isotope

American  

[ahy-suh-tohp] / ˈaɪ səˌtoʊp /

noun

Chemistry.

1. any of two or more forms of a chemical element, having the same number of protons in the nucleus, or the same atomic number, but having different numbers of neutrons in the nucleus, or different atomic weights. There are 275 isotopes of the 81 stable elements, in addition to over 800 radioactive isotopes, and every element has known isotopic forms. Isotopes of a single element possess almost identical properties.

---

isotope British  

/ ˌaɪsəˈtɒpɪk, ˈaɪsəˌtəʊp, aɪˈsɒtəpɪ /

noun

1. one of two or more atoms with the same atomic number that contain different numbers of neutrons

---

isotope Scientific  

/ ī′sə-tōp′ /

1. One of two or more atoms that have the same atomic number (the same number of protons) but a different number of neutrons. Carbon 12, the most common form of carbon, has six protons and six neutrons, whereas carbon 14 has six protons and eight neutrons. Isotopes of a given element typically behave alike chemically. With the exception of hydrogen, elements found on Earth generally have the same number of protons and neutrons; heavier and lighter isotopes (with more or fewer neutrons) are often unstable and undergo radioactive decay.

---

isotope Cultural  

1. In physics, different forms of the same element, with nuclei that have the same number of protons but different numbers of neutrons. Isotopes are distinguished from each other by giving the combined number of protons and neutrons in the nucleus. For example, uranium 235 is the isotope of uranium that has 235 protons and neutrons in its nucleus rather than the more commonly occurring 238. All elements have isotopes.

---

Other Word Forms

• isotopic
  adjective
• isotopically
  adverb
• isotopy
  noun

Etymology

Origin of isotope

First recorded in 1910–15; iso- + -tope, from Greek tópos “place”; cf. Utopia ( def. )

Compare meaning

How does isotope compare to similar and commonly confused words? Explore the most common comparisons:

• isotope vs. isomer

Explanation

An isotope of a chemical element is an atom that has a different number of neutrons (that is, a greater or lesser atomic mass) than the standard for that element. The atomic number is the number of protons in an atom's nucleus. Atomic mass adds to that the number of neutrons in the nucleus. Each element has a typical atomic mass, but when the number of protons stays the same and the number of neutrons changes, you have an isotope. These can be stable, like Deuterium, an isotope of Hydrogen that has one extra neutron, or they can be radioactive, like Plutonium-239, which is a component of nuclear waste.

Example Sentences

Subsidiary Atomic Alchemy secured a license to begin sales from its radiochemistry laboratory in Idaho, and noted that its first isotope customer was “pending.”

From Barron's • May 15, 2026

But some forms of water contain deuterium, an isotope of hydrogen that includes both a proton and a neutron.

From Science Daily • May 8, 2026

Stable isotope analysis lets us trace diet and migration.

From The Wall Street Journal • May 1, 2026

The isotope was then placed into a batch-mode ion source, where it was ionized, accelerated to high energies, and delivered to the target.

From Science Daily • Apr. 14, 2026

Scientists talk about the life of an isotope in terms of probability.

From "Meltdown" by Deirdre Langeland