Difference Between Stable Isotopes and Radioisotopes

Key Difference – Stable Isotopes vs Radioisotopes
 

Isotopes are different forms of the same chemical element having different atomic masses. This means isotopes of a certain chemical element have the same atomic number but different atomic masses. This is because these isotopes have different numbers of neutrons in their atomic nuclei. Some isotopes are stable whereas some are unstable. Stable isotopes are naturally occurring forms of chemical elements. These stable isotopes can occur naturally in atomic form or in combination with other atoms. Unstable isotopes undergo radioactive decay until they get a stable state. These isotopes are known as radioisotopes. The key difference between stable isotopes and radioisotopes is that stable isotopes do not undergo radioactive decay whereas radioisotopes undergo radioactive decay.

CONTENTS

1. Overview and Key Difference
2. What are Stable Isotopes 
3. What are Radioisotopes
4. Side by Side Comparison – Stable Isotopes vs Radioisotopes in Tabular Form
5. Summary

What are Stable Isotopes?

Stable isotopes are different forms of the same chemical element, having stable nuclei. These atoms have the same atomic number (number of protons in the atomic nuclei) because they belong to the same chemical element, but the atomic masses are different from each other because they have different numbers of neutrons in the atomic nuclei.

Stable isotopes are non-radioactive due to the stability of the atomic nuclei. Therefore, these atoms do not emit radiation. A particular chemical element can have more than one stable isotope. But in some chemical elements, all the isotopes are unstable; hence, they are radioactive.

Figure 1: Periodic Table Colored Based on the Number of Stable Isotopes

The stability of atomic nuclei depends on two basic factors:

1. Ratio between protons and neutrons
2. Sum of protons and neutrons

“Magic numbers” is a chemical concept that is used to determine the stability of a certain atomic nucleus. It gives the number of electrons present in stable isotopes. The magic number can be either the number of protons or even the number of neutrons present in the nucleus.

Magic numbers: 2, 8, 20, 28, 50, 82 and 126

If the atomic number of an isotope is equal to one of above numbers, then it is a stable isotope. In addition, if an isotope has 114 protons, it is a stable isotope. Moreover, if there are 126 or 184 neutrons present, they are also stable isotopes. In addition, if the ratios between protons and neutrons in an atom are even numbers, then these isotopes are most probably stable isotopes.

What are Radioisotopes?

Radioisotopes are unstable isotopes of chemical elements that undergo radioactive decay. These isotopes undergo radioactive decay since they have unstable atomic nuclei. Most chemical elements have one or more radioactive isotopes whereas some chemical elements only have radioactive isotopes (Ex: Uranium).

Figure 2: Different Radioisotopes and Their Radioactive Decay

Radioactive isotopes are unstable due to several reasons:

1. Presence of a high number of neutrons in the atomic nucleus compared to the number of protons

• In these radioisotopes, neutrons are converted into protons and electrons during the radioactive decay.

2. Presence of a high number of protons in the atomic nucleus

• In these radioisotopes, protons are converted into neutrons and positrons.

3. Presence of a high number of protons and electrons

• These radioisotopes undergo alpha decay where two protons and two neutrons are emitted as alpha particles

What is the Difference Between Stable Isotopes and Radioisotopes?

Summary – Stable Isotopes vs Radioisotopes

Isotopes are atoms of the same chemical element having identical atomic numbers but different atomic masses. Some isotopes are stable whereas others are unstable. Stable isotopes are the naturally occurring forms of those chemical elements. Unstable isotopes are also called radioisotopes because these isotopes undergo radioactive decay to obtain a stable state. The key difference between stable isotopes and radioisotopes is based on their ability to undergo radioactive decay.

Reference:

1. “Stable isotope ratio.” Wikipedia, Wikimedia Foundation, 27 Feb. 2018, Available here.
2. “What are radioisotopes?” Foro Nuclear, Available here.
3. Britannica, The Editors of Encyclopaedia. “Magic number.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 12 July 2017, Available here.

Image Courtesy:

1. “Periodic Table by Number of Stable Isotopes” By Nergaal at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Table isotopes en” By Table_isotopes.svg: Napy1kenobiderivative work: Sjlegg (talk) – Table_isotopes.svg (CC BY-SA 3.0) via Commons Wikimedia

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