revision preference src: Eighty elements have at least one stable isotope which is never observed to decay, amounting to a total of about 254 stable isotopes. However, thousands of isotopes have been characterized as unstable. <SEP> tgt: Eighty elements have at least one stable isotope which is never observed to decay, amounting to a total of about 254 stable isotopes. Thousands of isotopes have been characterized as unstable.