In periods 8 and above of the periodic table, relativistic and shell-structure effects become so strong that extrapolations from lighter congeners become completely inaccurate.
ɪn pˈiəɹɪədz ˈeɪt ænd əbˌʌv ʌvðə pˌiəɹɪˈɑːdɪk tˈeɪbəl, ɹˌɛləɾɪvˈɪstɪk ænd ʃˈɛlstɹˈʌktʃɚɹ ɪfˈɛkts bɪkˌʌm sˌoʊ stɹˈɔŋ ðæt ɛkstɹˈæpəlˈeɪʃənz fɹʌm lˈaɪɾɚ kəndʒˈɛnɚz bɪkˌʌm kəmplˈiːtli ɪnˈækjʊɹət.