The particle responsible for emitting radiation in radioactive decay is called a nucleon, which includes both protons and neutrons within an atomic nucleus. When an unstable nucleus decays, it releases energy in the form of radiation, which can include alpha particles, beta particles, or gamma rays. This process transforms the original element into a different one or a different isotope, resulting in changes to its atomic number or mass. Understanding radioactive decay is fundamental in fields such as nuclear science and medicine.