Difference Between Allotropes and Isotopes

Main Difference – Allotropes vs Isotopes

Chemical elements can occur naturally in several different forms. Sometimes, the elements are found in combination with other elements, and sometimes, elements are found in their elemental form such as gold (Au). However, some elements naturally occur in different forms but are in the same physical state. Such elements are called allotropes. There are also elements which have different forms of atomic structures. They are called Isotopes. The main difference between allotropes and isotopes is that allotropes are defined at their molecular level whereas isotopes are defined at their atomic level. 

Key Areas Covered

1. What are Allotropes
      – Definition, Properties, Examples
2. What are Isotopes
      – Definition, Properties, Examples
3. What is the Difference Between Allotropes and Isotopes
      – Comparison of Key Differences

Key Terms: Allotropes, Allotropy, Carbon, Gold, Hydrogen, Isotopes, Sulfur

What are Allotropes

Allotropes are different forms of the same chemical element that are stable in the same physical state. In allotropes, the atoms of the same element are bound to each other in different manners.  In other words, the spatial arrangement of atoms is different from one allotrope to another. An allotrope is composed only of atoms of the same element. There are no combinations of atoms of different elements.

The physical state of allotropes of the same chemical element is the same. But the molecular formulas of allotropes can be either equal or different from each other. Therefore, the chemical and physical properties of allotropes can be different from each other.

Allotropy is the term used to describe the presence or absence of allotropes for a particular chemical element. All chemical elements do not have allotropes. Only some elements show allotropy. Some common examples are discussed below.

Carbon (C)

Carbon is a major chemical element that shows allotropy. The most common allotropes of carbon are graphite and diamond. Both Graphite and diamond are composed of only carbon atoms. But the molecular structure, hybridization of carbon atoms and other physical properties of them are different from each other.

Figure 01: Chemical structures and the appearances of Diamond and Graphite

Oxygen

The allotropes of oxygen are Dioxygen (O₂) and Ozone (O₃). Both of them are in the gaseous phase in nature and are different from each other through the molecular structure, chemical and physical properties.

Sulfur

Sulfur in nature is found as S₈ units. These units are composed of eight sulfur atoms. Here, one sulfur atom is bonded to two other sulfur atoms forming a cyclic structure. These cyclic structures can be either in rhombic structure, needle form (Monoclinic) or orthorhombic form. The general structure of S₈ is the crown structure.

Figure 02: Crown structure of S8

Allotropy is defined for molecules in the physical state. Therefore, liquid water and ice are not allotropes even though both are composed of only water molecules (H₂O).

What are Isotopes

Isotopes are different forms of atomic structures of the same chemical element. Generally, an atom is made out of a nucleus and an electron cloud surrounding this nucleus. The nucleus is composed of protons and neutrons whereas the electron cloud is composed only of electrons. An element is composed of a unique number protons. The atomic number of an element is the number of protons. Therefore, each chemical element has a unique atomic number. The periodic table of elements is built based on the atomic numbers of elements. Here, the chemical elements are arranged in the ascending order of the atomic number. However, the number of neutrons present in the nucleus is not a unique value for elements. The atoms of the same element may have a different number of neutrons in their nucleus. These atoms are called isotopes.

The isotopes of a particular element can be either stable or unstable. Unstable isotopes may undergo radioactive decay to obtain a stable form. Some of the most common Isotopes are given below.

Hydrogen (H)

The atomic number of Hydrogen is 1. Therefore, it is composed of 1 proton. There are 3 common Isotopes of Hydrogen. They are Protium, Deuterium, and Tritium. Protium has no neutrons; Deuterium has one neutron and Tritium has two neutrons in their nucleus.

Figure 03: Isotopes of Hydrogen

Helium

Helium is composed of two protons. Naturally occurring isotopes of Helium have 1 neutron or 2 neutrons.

Figure 04: Isotopes of Helium

 Carbon

Carbon atoms also occur in isotope forms. The most common isotope of Carbon is composed of 6 neutrons. Some carbon isotopes have 7 or 8 neutrons.

Difference Between Allotropes and Isotopes

Definition

Allotropes: Allotropes are different forms of the same chemical element, which are stable in the same physical state.

Isotopes: Isotopes are different forms of atomic structures of the same chemical element.

Nature

Allotropes: Allotropes describe molecular structures.

Isotopes: Isotopes describe atomic structures.

Mass

Allotropes: The molar mass of allotropes can be either equal or different from each other.

Isotopes: The atomic number of isotopes are the same, but the atomic masses are different from each other.

Abundance

Allotropes: Allotropes are not found in all chemical elements.

Isotopes: Isotopes are found in almost all elements.

Chemical Properties

Allotropes: The chemical properties of allotropes are different from each other.

Isotopes: The chemical properties of isotopes are similar due to the presence of equal numbers of electrons.

Stability

Allotropes: Allotropes are stable molecules that are found naturally.  

Isotopes: Some Isotopes are stable whereas as others are unstable.

Conclusion

Both allotropes and isotopes refer to different forms of a particular chemical element. Allotropes explain the differences in molecular structures. Isotopes explain the differences in atomic structures. This is the main difference between allotropes and isotopes. Allotropes can either have very slight differences in their properties or big differences. But most isotopes are different from each other according to their stability rather than other properties. The chemical properties of isotopes would be same because they have the same number of electrons. Almost all chemical properties are dependent on the number and arrangement of electrons.

References:

1.Helmenstine, Anne Marie. “What Is an Isotope? Definition and Examples.” ThoughtCo. N.p., n.d. Web. Available here. 20 July 2017. 
2.”Isotopes and Allotropes.” GKToday. N.p., n.d. Web. Available here. 20 July 2017.

Image Courtesy:

1.”Diamond and graphite2″ By Diamond_and_graphite.jpg: User:Itubderivative work: Materialscientist (talk) – Diamond_and_graphite.jpgFile:Graphite-tn19a.jpg (CC BY-SA 3.0) via Commons Wikimedia
2. “Cyclooctasulfur-above-3D-balls” (Public Domain) via Commons Wikimedia
3. “Blausen 0530 HydrogenIsotopes” By BruceBlaus – Own work (CC BY 3.0) via Commons Wikimedia
4. “Helium-3 and Helium-4” By Uwe W. – Own work (Public Domain) via Commons Wikimedia