Allotropy - Definition of Allotropy, Properties of Allotropes, Examples, Difference between Polymorphism and Allotropy and Uses

Look at this image carefully and try to visualise her in a vivid range of attires and makeovers. For example, imagine the same woman in a royal red gown resembling a queen, or in an all-white cloak resembling a nun, or try to visualise her in a corporate suit and tie resembling a businesswoman. And now let's come back to the same look that she is carrying here, of a simple Indian woman. Yes! The woman in all the portraits that you have imagined of her, flaunting different hairstyles, dresses and gestures, is actually the same one person. In fact it is just one person showcasing so many different personalities. The underlying feature here is quite similar to the phenomenon of Allotropy.

Some of the elements in our periodic table are known to have this property, which allows an element to have multiple physical states in the same physical state. The word "allotropy" comes from the Greek word "allottropia," which means "changeable."

The idea of allotropy was initially put forth by Swedish scientist Baron Jöns Jakob Berzelius in the year 1841. Allotropes are different structural variations of a chemical element that primarily exist in the same physical state and in which the element's atoms are connected to one another in various ways.

Let's explore the vibrant diversity that allotropes exhibit further.

TABLE OF CONTENTS

Definition of Allotropy
Reason of Allotropy
Properties of Allotropes
Examples of Allotropy
Types of Allotropy
Practice Problems
Frequently Asked Questions-FAQs

Definition of Allotropy

The ability of some chemical elements to exist in two or more distinct forms, known as allotropes of the element, while yet existing in the same physical state is known as allotropy.

Allotropes are distinct structural variations of an element in which the bonding between the atoms is altered.

For Example- The carbon atoms are bound together to create sheets of a hexagonal lattice in graphite, single sheets of graphite in graphene, and fullerenes, for instance, as well as a cubic lattice of tetrahedral in diamond (the carbon atoms are bonded together in spherical, tubular, or ellipsoidal formations).

Here are a few facts on allotropy:

Allotropy is a phenomenon that is only restricted to elements and not compounds.

For compounds we use a separate terminology and the phenomenon is polymorphism, but its usage is limited to solid crystals alone.

Only various incarnations of an element within the same physical phase are referred to as allotropy (the state of matter, such as a solid, liquid or gas).

These states of matter's differences do not stand alone as examples of allotropy.

Chemical elements' allotropes are frequently referred to as the element's polymorphs or phases.

For some elements, allotropes differ in their physical phases as well as their molecular formulas or crystalline structures. For instance, two oxygen allotropes, dioxygen (O₂) and ozone (O₃), can both exist in the solid, liquid, and gaseous states. Some elements, like phosphorus, have several solid allotropes that all transform into the same P4 form when heated to a liquid state. Other elements, however, do not preserve unique allotropes in various physical phases.

Reasons for Allotropy

Allotropes are various structural configurations of the same element that can display a wide range of physical and chemical characteristics. The same forces that impact other structures, such as pressure, light, and temperature, can cause changes in allotropic forms and are hence the main reason why allotropes form. Hence allotropes of varying stability can transform from one another due to the following scenarios:

Usually, elements with a wide range of coordination numbers and/or oxidation states show more allotropic forms.
The ability of an element to catenate is another aspect. Allotropes typically consist of covalent molecules with strong catalytic properties. The ability of elements to create lengthy chains is referred to as catenation power.
An atom's stability is impacted by the force it receives per square unit due to the environment's pressure.
The substance is heated to a temperature that is significantly higher than the state it is "usually" in.
When a pure substance (such as pure carbon or pure oxygen) receives a significant amount of energy, the chemical structure is completely altered.

Properties of Allotropes

Allotropes are multiple structural forms of the same element, and as a result, they can display various physical and chemical characteristics. The same forces that affect other structures like light, pressure, and temperature also affect allotropic forms. The stability of the various allotropes thus depends on particular circumstances.

The two allotropes of carbon, diamond and graphite, differ in terms of their appearances, hardness ratings, melting points, boiling points, and reactivities.

Some elements have several solid allotropes and some exist in all three phases. For example, phosphorus has a number of solid allotropes (white, red, black phosphorus) and they all revert to the same P4 in their molten/liquid state. Also oxygen and ozone can are two allotropes that can both exist in solid, liquid and gaseous states.

The stability of a specific allotrope depends on a set of circumstances.

For instance, iron suffers an alteration known as ferrite that transforms it from a body-centred cubic structure to a face-centred cubic structure (austenite) over 906 °C, while tin experiences a tin pest that transforms it from a metallic form to a semiconductor below 13.2 °C (55.8 °F).

Ozone (O₃), which exhibits different chemical behaviour from dioxygen (O₂), is an example of an allotrope.

Examples of Allotropy

Allotropes of carbon: Diamond, Graphite, Fullerenes (Buckminsterfullerene). Due to the property of catenation and p𝛑–p𝛑 bond formation, carbon is able to show two allotropic forms namely crystalline and amorphous.

In crystalline form, the constituent particles are arranged in a definite pattern (e.g- diamond and graphite). In amorphous form, there is an irregular arrangement of constituent particles (e.g. fullerenes).

Graphite has a layered structure held by van der Waals forces, and the distance between two layers is 340 pm. Each layer is composed of planar hexagonal rings of sp² hybridised C atoms. Each C-atom in the hexagonal ring makes, three sigma bonds with three neighbouring carbon atoms, and the fourth electron forms a 𝛑 bond.

Diamond has a crystalline lattice in which each carbon atom is sp3 hybridised. Each C-atom is linked to four, other carbons and the C-C bond length is 154 pm. The structure extends in space and produces a rigid 3D network of carbon atoms. It is very difficult to break extended covalent bonding, which makes diamond one of the hardest substances on the earth.

Fullerenes, the amorphous allotropes of carbon, are the only pure form of carbon because they have a smooth structure without ‘dangling’ bonds. Fullerenes are cage-like molecules., C60 molecule has a shape like a soccer ball and is known as Buckminsterfullerene.

In the history of nanotechnology, the discovery of C60, a molecular allotrope of carbon, was a turning point. Twenty-five years later, carbon is still the preferred ingredient for making straightforward yet useful materials.

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.

Allotropes of oxygen: Oxygen has majorly two allotropes namely oxygen and ozone.

Allotropes of sulphur: Rhombic (α-Sulphur.), Monoclinic (β-Sulphur), Plastic sulphur (δ-sulphur).

Types of Allotropy

Enantiotropic Allotropy

Enantiotropy is an allotropy in which the various forms are stable under diverse conditions and are capable of reversible interconversion under specific pressures and temperatures.

Rhombic and monoclinic are the two crystalline allotropes of sulphur that can exist in solid form. Any needle-shaped crystals of monoclinic sulphur that cool below this temperature will progressively change into the rhombic form since the rhombic form is stable below 95.5°C.
The monoclinic form, on the other hand, is stable between 95.5°C and the melting point (119.25°C), and when kept at a temperature in this range, the rhombic form gradually transforms into the monoclinic form.
Rhombic sulphur, on the other hand, melts at 112.8°C when heated rapidly and produces an amber-colored liquid.

Monotropic Allotropy

Monotropic allotropes have a single form that is always the most stable under everyday circumstances.

In contrast, the only stable solid allotrope of carbon is graphite, and at all temperatures, the transformation of diamond into graphite is extraordinarily slow.
If only one of these allotrope pairs is stable across the whole temperature range, they are considered to be monotropic.
The fact that diamond exhibits greater resilience to chemical reactions and has a stronger molecular structure.
The various configurations of the carbon atoms in space may be responsible for all of the differences between the two allotropes.
In order for the diamond lattice to stretch in all three dimensions, each carbon atom in a diamond is connected to four nearby carbon atoms by valence bonds that are each the same length.

Dynamic Allotropy

Dynamic allotropy is the third category of allotropy. The two types are in dynamic equilibrium in this. Temperature and occasionally pressure affect how many of the two allotropes are in equilibrium with one another. This kind of allotropy exists between the two liquid forms of sulphur, and when switching between them, the colour and viscosity vary as well (resistance to flow).

The two liquid allotropes of sulphur are referred to as λ-sulphur and μ-sulphur. The fraction of λ-sulphur is fairly large at temperatures near the melting point, but as the temperature increases, more and more of the λ-sulphur form transforms into μ-sulphur. In the molecules of both forms, eight atoms are grouped in a ring that, when heated, breaks apart to produce chains that still contain eight atoms. Their interaction between these chains is what causes the viscosity to grow at 180°C.
Oxygen atoms can be found in molecules with two or three atoms. While the latter (O₃) is known as ozone, the former (O₂) is also known as oxygen. Additionally, these molecules display dynamic allotropy. Ozone is more likely to form under higher pressure since there are fewer molecules that must be accommodated: 3O₂ 2O₃

Practice Problems

Q.1. What is the difference between allotropy and polymorphism?

Answer: While allotropy is the ability of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of these elements, polymorphism is the capacity of a solid material to exist in more than one form or crystal structure.

When a chemical element can remain steadily in more than one crystal form, it is said to have allotropy. The same phenomena that a chemical substance displays is polymorphism.

Q.2. Which is the most thermodynamically stable allotrope of phosphorus?

White Phosphorus
Black Phosphorus
Red Phosphorus
Scarlet Phosphorus

Answer: (B)

Solution:

The lattice structure of black phosphorus is an interlinked ring of six P atoms. Here, each phosphorus atom is bonded to three other phosphorus atoms. This makes it a puckered sheet-like strongly interlinked structure, which is difficult to break. Hence, it is the most stable allotrope. This allotrope has the maximum amount of interlinking.

Q.3. Catenation, which is a major cause of allotropy in carbon, is due to formation of?

dπ-dπ
p𝛑-p𝛑
p𝛑-d𝛑
None of the above

Answer: (C)

Solution:

Allotropes of Carbon are : Diamond, Graphite, Fullerenes, Buckminsterfullerene. Due to the property of catenation and p𝛑–p𝛑 bond formation, carbon is able to show two allotropic forms namely crystalline and amorphous.

Q.4. Which type of allotropy is shown by dioxygen and ozone?

Monotropic Allotropy
Enantiotropic Allotropy
Dynamic Allotropy
Polymorphism

Answer: Oxygen atoms can be found in molecules with two or three atoms. (O₃) is known as ozone and

(O2) is also known as oxygen. Additionally, these molecules display dynamic allotropy. Ozone is more likely to form under higher pressure since there are fewer molecules that must be accommodated: 3O₂ 2O₃

So option C is the correct answer.

Frequently Asked Questions-FAQs

Q.1. What are nano allotropes?
Answer: Professor Rafal Klajn of the Weizmann Institute of Science's Organic Chemistry Department first put up the idea of nano allotropy in 2017. Nanoporous materials with the same chemical makeup (such as Au) but different nanoscale architectural characteristics are known as nano allotropes (that is, on a scale 10 to 100 times the dimensions of individual atoms). Such nano allotropes might aid in the development of incredibly small electrical devices and find use in various industrial settings. Surface-enhanced Raman scattering on numerous different nano allotropes of gold served as a demonstration of how the various nanoscale structures translate into various characteristics. Additionally, a two-step procedure for producing nano allotropes was developed.

Q.2. What are the major allotropes of Group 16 elements?
Answer: Group 16 elements are all allotropic. For instance, oxygen can be found as both an oxygen and an ozone molecule. There are several allotropic forms of sulphur, but the yellow ortho-rhombic, alpha, and beta-monoclinic forms are the most significant. These sulphate allotropes are all non-metallic. There are eight allotropic types of selenium. Three of these eight allotropic forms—the red monoclinic forms—are allotropic. Se8 rings can be found in these allotropic forms of selenium. Grey-hexagonal "metallic" selenium, which is made up of polymeric helical chains, is the form that is thermodynamically stable. The substance is a prevalent form of black, amorphous selenium. The only selenium allotrope that conducts electricity is grey selenium. Tellurium only exists in one crystalline form with a chain structure resembling grey selenium.

Q.3. Give some examples of polymorphism.
Answer: Polymorphism is for compounds (solid crystalline state) and allotropy is for single chemical elements.

There are two types of silver nitrate (AgNO₃): rhombohedral and orthorhombic. The crystalline forms of potassium nitrate (KNO₃) are rhombohedral and orthorhombic, respectively. There are two crystalline types of calcium carbonate (CaCO₃): trigonal and orthorhombic.

Q.4. Name the allotropes of Silicon.
Answer: The major allotropes of silicon are crystalline silicon which has diamond cubic structure. Amorphous silicon and silicene which is similar to graphene and is like a buckled planar single layer of silicon atoms.

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