An isotope is any form of a chemical element that has the same number of protons in the nucleus, or the same atomic number but has a different number of neutrons in the nucleus.
For example, three isotopes of carbon are found in nature- carbon-12, carbon-13 and carbon-14. All three have six protons, but their neutron numbers differ, being 6, 7, and 8 respectively. Isotopes may be stable or unstable. If unstable, they will be radioactive. The term isotope is a combination of the Greek word ‘isos’, meaning equal, and ‘typos’ which means place.
Radiochemist Frederick Soddy was the first to suggest the existence of isotopes in 1913. He made this inference based on studies of radioactive decay chains. Soddy was also the first to isolate isotopes by degenerating uranium.
The first evidence for multiple isotopes of a stable, non-radioactive element was found by J. J. Thomson in 1913. The effect of isotopes on atomic mass was discovered by Harold Urey and G. M. Murphy in 1931.
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