An element is radioactive because its atomic nucleus is unstable. This instability occurs when there is an imbalance in the number of neutrons and protons in the nucleus, leading to excess internal energy. To reach a more stable state, the nucleus spontaneously releases energy in the form of particles (like alpha or beta particles) or electromagnetic radiation (such as gamma rays). This process is called radioactive decay, and it changes the original atom into a different isotope or even a different element. Typically, heavier atoms with a large number of protons are more prone to radioactivity due to the competing forces within the nucleus—strong nuclear forces that hold the nucleus together versus electromagnetic repulsion among protons. When the electromagnetic repulsion overcomes the strong nuclear force in large nuclei, the atom becomes unstable and radioactive.
