Caesium was also thought to be radioactive in the early 20th century, although it has no naturally occurring radioisotopes.
sˈiːsiəm wʌz ˈɔːlsoʊ θˈɔːt təbi ɹˌeɪdɪoʊˈæktɪv ɪnðɪ ˈɜːli twˈɛntiəθ sˈɛntʃɚɹi, ɔːlðˈoʊ ɪt hɐz nˈoʊ nˈætʃɚɹəli əkˈɜːɹɪŋ ɹˌeɪdɪoʊˈaɪsətˌoʊps.