms made of two types of carbon rings (5-carbon and 6-carbon rings). The chemical formula of fullerene is C60. 

Read Also: Periodic Properties Of Elements

• Sulphur

Although sulphur has various allotropes, the most common are as follows.

1. Yellow Rhombic Sulphur: This allotrope has a yellow colour, a melting point of 385.8 K, and a specific gravity of 2.06. When roll sulphur solution in CS₂ is evaporated, rhombic sulphur crystals emerge. It is insoluble in water but somewhat soluble in benzene, alcohol, and ether.
2. Monoclinic Sulphur: It has a melting point of 393 K and a specific gravity of 1.98. It dissolves in CS₂. This type of sulphur is made by melting rhombic sulphur in a dish and cooling it until a crust forms. The leftover liquid is poured out via the holes drilled in the crust. When the crust is removed, colorless needle-shaped crystals of β-sulphur develop. It is stable at 369 K and converts into β-sulphur above that temperature.

• Phosphorus

Phosphorus exists in various allotropic forms and out of those, the three main allotropes of phosphorus are as follows: White Phosphorus, Red Phosphorus, and Black Phosphorus. 

1. White Phosphorus: It is a highly reactive allotrope of Phosphorus. It has to be kept in water to avoid contact from air because it spontaneously burns once exposed to air by giving white fumes of P₄O₁₀. Each phosphorus atom is bonded with three P atoms (because of the P₄ units) via non-metallic covalent bonds to complete the valence shells. Thus, the P4 tetrahedra causes an extreme angular strain of 60 degrees that leads to instability and high reactivity of white phosphorus.

Allotropes of Phosphorus

2. Red Phosphorous: It is a fairly stable allotrope of Phosphorus. It has a polymeric shape consisting of chains of P4 atoms. Interestingly, red phosphorus can be obtained from white phosphorus by heating white phosphorus for several days at 573 K. 
3. Black Phosphorus: It is the most stable allotrope of phosphorus that can be obtained by heating red phosphorus under high pressure for a long while. It has two more forms: α-black phosphorus and β-black phosphorus. The β-black phosphorus is prepared by heating white phosphorus at 473 K under high pressure. 

Read Also: Sulphur Dioxide

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Things To Remember

1. Allotropy refers to the property of an element to exist in two or more forms by usually retaining its physical state but with different physical and chemical properties. 
2. The different forms of elements exhibiting allotropism are called allotropes. The changes in their forms are caused by the influence of light, temperature and pressure. 
3. For example, the allotropes of Carbon are diamond, graphite and Fullerene. 
4. Phosphorus has white phosphorus, red phosphorus and black phosphorus as its allotropes. 
5. Oxygen has diatomic oxygen and ozone as its allotropes.