BharatNotes What is Isotopes?
Isotopes are atoms of the same element that share the same atomic number (number of protons) but differ in their mass number because they hold different numbers of neutrons. Since the number of protons fixes the element's chemical identity, all isotopes of an element occupy the same position in the periodic table and behave almost identically in chemical reactions. They differ, however, in atomic mass and in physical and nuclear properties.
The classic example is carbon: carbon-12, carbon-13 and carbon-14 all have 6 protons, but carry 6, 7 and 8 neutrons respectively. Isotopes can be stable (do not decay) or radioactive (radioisotopes), which emit radiation as their unstable nuclei decay.
Isotopes, Isobars and Isotones (UPSC distinction)
A frequent point of confusion in the exam is the difference between three "iso" terms:
| Term | What is same | What differs | Example |
|---|---|---|---|
| Isotopes | Atomic number (protons) | Mass number (neutrons) | C-12, C-13, C-14 |
| Isobars | Mass number | Atomic number (different elements) | Ar-40, K-40, Ca-40 |
| Isotones | Number of neutrons | Protons and mass number | S-36, Cl-37, Ar-38, K-39, Ca-40 (each with 20 neutrons) |
Mnemonic: isotoPes differ in Protons-to-neutron count but share protons; isobArs share mass (A); isotonNes share Neutrons.
Significance and Applications
Radioisotopes have wide-ranging uses:
- Medicine: Cobalt-60 gamma sources are used in cancer radiotherapy; Iodine-131 treats hyperthyroidism and thyroid cancer; Technetium-99m (the decay product of Molybdenum-99) is the workhorse of diagnostic nuclear imaging, used in about 80% of nuclear-medicine procedures worldwide; Lutetium-177 and Samarium-153 are used in therapy.
- Archaeology/Geology: Carbon-14 dating, developed by Willard Libby (Nobel Prize in Chemistry, 1960), uses C-14's half-life of about 5,730 years to date organic remains up to roughly 50,000 years old.
- Industry & agriculture: Gamma radiometry (Co-60, Cs-137) detects defects in materials; radiation is used for sewage hygienisation and food preservation.
Current Status in India (as of mid-2026)
India's radioisotope programme began with the Apsara research reactor (commissioned August 1956), followed by the CIRUS and the 100 MW Dhruva reactor (operational 1985), which improved both quality and quantity of production. The Board of Radiation and Isotope Technology (BRIT), an industrial unit of the Department of Atomic Energy, produces radiopharmaceuticals, Tc-99m generators, immunoassay kits and labelled compounds. Indigenous achievements include the Bhabhatron computerised Cobalt-60 teletherapy machine for cancer treatment, and BARC's SHRI facility (Vadodara) for Co-60 gamma hygienisation of sewage sludge. BRIT and DAE are working to expand domestic production of key medical isotopes such as Mo-99 and Lu-177 to reduce import dependence.
