The term used to describe an unstable atomic nucleus is radioactive. These nuclei are prone to changing into more stable forms through the emission of radiation. Such changes occur via processes like alpha decay, beta decay, or gamma decay. The instability arises from an imbalance in the number of protons and neutrons within the nucleus, leading to the nucleus seeking a more stable configuration. Radioactive elements, such as uranium and radium, emit radiation that can be detected and measured, serving practical applications in fields like nuclear energy and medicine for both diagnostic and therapeutic purposes.