Difference between allotropes and isomers

Introduction

Jöns Jakob Berzelius, the Swedish chemist proposed the concept of isomerism and allotropes. The concept of allotropy was proposed in the year 1841. The allotropes of Oxygen in the O₂ and O₃ has been found in the year 1860. In 1827 the property of isomerism was first noticed and was made by the preparation of cynic acid by Friedrich Woehler. Since the properties of cyanic acid are different from fulminic acid even though both have the same elemental composition. After some additional discoveries made by Woehler that urea and ammonium cyanate were the same, Jöns Jakob Berzelius introduced the term Isomerism. These two terms are very important concepts and have many applications in the science field.

What are Isomers?

The presence of molecules that have different chemical and physical properties but an equal number of similar atoms or the same molecular formula is Isomerism. And the chemical compounds that show this property are called Isomers. All the molecules are mobile compounds that can make several motions and that can lead to different shapes. And this rotational motion of molecules present in a compound is responsible for Isomerism. There are two types of isomerism. And are Constitutional isomers and stereoisomers.

Constitutional isomers are also provided with the name structural isomers since they differ in the connectivity of molecules present in the compound. The constitutional isomers further include functional isomers, position isomers, and chain isomers. Hydrocarbons such as methane(CH₃), ethane(CH₃ CH₃), and propane(CH₃ CH₂ CH₃) have no constitutional isomers. Since their connectivity cannot be changed into any form. While the fourth and fifth members of hydrocarbons have constitutional isomers. They are,

1840460mahesh, Alkane structure 3, CC BY-SA 4.0

So it is easily observable that as the number of atoms in a molecule increases the chance of constitutional isomers will also increase.

The chemical compounds that have different spatial orientations but the same chemical composition are stereoisomers. The stereoisomers are further classified into enantiomers and diastereomers. Cis-trans isomer is a class of diastereomers. For example in ethane molecules, the rotation of molecules in the single bond will result in the formation of two types of stereoisomers. They are staggered ethane and eclipsed ethane. It is shown in the below figure

Dissolution, Staggered and eclipsed, CC BY-SA 3.0

Examples of Isomers

Examples of isomers can be given as 1-pentene and 2-pentene are two structural isomers.

$$\mathrm{CH_3-CH_2-CH_2-CH=CH_2 -1-pentene.}$$

$$\mathrm{CH_3-CH_2-CH=CH-CH_2-2-pentene.}$$

Ethyl alcohol and Dimethyl ethers are examples of functional isomers. That is,

The cis and trans isomers but-2-ene are shown below.

What are Allotropes?

Different chemical compounds contain only one type of atoms allotropes. And the property or the ability of such elements to exist in these forms is called allotropism. The elements present in group 13 to group 16 are more fond of allotropism. So the elements which show allotropism has different forms. So the existence of chemical elements in two or more different forms is allotropism or allotropy. The physical and chemical properties of the allotropes formed in this way are different. The allotropes of carbon are commonly heard one they are Diamond and Graphite. As we know from their appearance itself, these two compounds are entirely different. Diamond is the hardest one with a shiny appearance while graphite is a soft material with no shine. Even though both contain the element the carbon the bonding between them made them different compounds. The bonding in diamonds is in a way that the carbon atoms are tetrahedrally bonded. While in the case of graphite it has a layered or sheet-like structure.

Diamond_and_graphite.jpg: User:Itub derivative work: Materialscientist (talk), Diamond and graphite2, CC BY-SA 3.0

Graphene and Fullerenes are also allotropes of carbon. The allotropes of Oxygen are O₂ (dioxygen), and O₃ (Ozone). And these allotropes have a characteristic property in which they can exist in different phases as in the solid, liquid, and gaseous forms.

Examples of Allotropes

Some of the allotropes of some elements are listed in the following table.

Element name	Allotropes
Carbon	Diamond Charcoal Graphite Coal Fullerenes Graphene
Arsenic	Yellow Arsenic White Arsenic Gray Arsenic
Phosphorous	White Phosphorous Black Phosphorous Red Phosphorous
Oxygen	O2, dioxygen O3, Ozone
Tin	White Tin Gray Tin
Sulfur	Rhombic sulfur Monoclinic sulfur
Silicon	Amorphous silicon Crystalline silicon

What is the Difference Between Isomers and Allotropes?

The difference between allotropes and Isomers is listed in the following table.

Isomers	Allotropes
The compounds have different structural formulas but the same chemical formula.	The various forms of the same chemical element are allotropes.
They have similar molecular formulas.	Different chemical formula but same element.
The molecular structure is different.	Atomic structure is different.
Several atoms are equal for isomers.	Several atoms are different.
Some sort of similar chemical properties but their physical properties are different.	Different chemical and physical properties.
Butane and isobutane are examples of isomers.	Graphite and Diamond is an allotrope of carbon.

Fun Facts about Allotropes and Isomers

Allotropes of carbon are the most common and discussed topic. Many graphite and diamond. Carbon is nonmetallic with a nonshiny appearance. But the allotropes formed by the linkage of carbon atoms show different appearances. The allotrope of carbon, diamond is an allotrope of carbon but it has a shiny appearance. Graphite is also an allotrope of carbon in which it has no such appearance. The next important characteristic is diamond is the hardest material and is used for cutting glasses. While graphite is a soft material. And is used in pencils. So such a large difference in properties has been shown by graphite and diamonds even though they are made up of the basic element carbon.

The isomers have different chemical and physical properties. For example, the isomers butane and isobutane have different properties. The boiling point of butane is 1 degree Celsius while that of -11 degrees Celsius. Butane has a gasoline-like smell while isobutane is odourless. So even though they have the same molecular formula and the same number of constituent atoms their properties are entirely different.

Conclusion

Allotropes and isomers are two important terms associated with chemical compounds. The different chemical compounds which have only one type of atom but the different molecular formulas are allotropes. While the isomers are compounds that contain similar molecular formulas and several atoms but the different structural formulas. Allotropes are entirely different chemical compounds in the physical and chemical means. But isomers somewhat show similarities. Diamond and graphite is an important allotrope of carbon. Butane and isobutane are isomers. Different chemical compounds show isomerism that may be constitutional isomers or stereoisomers. Different elements such as Oxygen, Phosphorous, Tin, Carbon, Sulfur, etc. show different allotropes.

FAQs

1. What is polymorphism? How does it differ from allotropism?

Polymorphism describes the presence of several crystalline forms of one chemical compound. Crystalline chemical compounds show this phenomenon. While allotropism deals with the different forms of the same chemical element. The two polymorphs of calcium carbonate are calcite and antagonise.

2. Which element has the most allotropes?

Carbon is the element that has the highest allotropes. More than 40 allotropes of carbon have been found. And are commonly in amorphous forms.

3. Is water an allotrope?

Water is a chemical compound containing one Oxygen and two Hydrogen atoms with the chemical formula H₂O. So it is not an allotrope but a chemical compound. Allotropes contain only one type of chemical element.

4. What are enantiomers and diastereomers?

The stereoisomers that are mirror images of each other are enantiomers. While the one that does not mirror images are diastereomers. The chemical compounds Dextro lactic acid and laevo lactic acid are enantiomers. The chemical compounds D-threose and D-erythrose are examples of diastereomers.

5. What are isotopes?

Isotopes are the different atoms that have the same mass number but different atomic numbers.. ^1H,^2H,^3H are isotopes of hydrogen