What does isotope mean?
An isotope is a form of a chemical element whose atoms contain the same number of protons but different numbers of neutrons. Because the proton number determines an element’s identity, isotopes have essentially the same chemical properties, but they differ in mass and can differ greatly in nuclear stability. Some, such as carbon-12, are stable, while others, such as carbon-14, are radioactive and decay over time. Isotope ratios can provide information about age, environment, diet, and geological history, making isotopes important in radiocarbon dating, nuclear science, medicine, and isotope analysis. The word is also used in technical discussions of isotope labeling, where selected atoms in a molecule are replaced with a particular isotope so that its movement or reaction can be followed.
nounAn isotope is one of two or more forms of the same chemical element that have identical numbers of protons but different numbers of neutrons. Isotopes therefore have nearly identical chemical properties but different atomic masses and may differ in stability and radioactivity.
- A form of an element characterized by a fixed number of protons and a particular number of neutrons.
- More broadly, an atom or nuclide belonging to such a form, especially when discussing nuclear properties, radioactive decay, or scientific measurement.
"Carbon-12 and carbon-14 are isotopes of carbon, with the latter used in radiocarbon dating."
"Scientists measured the ratio of carbon-13 to carbon-12 in the sample to study its origin."
"The reactor used uranium-235 and other uranium isotopes as fuel."
The plural is used for different forms of the same element, as in “The sample contains several radioactive isotopes.”
"Different isotopes of uranium undergo different nuclear reactions."
Every atom of an element shares a chemical identity, but isotopes can carry a different nuclear clock: carbon-14’s radioactivity makes it an archaeological timekeeper, while carbon-12 remains perfectly stable.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does isotope come from?
The word isotope was formed in the early 20th century from Greek iso-, meaning “equal,” and tópos, meaning “place,” reflecting the idea of isotopes occupying the same place in the periodic table. The related terms isobar, isotonic, and isomer also use the Greek prefix iso-, although they describe different relationships among scientific quantities or chemical structures. The modern terminology developed as scientists distinguished atoms of the same element by their differing neutron counts.
Related word forms
How isotope is actually used
Isotope is a countable noun. In technical contexts, the mass number is commonly given before the element name or symbol, as in uranium-235 or U-235. The term is used in chemistry, physics, medicine, geology, and related fields.
Easily confused with isotope
An isotope is a variant of an element with a different number of neutrons, whereas an isomer is a compound with the same molecular formula but a different arrangement of its atoms.
An isotope refers to atoms of an element that differ in neutron number, while an isotopologue is a molecule containing particular isotopic atoms; molecules with the same composition but different isotopic positioning can also be isotopologues.