Isotopes Definition and Examples in Chemistry

An Introduction to Isotopes

These are the isotopes of hydrogen.
These are the isotopes of hydrogen. OpenStax College / Wikimedia Commons / CC BY 3.0

Isotopes [ahy-suh-tohps] are atoms with the same number of protons, but differing numbers of neutrons. In other words, the have different atomic weights. Isotopes are different forms of a single element.

Key Takeaways: Isotopes

  • Isotopes are samples of an element with different numbers of neutrons in their atoms.
  • The number of protons for different isotopes of an element does not change.
  • Not all isotopes are radioactive. Stable isotopes either never decay or else decay very slowly. Radioactive isotopes undergo decay.
  • When an isotope decays, the starting material is the parent isotope. The resulting material is the daughter isotope.

There are 275 isotopes of the 81 stable elements. There are over 800 radioactive isotopes, some of which are natural and some synthetic. Every element on the periodic table has multiple isotope forms. The chemical properties of isotopes of a single element tend to be nearly identical. The exception would be the isotopes of hydrogen since the number of neutrons has such a significant effect on the size of the hydrogen nucleus. The physical properties of isotopes are different from each other since these properties often depend on mass. This difference may be used to separate isotopes of an element from each other by using fractional distillation and diffusion.

With the exception of hydrogen, the most abundant isotopes of the natural elements have the same number of protons and neutrons. The most abundant form of hydrogen is protium, which has one proton and no neutrons.

Isotope Notation

There are a couple of common ways to indicate isotopes:

  • List the mass number of an element after its name or element symbol. For example, an isotope with 6 protons and 6 neutrons is carbon-12 or C-12. An isotope with 6 protons and 7 neutrons is carbon-13 or C-16. Note the mass number of two isotopes may be the same, even though they are different elements. For example, you could have carbon-14 and nitrogen-14.
  • The mass number may be given in the upper left side of an element symbol. (Technically the mass number and atomic number should be stacked in line with each other, but they don't always line up on a computer.) For example, the isotopes of hydrogen may be written:
    11H, 21H, 31H

Isotope Examples

Carbon 12 and Carbon 14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons (both with 6 protons). Carbon-12 is a stable isotope, while carbon-14 is a radioactive isotope (radioisotope).

Uranium-235 and uranium-238 occur naturally in the Earth's crust. Both have long half-lives. Uranium-234 forms as a decay product.

Related Words

Isotope (noun), Isotopic (adjective), Isotopically (adverb), Isotopy (noun)

Isotope Word Origin and History

The term "isotope" was introduced by the British chemist Frederick Soddy in 1913, as recommended by Margaret Todd. The word means "having the same place" from the Greek words isos "equal" (iso-) + topos "place". Isotopes occupy the same place on the periodic table even though isotopes of an element have different atomic weights.

Parent and Daughter Isotopes

When radioisotopes undergo radioactive decay, the initial isotope may be different from the resulting isotope. The initial isotope is called the parent isotope, while the atoms produced by the reaction are called daughter isotopes. More than one type of daughter isotope may result.

As an example, when U-238 decays into Th-234, the uranium atom is the parent isotopes, while the thorium atom is the daughter isotope.

A Note About Stable Radioactive Isotopes

Most stable isotopes don't undergo radioactive decay, but a few do. If an isotope undergoes radioactive decay very, very slowly, it may be termed stable. An example is bismuth-209. Bismuth-209 is a stable radioactive isotope that undergoes alpha-decay, but has a half-life of 1.9 x 1019 years (which is more than a billion times longer than the estimated age of the universe). Tellurium-128 undergoes beta-decay with a half-life estimated to be 7.7 x 1024 years!

Historical References

Alexander Thomas Cameron, Radiochemistry (London, England: J. M. Dent & Sons, 1910), p. 141.

Soddy, Frederick. "Intra-atomic charge." Nature 92, Springer Nature Publishing AG, 1913..

Strömholm, Daniel and 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.

Thomson, J. J. (1912). "XIX. Further experiments on positive rays". Philosophical Magazine. Series 6. 24 (140): 209.