What Are Isotopes?

by Carson

The element that an atom belongs to is defined by the number of protons in the atom, but do you know that a single element can contain very different types of atoms that exhibit unique properties that are not present in other kinds of that element? Therefore, isotopes will be explained in this article.

The Particles in an Isotope

To understand isotopes, we first have to explain an atom’s structure briefly. It comprises a nucleus and a surrounding area where electrons seemingly orbit them. We’re not going to talk about electrons in this article because they are unrelated to isotopes. Instead, the nucleus is what we’ll be focusing on.

A nucleus consists of some protons and neutrons. As mentioned earlier in this article, protons define the element that an atom belongs to. They place the atom in its corresponding location in the periodic table because the number of protons dictates how many electrons the atom needs so that the electric charge of the atom is neutralized. This is the first part of the definition of an isotope, and the element that the atom belongs to is listed in the representation of isotopes.

What about the role of neutrons in defining the isotope of an atom? It turns out that neutrons are the crucial particles to do so. The number of neutrons shapes the second part of the representation of the isotope. Specifically, this part of the notation mentions the number of protons and neutrons in the atom combined.

For instance, if an atom contains one proton and one neutron, it belongs to a hydrogen atom since it has one proton. Thus, the first part of the representation is “hydrogen”. The total amount of nucleons (protons and neutrons) is 2 in this nucleus, which means that the second part of the isotope is “2”. Blending the two parts, the name of the isotope is “hydrogen-2”. This applies to all other isotopes as well.

A diagram showing how the representation of “helium-4” works

Isotopes and Radioactivity

What isotope an atom belongs to determines the radioactivity of the atom. Some isotopes are stable. They are not radioactive and does not emit radiation by themselves. The most common isotopes in the Universe are stable, as well as the most common isotopes of most elements.

However, many other isotopes are radioactive. All atoms with more than 82 protons are known to be radioactive. If an atom is radioactive, its rate of radioactive decay is measured by its half-life, which is the time elapsed for half of a cluster of atoms of one isotope to undergo radioactive decay. Some isotopes undergo radioactive decay extremely slowly, such as bismuth-209, with a half-life longer than the age of the Universe. The most radioactive isotopes decay swiftly, with half-lives of less than a few milliseconds.

Different isotopes experience different types of radioactive decay as well. For instance, the alpha decay causes two protons and two neutrons to escape from the nucleus as a separate helium-4 nucleus. Other than that, the beta decay causes one neutron to be separated into an electron and a proton, from which the electron escapes while the proton stays inside the nucleus.

An illustration showing the alpha and beta decay


In this article, we discussed isotopes, which are types of atoms classified by the number of protons and neutrons in the nucleus. Remember that different isotopes, even those in the same element in the periodic table, can have very different properties, especially when it comes to radioactivity.

References and Credits

  1. Naomi Lubick. (2003, April 25.). Bismuth Not So Stable After All. Retrieved January 27, 2022, from https://www.science.org/content/article/bismuth-not-so-stable-after-all
  2. (n.d.). 21.3 Radioactive Decay. Retrieved January 27, 2022, from https://opentextbc.ca/chemistry/chapter/21-3-radioactive-decay/
  3. (2019, April 23). Types of Radioactive Decay. Retrieved January 27, 2022, from https://flexbooks.ck12.org/cbook/ck-12-chemistry-flexbook-2.0/section/24.2/primary/lesson/nuclear-decay-processes-chem/

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