If an isotope is radioactive then what part of it breaks down over time?

In a radioactive isotope, the unstable nucleus is the part that breaks down over time. This decay process involves the emission of radiation, which can take the form of alpha particles, beta particles, or gamma rays, and leads to the transformation of the original isotope into a more stable form or a different element altogether. The rate of this decay is characterized by the isotope's half-life, which is the time it takes for half of a sample to decay.