Isotopes
Middle School
Definition
Atoms of the same element with the same number of protons but different numbers of neutrons, giving them different mass numbers. Isotopes share chemical properties; for example carbon-12 and carbon-14 are both carbon, but carbon-14 is radioactive and used in dating.
Worked examples
\(^{12}\)C\(\) has 6 protons and 6 neutrons; \(^{14}\)C\(\) has 6 protons and 8 neutrons.
Both are carbon (same proton count) but different neutron counts give different mass numbers.
Hydrogen has three isotopes: protium \(^{1}\)H\(\), deuterium \(^{2}\)H\(\), and tritium \(^{3}\)H\(\).
All have 1 proton but 0, 1, or 2 neutrons respectively, so mass numbers differ.
Common mistakes
- Isotopes have different numbers of protons → Isotopes have the same number of protons, different numbers of neutrons Different proton counts means different elements, not isotopes of the same element.
- All isotopes of an element are radioactive → Only some isotopes are radioactive; many are stable Carbon-12 is stable; carbon-14 is radioactive. Most elements have both stable and radioactive isotopes.
- Isotopes have different chemical properties → Isotopes share the same chemical properties Chemical behavior depends on electrons and protons, not neutrons; isotopes react identically.
Where you'll use it next
You'll use isotopes in nuclear chemistry for radioactive decay and half-life calculations, in Earth science for radiometric dating of rocks and fossils, and in medical applications like PET scans and cancer treatment.
See also
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026