@tsh2 the radioactive elements that we have on Earth today - U-238, Th-232 and K-40 mainly - all have very long half-life times (times in which half of the material undergoes nuclear fission). The "nothing left" elements are all described above as "very radioactive", i.e. they decay very fast or have a short half-life, so that nothing is left of them today (except of some short-lived amount if one of the more stable radioactive nuclei decays into them).