isotope

Etymology

It was coined in 1913 by the British chemist Frederick Soddy,^{[2][3][4][5][6]} at the suggestion of his friend, the Scottish doctor and writer Margaret Todd.^[4][6] It is formed from English iso- + -tope,^{[7][8][9][10]} the first element from Ancient Greek ἴσος (ísos, “equal”),^[4][2] the second from τόπος (tópos, “place”),^[8][4][2][6] the source of topic.^[2][6] The name, literally "same place", was chosen because the isotopes of an element, though they differ in atomic weight, occupy the same place in the periodic table.^{[4][2][3][11][5]}

Earliest recorded meaning

A variety of a chemical element with a different mass number from its other varieties: 1913, in Soddy's letter to Nature of 4 December.^[8][2]

Semantic evolution

Soddy's word met the need, after the discovery of radioactive decay series, to name forms of an element that are chemically identical but differ in atomic weight;^[2][3] the Shorter Oxford explains the difference as a different number of neutrons in the nucleus, and adds that the word is often used loosely for any distinct kind of atom or nucleus, a nuclide.^[10] From 1945 biologists and biochemists used isotope by itself for a rarer, usually radioactive isotope used as a tracer, as against the ordinary form of the element.^[8][10][12]

Timeline

• 1913 – a variety of a chemical element with a different mass number, coined by Soddy in Nature.^[8][2]
• 1945 – in biology and biochemistry, a usually radioactive isotope used as a tracer.^[8][12]

Meanings

Noun

1. Each of two or more forms of a chemical element whose atoms have the same number of protons but different numbers of neutrons, so that they share the same atomic number and chemical behaviour but differ in atomic mass: Carbon-14 is a radioactive isotope of carbon.^{[7][13][10][8]}
2. In biology and biochemistry, a less common, usually radioactive isotope used as a tracer.^[8][10]

Word family

• isotopic
• isotopically
• radioisotope

Related words

• iso-
• topic
• isobar

References

1. ^ a b Wiktionary, the free dictionary, https://en.wiktionary.org
2. ^ a b c d e f g h Ayto, John (2005), Word Origins: The Secret Histories of English Words from A to Z, 2nd ed., London: A & C Black
3. ^ a b c Barnhart, Robert K. (1988), The Barnhart Dictionary of Etymology
4. ^ a b c d e Harper, Douglas. Online Etymology Dictionary, https://www.etymonline.com
5. ^ a b Stevenson, Angus, ed. (2010), Oxford Dictionary of English, 3rd ed., Oxford: Oxford University Press
6. ^ a b c d Klein, Ernest. A Comprehensive Etymological Dictionary of the English Language. 2 vols. Amsterdam: Elsevier, 1966–1967.
7. ^ a b Merriam-Webster Unabridged Dictionary (2024), online ed., Springfield, Mass.: Merriam-Webster
8. ^ a b c d e f g h Oxford English Dictionary Online (accessed March 2026), Oxford: Oxford University Press
9. ^ Hoad, T. F., ed. (1986), The Concise Oxford Dictionary of English Etymology, Oxford: Clarendon Press
10. ^ a b c d e Shorter Oxford English Dictionary on Historical Principles (2002), 5th ed., ed. William R. Trumble and Angus Stevenson, Oxford: Oxford University Press
11. ^ The American Heritage Dictionary of the English Language (online 2024), 5th ed., Boston: Houghton Mifflin Harcourt
12. ^ a b Kay, Christian; Roberts, Jane; Samuels, Michael; Wotherspoon, Irené, eds. (2009), Historical Thesaurus of the Oxford English Dictionary, Oxford: Oxford University Press; online edition (2024)
13. ^ Collins English Dictionary (2023), Glasgow: HarperCollins