Isotopes of the same element differ in the number of neutrons contained within the nucleus of the element's atoms, which causes them to differ in their atomic weights. Some isotopes are more radioactive and unstable than others.
Many isotopes are unstable and spontaneously shed particles through radioactive decay, including alpha particles, beta particles and gamma radiation. How long it takes for any given nucleus to decay is random, and the amount of time it takes for half of a sample of an isotope to decay is known as its half-life. Isotopes with shorter half-lives decay more quickly, emitting more particles.