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Do you know what an isotope is? It's basically an unstable isotope, that breaks down.
A radioactive isotope is one that decays, possibly because it has too many neutrons, and the nucleus is just too heavy. It's unstable because it's considered high energy. A nucleus that has high energy will try to release some of that energy in the form of a particle I/e alpha particle to become another element. Always Remember that the number of protons is technically what defines your element, and once that changes, so too does the identity of your element. Of course a nuclei can release energy in the form of a gamma ray which wouldn't change the identity of your atom.
For example, carbon 14 is a radioactive isotope. It's an isotope because carbon 14 has 8 neutrons (regular old carbon-12 has 6 neutrons) its radioactive so that means it decays, it changes to nitrogen and releases an electron (beta decay)/