A radioactive isotope is a variant or an element that contains an unstable nucleus, which means it emits radiation as it decays to a stable form. This decay process can be either alpha decay (where an alpha particle is released), beta decay (where a high-energy electron or positron is released) or gamma decay (where high-frequency electromagnetic waves are emitted). The emission of these particles and waves results in the loss of energy, which causes the nucleus to transform into a more stable form. Radioactive isotopes can be used for various purposes such as tracing chemical reactions, measuring time intervals or radiation therapy for cancer treatment.