Final answer:
When a parent isotope decays, it transforms into a daughter isotope through a process known as radioactive decay. The daughter isotope may be stable or may further decay in a series until a stable isotope is reached.
Step-by-step explanation:
As a parent isotope decays, the remains of the nucleus turn into what is known as the daughter isotope. This process is referred to as radioactive decay. During decay, such as alpha decay, the parent nucleus emits particles; for instance, in alpha decay, it loses two protons and two neutrons, leading to a change in both atomic and mass numbers.
The daughter nucleus that results from this decay may be stable or it may undergo further decay until a stable nucleus is formed. In some decay series, such as the one that starts from Uranium-238 (238 U), a series of nuclear decays occurs until a stable isotope like Lead-206 (206 Pb) is produced.