[ ahy-suh-tohp ]
/ ˈaɪ səˌtoʊp /

noun Chemistry.

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.

Nearby words

  1. isotone,
  2. isotonia,
  3. isotonic,
  4. isotonic exercise,
  5. isotonic sodium chloride solution,
  6. isotope geology,
  7. isotopic number,
  8. isotopic spin,
  9. isotopy,
  10. isotretinoin

Origin of isotope

1910–15; iso- + -tope < Greek tópos place

Related formsi·so·top·ic [ahy-suh-top-ik] /ˌaɪ səˈtɒp ɪk/, adjectivei·so·top·i·cal·ly, adverb Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for isotope

British Dictionary definitions for isotope


/ (ˈaɪsəˌtəʊp) /


one of two or more atoms with the same atomic number that contain different numbers of neutrons
Derived Formsisotopic (ˌaɪsəˈtɒpɪk), adjectiveisotopically, adverbisotopy (aɪˈsɒtəpɪ), noun

Word Origin for isotope

C20: from iso- + Greek topos place

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for isotope



1913, literally "having the same place," introduced by British chemist Frederick Soddy (1877-1956) on suggestion of Margaret Todd, from Greek isos "equal" (see iso-) + topos "place" (see topos); so called because despite the different atomic weights, the various forms of an element occupy the same place on the periodic table.

Online Etymology Dictionary, © 2010 Douglas Harper

Medicine definitions for isotope


[ īsə-tōp′ ]


One of two or more atoms having the same atomic number but different mass numbers.
Related formsi′so•topic (-tŏpĭk) adj.

The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Science definitions for isotope


[ īsə-tōp′ ]

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.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Culture definitions for isotope


[ (eye-suh-tohp) ]

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.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.