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Jai Ganesh

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Allotrope/Allotropy

Allotrope/Allotropy

Gist

Allotropes are different structural forms of the same element in the same physical state (solid, liquid, or gas). They possess distinct physical and chemical properties due to variations in how their atoms are bonded and arranged. Common examples include carbon (diamond, graphite, graphene) and oxygen (dioxygen, ozone).

Allotropes are different structural forms of the same element in the same physical state (solid, liquid, or gas). While they share identical chemical properties, they possess distinct physical properties (e.g., density, hardness, electrical conductivity) due to variations in how their atoms bond or arrange themselves, such as in diamond and graphite.

Summary

Allotropy or allotropism is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: the atoms of the element are bonded together in different manners. For example, the allotropes of carbon include diamond (the carbon atoms are bonded together to form a cubic lattice of tetrahedra), graphite (the carbon atoms are bonded together in sheets of a hexagonal lattice), graphene (single sheets of graphite), and fullerenes (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).

The term allotropy is used for elements only, not for compounds. The more general term, used for any compound, is polymorphism, although its use is usually restricted to solid materials such as crystals. Allotropy refers only to different forms of an element within the same physical phase (the state of matter, i.e. plasmas, gases, liquids, or solids). The differences between these states of matter would not alone constitute examples of allotropy. Allotropes of chemical elements are frequently referred to as polymorphs or as phases of the element.

For some elements, allotropes have different molecular formulae or different crystalline structures, as well as a difference in physical phase; for example, two allotropes of oxygen (dioxygen, O2, and ozone, O3) can both exist in the solid, liquid and gaseous states. Other elements do not maintain distinct allotropes in different physical phases; for example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

Details

Have you ever wondered looking at a chameleon (if at all you could recognise one sitting on a branch of a tree or so) how quickly it can change its colour. Indeed this is intriguingly admirable how one single species can exist in multiple physical forms without changing its core properties or values!

Certain elements in our periodic table are known to exhibit such properties wherein a particular element in the same physical state can exhibit more than one physical form. Allotropy originated from the Greek word ‘allottropia’ meaning ‘changeable’.

In the year 1841, Swedish scientist Baron Jöns Jakob Berzelius first proposed the concept of Allotropy.

Let’s dig deeper into the vivid variations shown by allotropes!

Allotropes are different structural modifications of a chemical element existing mostly in the same physical state; wherein the atoms of the element are bonded together in a different manner.

What is Allotropy?

Allotropes are different structural modifications of the same element and can exhibit quite different physical properties and chemical behaviours. The change between allotropic forms is caused by physical forces like pressure, light, and temperature. Hence, the stability of a particular allotrope of an element depends on particular conditions and its structural composition.

Many elements (especially non-metals) from the p-block of the periodic table exhibit allotropy. For example, carbon, oxygen, phosphorus, sulphur, and selenium from the p-block exhibit allotropy. Allotropes of carbon include diamond, graphite, graphene, fullerenes, carbon nanotubes, etc. Phosphorus also has many solid state allotropes and also a gaseous phase allotrope.

Properties of Allotropes

At different temperatures, pressure conditions and atmospheric conditions, an element finds stability in different geometries where atoms are bonded in different ways. Hence, these elements show allotropy.

* Allotropes have different structural features belonging to the same element and therefore can exhibit different physical and chemical properties.
* The change between one allotropic form to another is caused by physical forces like pressure, light, and temperature.
* Stability of the different allotropes relies on specific conditions.
* For example, diamond and graphite (two allotropes of carbon) have different appearances, hardness values, melting points, boiling points, and reactivities.
* Allotropes of some elements have different molecular formulae or different crystalline structures, as well as they differ in physical phase. For example, two allotropes of oxygen (dioxygen, O2 and ozone, O3) can both exist in the solid, liquid and gaseous states.
* All elements showing allotropy do not maintain distinct allotropes in different physical phases. For example, phosphorus has numerous solid allotropes, which all revert to the same P4 form when melted to the liquid state.

Allotropes of Phosphorus

Allotropes of phosphorus are originally P4 and there are around 12 allotropes of phosphorus. The major ones are white phosphorus, red phosphorus, black phosphorus, diphosphorus (a gaseous allotrope), scarlet and violet phosphorus.

Phosphorus is a solid non-metallic compound at room temperature. The most common (and reactive) of all its allotropes is white (or yellow) phosphorus which looks like a waxy solid or plastic. The other common form of phosphorus is red phosphorus which is much less reactive and is one of the components of the matchstick head. Red phosphorus can be transformed into white phosphorus by careful heating.

Frequently Asked Questions - FAQs

Question 1. Why white phosphorus has the structure of P4 and sulphur can exist as S2?
Answer 1: White phosphorus has the structure of P4 as one phosphorus atom can form three bonds at a time. Thus, phosphorus forms a P4 white phosphorus tetrahedron (being sp3hybridised), while sulphur can only form two bonds. Hence, sulphur only forms rings and chains.

Question 2. Which allotrope of phosphorus is poisonous in nature?
Answer 2: The least stable, the most reactive, the most volatile, the least dense, and the most toxic of the allotropes is white phosphorus. It eventually changes to red phosphorus, a light and heat-accelerated transition.

Question 3. Which phosphorus allotrope is used in matchstick?
Answer 3: Red phosphorus. The striking surface of a matchbox is made up of red phosphorus and powdered glass, which on friction with the stick converts to white phosphorus and ignites a flame in the air.

Question 4. Why do some of the beaches show chemiluminescence?
Answer 4: White phosphorus present in the ocean helps in the production of microbes and tiny marine plants called phytoplankton. When white phosphorus is particularly abundant in the water, phytoplankton produce and store a form of phosphorus called polyphosphate to use later during times of phosphorus scarcity. This is why phytoplankton rich beaches produce chemiluminescence.

Additional Information

Allotropy is the existence of a chemical element in two or more forms, which may differ in the arrangement of atoms in crystalline solids or in the occurrence of molecules that contain different numbers of atoms. The existence of different crystalline forms of an element is the same phenomenon that in the case of compounds is called polymorphism. Allotropes may be monotropic, in which case one of the forms is the most stable under all conditions, or enantiotropic, in which case different forms are stable under different conditions and undergo reversible transitions from one to another at characteristic temperatures and pressures.

Elements exhibiting allotropy include tin, carbon, sulfur, phosphorus, and oxygen. Tin and sulfur are enantiotropic. The former exists in a gray form, stable below 13.2 °C, and a white form, stable at higher temperatures. Sulfur forms rhombic crystals, stable below 95.5 °C, and monoclinic crystals, stable between 95.5 °C and the melting point (119 °C). Carbon, phosphorus, and oxygen are monotropic. Graphite is more stable than diamond, red phosphorus is more stable than white, and diatomic oxygen, having the formula O2, is more stable than triatomic oxygen (ozone, O3) under all ordinary conditions.

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