Isotope Definition and Examples in Chemistry

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

There are 250 isotopes of the 90 naturally occurring elements and there are over 3,200 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 exceptions are 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 because 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 isotope 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.

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 the isotopes of an element have different atomic weights.

Related Words

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

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 isotope, 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.