Why is the study of radioactivity labeled nuclear chemistry?
Radioactivity is a result of changes in the nucleus of an atom.
Nuclear chemistry is the study of the atomic structure of elements. It includes isotopes – many of which are radioactive – and transmutation, which is the build-up of heavier elements by the energetic fusion of two nuclei (fusion). Both radioactive processes and fusion can release large amounts of energy according to Einstein's famous equation.
This equation reduces to
See also: https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence for more details and references.