What Is an Isotope? Definition and Examples

Hydrogen Isotopes
The three isotopes of hydrogen are protium, deuterium, and tritium. (Dirk Hünniger / Wikimedia Commons)

Isotopes are forms of an element that have different numbers of neutrons. All isotopes of an element have the same atomic number and number of protons, but they have different atomic masses from each other. Isotopes of an element share similar chemical properties, but have different nuclear properties.

Every element has isotopes. The 81 stable elements have 275 isotopes. But, elements with stable isotopes also have radioactive isotopes or radioisotopes. The radioactive elements, on the other hand, have no stable isotopes. Over 800 radioactive isotopes have been identified. Some of the radioactive isotopes are natural, while others have only been produced in the laboratory.

Word Origin

The term isotopes was coined by Scottish doctor Margaret Todd in 1913. She suggested the word to chemist Frederick Soddy. He adopted it for use in chemistry and physics. The word comes from the Greek words isos (equal) and topos (place). Isotopes of an element occupy the same position on the periodic table. The atomic mass of an element on the periodic table is an average of the masses of the naturally occurring isotopes of that element.

Isotope Notation

There are two common methods of writing isotopes:

(1) The element name or symbol is listed first, followed by a dash and then the mass number of the isotope. For example, hydrogen-3 or H-3 refers to the hydrogen isotope with 1 proton and 2 neutrons, which add together to give a mass number of 3. Carbon-12 or C-12 refers to the stable isotope of carbon with 6 protons and 6 neutrons.

(2) The mass number or both the mass number and atomic number are cited on the upper lefthand side of an element symbol. For example, the isotope of carbon with 6 protons and 6 neutrons is 126C . Ideally, the mass number is positioned directly over the atomic number, but this is not always possible for typed notation.

Isotope Examples

The isotopes of hydrogen are hydrogen-1 (protium, which is a stable isotope), hydrogen-2 (deuterium, which is another stable isotope), and hydrogen-3 (tritium, which is a radioisotope).

Uranium-235 and uranium-238 are two isotopes of uranium. Both are natural isotopes that are found in the Earth’s crust.

Carbon-12 and carbon-14 are two carbon isotopes. Carbon-12 is stable, while carbon-14 is radioactive.

Parent and Daughter Isotopes

When a radioisotope undergoes radioactive decay, the starting isotope is called the parent isotope. Decay produces one or more daughter isotopes. For example, uranium-238 is the parent isotope that decays into the daughter isotope thorium-234.

Isotope vs Nuclide

An isotope refers to a sample of atoms. When the number of protons and neutrons of an individual atom is studied, it is called a nuclide of the element. In nuclear science, the term nuclide is preferred over the term isotope. Nuclides with the same mass number as each other are called isobars. For example, argon-40, potassium-40, and calcium-40 are isobars.


  • Nagel, Miriam C. (1982). “Frederick Soddy: From Alchemy to Isotopes.” Journal of Chemical Education. 59 (9): 739–740. doi:10.1021/ed059p739
  • Soddy, Frederick (1913). “Intra-atomic charge.” Nature 92 (2301), Springer Nature Publishing AG. doi:10.1038/092399c0
  • Strömholm, Daniel; Svedberg, Theodor (1909). “Untersuchungen über die Chemie der radioactiven Grundstoffe II.” (Investigations into the chemistry of the radioactive elements, part 2). Zeitschrift für anorganischen Chemie63: 197–206.