Atoms have a fixed number of protons and electrons, however, atoms can have various amounts of neutrons. When an atom has a number of neutrons that is uncommon it is called an isotope.
The isotopes that are not radioactive are called stable isotopes. Isotopes that are unstable are called radioactive isotopes. Radioactive isotopes emit radiation in various forms.
Smoke detectors use americium-241, a radioactive element, in order to detect smoke. Americium-241 emits alpha particle radiation which is used to distinguish when smoke particles are present.
Carbon-14 is a unstable isotope used to find the age of carbon materials that are thousands of years old.
