Isotopes explained
Every element is defined by its atomic number (Z) — the number of protons in its nucleus. All carbon atoms have 6 protons. But the number of neutrons can vary. Carbon-12 has 6 neutrons; carbon-13 has 7; carbon-14 has 8. All three are carbon; all three behave chemically the same way. They differ only in mass.
The notation is: element name followed by the mass number (protons + neutrons). So carbon-14 means: atomic number 6 (carbon), mass number 14, therefore 8 neutrons.
Mass number (A)
= protons + neutrons
Atomic number (Z)
= protons only (defines element)
Read our atomic structure guide for the full picture of protons, neutrons and electrons.
Stable vs radioactive isotopes
Some isotopes are stable — their nuclei stay intact indefinitely. Others are radioactive: their neutron-to-proton ratio is outside the band of stability, and the nucleus rearranges itself by emitting radiation.
- Alpha decay — emits an alpha particle (2 protons, 2 neutrons). Common in heavy elements. Stopped by a sheet of paper.
- Beta decay — emits an electron (β⁻) or positron (β⁺) as a neutron converts to a proton or vice versa. Stopped by a few mm of aluminium.
- Gamma emission — high-energy electromagnetic radiation released after alpha or beta decay. Requires lead shielding.
The half-life is the time for half of a radioactive sample to decay. It ranges from fractions of a second (synthetic superheavy elements) to billions of years (uranium-238: 4.5 × 10⁹ years).
Selected isotopes
Applications
Isotopes are tools across science and medicine:
- Radiocarbon dating — Carbon-14 has a half-life of 5,730 years. Living organisms maintain a constant C-14 / C-12 ratio. After death, C-14 decays without being replaced, letting archaeologists date organic material up to ~50,000 years old.
- Medical imaging — Technetium-99m (half-life 6 hours) emits gamma rays detectable by SPECT scanners, revealing bone or organ function.
- Cancer therapy — Iodine-131 concentrates in the thyroid gland and destroys thyroid cancer cells with beta radiation.
- Nuclear power — Uranium-235 is the fissile isotope used in reactors; it splits when struck by a neutron, releasing energy.
- Stable isotope tracing — Carbon-13 is used in ¹³C breath tests to diagnose H. pylori infection without radiation exposure.
See the element 119 article to understand how isotope synthesis drives the search for new elements.