Final answer:
The spontaneous process by which unstable nuclei emit radiation to become more stable is called radioactive decay, transforming a parent nuclide into a daughter nuclide.
Step-by-step explanation:
The process of unstable nuclei changing spontaneously and emitting radiation to become more stable is known as radioactive decay. During this process, the original nucleus, or parent nuclide, transforms into a daughter nuclide, which may be stable or may undergo further decay. This transformation is a part of a sequence of decays known as decay chains. An example includes the decay of uranium-235 which emits an alpha particle to form thorium-231. Such radioactive decay processes contribute to the transmutation of elements, where after losing or gaining particles, a new element is formed.
Radioactive decay is spontaneous and does not require external energy. It results in the emission of various types of particles and radiation, such as alpha particles, beta particles, and gamma rays, depending on the decay route of the unstable nuclide. The concept of the band of stability helps predict the type of decay an unstable isotope will undergo, moving it closer to a stable configuration in the process.