In 1915, the reason for nuclear charge being quantized in units of Z, which were now recognized to be the same as the element number, was not understood.
ɪn nˈaɪntiːnhˈʌndɹɪd fˈɪftiːn, ðə ɹˈiːzən fɔːɹ nˈuːklɪɹ tʃˈɑːɹdʒ bˌiːɪŋ kwˈɔntaɪzd ɪn jˈuːnɪts ʌv zˈiː, wˌɪtʃ wɜː nˈaʊ ɹˈɛkəɡnˌaɪzd təbi ðə sˈeɪm æz ðɪ ˈɛlɪmənt nˈʌmbɚ, wʌz nˌɑːt ˌʌndɚstˈʊd.