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What is an isotope?

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Quick answer

Isotopes are versions of the same chemical element that have the same number of protons but different numbers of neutrons. They behave almost identically in chemistry, but differ in mass and in whether they are radioactive. Same element, same protons — but a different neutron count, so a different mass.

Key things to understand

  1. Same element, same protons — but a different neutron count, so a different mass.
  2. For example, carbon-12 and carbon-14 are both still carbon.
  3. Some isotopes are stable; others are radioactive and decay over time.
  4. Radioactive isotopes power carbon dating and medical imaging.
  5. Chemically they react the same, because chemistry depends on electrons, not neutrons.

People also ask

What's the difference between isotopes of an element?
Only the number of neutrons, which changes the atom's mass but not its chemical identity.
What is a radioactive isotope?
An unstable isotope whose nucleus decays over time, emitting radiation — useful for dating, imaging, and energy.
How is carbon-14 used?
It decays at a known rate, so the amount left in ancient organic material reveals its age (radiocarbon dating).