allotropism, allotropy

There are a variety of forms in which certain elements can exist. Do you know that diamond and graphite are both the same - just pure carbon? And yet they are so different. As diamond is the hardest, graphite is one of the softest. But how and why are they different, if both are made of the same element? 

This is what we are going to learn in this lesson. 

Learning Objectives 

By the end of this lesson, you should be able to:

• Describe the meaning of allotropy.
• Describe allotropes of different elements.
• Explain the features of different allotropes.
• Different types of allotropes.
• Difference between allotropism and polymorphism.

What is allotropy?

Allotropy, which is also known as allotropism, refers to the property of the existence of some chemical elements in two or more different forms. These different forms are known as the allotropes of the elements. Allotropes are different structural modifications of an element. This is due to the fact that the atoms of the element are bonded together in a different manners. 

For example, the allotropes of carbon include diamond, graphite, graphene, and fullerene.

Do all elements have allotropes? The answer is No. Only some elements have allotropes. 

The term allotropy is used for elements only, not for compounds. Allotropy refers only to different forms of an element within the same state (i.e., different solid, liquid, or gas forms); these different states are not, themselves, considered examples of allotropy.

Allotropes have different molecular formulas in some elements despite the difference in phase. For example, in oxygen, two allotropes: dioxygen O2 and ozone O3 can both exist in different states, solid, liquid, or gas.

 

Types of allotropy: Monotropic and Enantiotropic

Allotropes can be monotropic or enantiotropic.

• In monotropic allotropes, only one of the forms is most stable under different conditions. It happens when a particular form of an element maintains its stability under normal conditions, while the other form(s) change, eventually converting into the stable one. Graphite and diamonds, as mentioned earlier, are allotropes of carbon, with graphite being the stable one and diamond converting slowly into graphite. 
• In the enantiotropic form, different forms are stable at different conditions and undergo reversible transitions. It happens when a specific condition such as temperature or pressure causes one form of a substance to transition into another form. For example, there are two allotropes of tin - white tin and gray tin. White tin is stable at temperatures above 13.2° and gray tin is stable at temperatures below 13.2°. At temperatures below 13.2° C, it changes from white tin to gray tin. 

 

Allotropism Versus Polymorphism

Allotropism refers only to the different forms of pure chemical elements. The phenomenon in which compounds display different crystalline forms is called polymorphism.

 

Allotropes in Periodic Table

Allotropes occur only with certain elements, in Groups 13 through 16 in the Periodic Table.

Group 13

Boron (B), the second hardest element, is the only allotropic element in Group 13. It is second only to carbon (C) in its ability to form element-bonded networks.

Allotropes of Boron

• amorphous boron
• red crystalline α-rhombohedral boron
• black crystalline β-rhombohedral boron (the most thermodynamically stable allotrope)
• black crystalline β-tetragonal boron

Group 14

In Group 14, only carbon and tin exist as allotropes under normal conditions.

Allotropes of Carbon

The allotropes of carbon include:

• Diamond, where the carbon atoms are bonded together in a four-cornered lattice arrangement
• Graphite, where the carbon atoms are bonded together in sheets of a six-sided lattice
• Graphene, single sheets of graphite; and
• Fullerene, where the carbon atoms are bonded together in spheres, cylinders, or egg-shaped formations

Diamond and graphite are the most well-known allotropes of carbon. The properties of diamond and graphite are very different with diamond being transparent and very hard while graphite is black and soft (soft enough to write on paper).

Graphite is the most thermodynamically stable form of carbon. Graphite is a dark, waxy solid, used extensively as a lubricant. It is also a very good conductor of electricity and can be used as the material in the electrodes of an electrical arc lamp. Graphite is the most stable form of solid carbon ever discovered. It also comprises the “lead” in pencils.

Diamond has the highest melting point and is the hardest of the naturally occurring solids. Its hardness and high dispersion of light make it good for use in jewelry. It also has industrial uses. Its hardness makes it an excellent abrasive.

Allotropes of Tin

Tin has two main allotropes:

• At room temperature, the stable allotrope is β-tin, a silvery-white, malleable metal. It is also known as white tin. It has a body-centered tetragonal structure.
• at low temperatures, it transforms into the less dense gray α-tin, which has a diamond cubic structure. It is also known as gray tin. It has a diamond-type lattice structure.
  
  Two more allotropes, γ and σ, exist at temperatures above 161°C (322 °F) and pressures above several GPa.  In cold conditions, β-tin tends to transform spontaneously into α-tin, a phenomenon known as “tin pest” or “tin disease”. Tin pest was a particular problem in Northern Europe in the 18th century as organ pipes made of tin allows would sometimes be affected during long cold winters.

Group 15

There are two allotropic elements in Group 15, phosphorous and arsenic.

Allotropes of Phosphorus

The main allotropic forms of phosphorous forms are:

• White or yellow phosphorus – It is a waxy solid and very poisonous. It is the most volatile, most reactive, and most toxic, but the least thermodynamically stable form of phosphorous. It glows in the dark and can spontaneously combust in the air.
• Red phosphorus – It is found on the side of matchboxes. Red phosphorous is formed when white phosphorus is heated to 250°C and forms a vapor. The vapor is then collected underwater.
• Black phosphorus – This is the most thermodynamically stable, and least reactive, form of phosphorous. It exists as three crystallines (orthorhombic-, rhombohedral- and metallic, or cubic) and one amorphous, allotrope.
• Violet phosphorus – It is also known as monoclinic phosphorus or Hittorf’s phosphorus after its discoverer.

Only white and red phosphorus are of industrial importance.

Allotropes of Arsenic

Arsenic exists in a number of allotropes. Its two most common allotropes are – yellow and metallic gray.

• Gray arsenic is a brittle shiny solid.
• Yellow arsenic is soft and waxy. It is reactive and very toxic and converts to gray arsenic when exposed to light at room temperature.
• Another allotrope is black arsenic.

Group 16

There are only three allotropic elements in Group 16 – oxygen, sulfur, and selenium.

Allotropes of Oxygen

A diatomic molecule made up of 2 oxygen atoms with the molecular formula O2 commonly referred to as molecular oxygen or dioxygen. It is the most common form of elemental oxygen. It is a colorless gas at room temperature and forms about 21% of the earth’s atmosphere. It exists as a diradical and is the only allotrope with unpaired electrons.

A triatomic molecule made up of 3 atoms of oxygen with the molecular formula O3 is referred to as ozone. Ozone is thermodynamically unstable and highly reactive. It was discovered in 1840, by Christian Friedrich Schonbein, and exists as a pale blue gas at normal temperature and pressure conditions.

Both allotropes of oxygen, dioxygen, and ozone, are made up only of oxygen atoms, but they differ in the arrangement of the oxygen atoms:

• Molecular oxygen (dioxygen), O2, is a linear molecule
• Ozone, O3, is a bent molecule.  It is highly reactive.

Ozone functions as a protective shield for the biosphere against the mutagenic and harmful effects of UV radiation.

Tetraoxygen is another allotrope of oxygen. It is also known as oxozone. It exists as a deep red solid that is created by pressurizing O2 to the order of 20 GPa.

Allotropes of Sulfur

At present, about 30 well-characterized sulfur allotropes are known.

α-sulfur forms yellow, rhombic crystals out of 8-membered rings of sulfur atoms (S8). It is also known as rhombic sulfur, and is the predominant form found in “flowers of sulfur”, “roll sulfur”, and “milk of sulfur”.

β-Sulfur is a yellow solid with a monoclinic crystal form and is less dense than α-sulfur. It is also known as monoclinic sulfur. It is unusual because it is only stable above 95.3 °C, below this it converts to α-sulfur. 

γ-sulfur forms yellow, monoclinic, needle-like crystals out of 8-membered rings of sulfur atoms (S8). It is sometimes called “nacreous sulfur” or “mother of pearl sulfur” because of its appearance. It is the densest form of the three.

Allotropes of Selenium

Selenium (Se) also exists in several allotropic forms – gray (trigonal) selenium, rhombohedral selenium, three deep-red monoclinic forms (α -, β -, and γ –selenium), amorphous red selenium, and black vitreous selenium. The most thermodynamically stable and densest form is gray (trigonal) selenium, which contains infinite helical chains of selenium atoms. All other forms revert to gray selenium on warming. In keeping with its density, gray selenium is regarded as metallic, and it is the only form of selenium that conducts electricity. A slight distortion of the helical structure would produce a cubic metallic lattice.

 

Different properties of the allotropes 

Allotropes of the same element can show different physical and chemical behaviors. The change in allotropic forms is facilitated by the same forces that affect other structures, they include temperature, pressure, and light. For example, the chemical behavior of ozone is different from that of dioxygen; ozone is a stronger oxidizing agent than dioxygen.

Lesson Summary

• Allotropes are different structural modifications of an element.
• Allotropism is the existence of some chemical elements in two or more different forms.
• Allotropism is a result of the difference in the bonding of atoms of an element.
• The term allotropy is only used in elements but not for compounds.
• Allotropes of the same element can show different physical and chemical properties.