Isotopes have the same proton number (so they are the same element with the same chemical properties) but different nucleon numbers (different masses).
Some isotopes are stable; others are radioactive (unstable) and decay by emitting alpha, beta or gamma radiation.
Examples: carbon-12 (C, stable) and carbon-14 (C, radioactive); uranium-235 and uranium-238 (both radioactive, different half-lives).
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