Final answer:
The process of splitting an atom's nucleus into smaller parts is known as nuclear fission, not a physical or chemical process or melting.
Step-by-step explanation:
The nucleus of an atom can be split into smaller components by a process known as nuclear fission. This is neither a physical process nor a chemical process, and certainly not melting. Rather, it is a part of nuclear chemistry, which is the study of the structure of atomic nuclei and the processes that change nuclear structure. Nuclear fission is a type of nuclear reaction where the heavier nucleoids, specifically those with a mass number higher than 92, split into two smaller nucleoids that are more stable and release a large amount of energy.