Among other things, Moseley demonstrated that the lanthanide series (from lanthanum to lutetium inclusive) must have 15 members—no fewer and no more—which was far from obvious from known chemistry at that time.
ɐmˌʌŋ ˈʌðɚ θˈɪŋz, mˈoʊsli dˈɛmənstɹˌeɪɾᵻd ðætðə lˈænθɐnˌaɪd sˈɪɹiz fɹʌm lˈænθænəm tə luːtˈiːɾiəm ɪŋklˈuːsɪv mˈʌstɐv fˈɪftiːn mˈɛmbɚz—nˈoʊ fjˈuːɚ ænd nˈoʊmˌoːɹ—wˌɪtʃ wʌz fˈɑːɹ fɹʌm ˈɑːbviəs fɹʌm nˈoʊn kˈɛmɪstɹi æt ðæt tˈaɪm.