There are about 118 elements in the periodic table according to their atomic number. An element is a chemical substance that consists of only a single type of atoms; hence, they are pure. Allotropes are different forms of the same compound while isotopes are different forms of the same element.
What are Allotropes?
Allotropes are different forms of the same element in the molecular level. They show different physical properties. Among all the chemical elements, carbon, oxygen, sulfur and phosphorous are the major elements that have allotropes. Carbon has a large number of allotropes. The eight allotropes of carbon differ largely from each other. For example, diamond is the strongest allotrope of carbon whereas graphite is less strong. Carbon nanotubes, fullerene, and amorphous carbon are some other allotropes of carbon.
For the element oxygen, there are two common allotropes as O2 and O3. O2 is abundant than O3. Normally, in nature, some allotropes are more abundant than others because of their stability. Phosphorus has three allotropes as red, white and black phosphorus. From these, red and white phosphorus are the most common. Allotropes differ from each other due to the atomic arrangement, the number of atoms, etc.
What are Isotopes?
Isotopes are different forms of atoms of the same chemical element. They are different from each other as they have a different number of neutrons. Since the neutron number is different, their mass number also differs.
However, the isotopes of the same element have the same number of protons and neutrons. Different isotopes are present in varying quantities, and we can give this as a percentage value called relative abundance. For example, hydrogen has three isotopes as protium, deuterium and tritium. Their number of neutrons and relative abundances are as follows.
The number of neutrons a nucleus can hold differs from element to element. Among these isotopes, only some are stable. For instance, oxygen has three stable isotopes, and tin has ten stable isotopes. Most of the time, simple elements have the same neutron number as the proton number. But, in heavy elements, there are more neutrons than protons. The number of neutrons is important to balance the stability of the nuclei. When the nuclei are too heavy, they become unstable; therefore, those isotopes become radioactive. For example, 238U emits radiation and decays to much smaller nuclei. Isotopes may have different properties because of their different masses. For example, they may have different spins; thus their NMR spectra differ. However, their electron number is similar, giving rise to similar chemical behaviour.
What is the Difference Between Allotropes and Isotopes?
Allotropes are different forms of the same element in the molecular level. Isotopes are different forms of atoms of the same chemical element. The key difference between allotropes and isotopes is that allotropes are considered at the molecular level, whereas isotopes are considered at the atomic level. Moreover, a further difference between allotropes and isotopes is that the allotropes differ from each other on the way atoms are arranged while isotopes differ from each other in the number of neutrons.
The below infographic shows more details on the difference between allotropes and isotopes.
Summary – Allotropes vs Isotopes
Although the terms allotrope and isotope sound similar, they are very distinct from each other according to their meanings. The key difference between allotropes and isotopes is that allotropes are considered at the molecular level, whereas isotopes are considered at the atomic level.
1. Helmenstine, Anne Marie, “Allotrope Definition and Examples.” ThoughtCo, Jan. 13, 2019, Available here.
2. Helmenstine, Anne Marie, “Isotopes Definition and Examples in Chemistry.” ThoughtCo, Oct. 22, 2018, Available here.