Difference Between Stable and Unstable Isotopes

Main Difference – Stable vs Unstable Isotopes

Isotopes are atoms of the same element that have different atomic structures. Isotopes of the same element have the same atomic number since they are different forms of the same element. They differ from each other according to the number of neutrons they have in their nuclei. The atomic mass of an element is determined by the sum of the number of protons and the number of electrons. Therefore, the atomic masses of isotopes are different from each other. Isotopes can be divided mainly into two groups as stable isotopes and unstable isotopes. The main difference between stable and unstable isotopes is that stable isotopes have stable nuclei whereas unstable isotopes have unstable nuclei.

Key Areas Covered

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

Key Terms: Alpha Decay, Belt of Stability, Electrons, Helium, Isotopes, Magic Numbers, Neutrons, Protons, Radioactivity, Uranium

What are Stable Isotopes

Stable isotopes are atoms having stable nuclei. They are non-radioactive due to the stability of their nuclei. Therefore, stable nuclei do not emit radiation. A particular element can have more than one stable isotope. For some elements such as Uranium, all isotopes are unstable. The two main facts that determine the stability of nuclei are the ratio of protons to neutrons and the sum of protons and neutrons.

The phenomenon of “Magic numbers” is a concept in chemistry which describes the atomic numbers of most stable isotopes. The magic number can be either the number of protons or the number of neutrons. If a particular element has a magic number of protons or neutrons, they are stable isotopes.

Magic Numbers: 2, 8, 20, 28, 50, 82

Protons: 114

Neutrons: 126, 184 are magic numbers.

Furthermore, if the numbers of both protons and neutrons are even numbers, those isotopes are most probably stable. Another way is to calculate the proton: neutron ratio. There is a standard graph of number of neutrons vs. number of protons. If the proton: neutron ratio fits the region for stable isotopes in that graph, then those isotopes are essentially stable.

Figure 1: The graph of number of neutrons vs. number of protons. The colored region is called the belt of stability.

Although stable isotopes are not radioactive, they have many applications. For example, hydrogen element has three major isotopes. They are Protium, Deuterium, and Tritium. Protium is the most stable and most abundant isotopes among them. Tritium is the most unstable isotope. Deuterium is also stable but is not that much abundant in nature. However, Protium is an isotope found almost everywhere. Deuterium can be used in the form of heavy water for laboratory applications.

Some elements have only one stable isotope. These elements are called monoisotopic. There are 26 known monoisotopic elements. Other elements have more than one stable isotopes. For example, Tin (Sn) has 10 stable isotopes.

What are Unstable Isotopes

Unstable isotopes are atoms that have unstable nuclei. These are radioactive isotopes. Therefore, they are also called radioactive isotopes. Some elements such as Uranium has only radioactive isotopes. Other elements have both stable and unstable isotopes.

An unstable element can be unstable due several reasons. The presence of a high number of neutrons compared to the number of protons is one such reason. In this type of isotopes, radioactive decay occurs in order to obtain a stable state. Here, neutrons are converted into protons and electrons. This can be given as below.

¹₀n     →      ¹₁p     +       ⁰_-1e

n is a neutron, p is a proton and e is an electron. The mass of the particle is given in the upper case number and the electrical charge is given in the lower case number.

Some isotopes are unstable due to the presence of a high number of protons. Here, a proton can be converted into a neutron and a positron. A positron is similar to an electron but the electrical charge is +1.

¹₁p    →    ¹₀n  +   ⁰₁e

Here ⁰₁e indicates the positron.

Sometimes, there can be too many protons and too many electrons. This indicates that the atomic mass is very high. Then two protons and two neutrons are emitted as a Helium atom. This is called alpha decay.

Figure 2: Alpha decay of Radium-226

Radioactive elements have many applications in research work. For example, these can be used in determining the age of fossils, in DNA analysis, or for medicinal purposes, etc.

In unstable isotopes, the radioactive decay can be measured by their half-life. The half-life of a substance is defined as the time taken by that substance to become one half of its initial mass due to decay.

Difference Between Stable and Unstable Isotopes

Definition

Stable Isotopes: Stable isotopes are atoms having stable nuclei.

Unstable Isotopes: Unstable isotopes are atoms having unstable nuclei.

Radioactivity

Stable Isotopes: Stable isotopes do not show radioactivity.

Unstable Isotopes: Unstable isotopes show radioactivity.

Magic Numbers

Stable Isotopes: Magic numbers indicate the number of protons or number of neutrons present in the most stable isotopes.

Unstable Isotopes: Magic numbers do not indicate the numbers of protons or electrons in unstable isotopes.

Applications

Stable Isotopes: Stable isotopes are used for applications where radioactivity should not be present.

Unstable Isotopes: Unstable isotopes are used in applications where radioactivity is important such as in DNA analysis.

Half Life

Stable Isotopes: The half-life of a stable isotope is very long or it doesn’t have a half-life at all.

Unstable Isotopes: The half-life of unstable isotope is short and can be calculate easily.

Conclusion

All elements on the earth can be divided into two groups as stable isotopes and unstable isotopes. Stable isotopes are naturally occurring forms of elements that are non-radioactive. Unstable isotopes are atoms having unstable nuclei. Therefore, these elements undergo radioactivity. This is the main difference between stable and unstable isotopes. Radioactivity is useful in many applications but is not good for our health since radiation can cause mutations in our DNA that can lead to the formation of cancerous cells.

References:

1. “Nuclear Stability.” EasyChem – The Best HSC Chemistry Notes, Syllabus Dot-Points, Past Papers and Videos. N.p., n.d. Web. Available here.  27 July 2017. 
2. Libretexts. “Nuclear Magic Numbers.” Chemistry LibreTexts. Libretexts, 05 June 2017. Web. Available here. 27 July 2017. 

Image Courtesy:

1. “Isotopes and half-life” By BenRG – Own work (Public Domain) via Commons Wikimedia
2. “Alpha-decay” By PerOX – (CC0) via Commons Wikimedia