The conventional symbol Z comes from the German word  'number', which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order was then approximately, but not completely, consistent with the order of the elements by atomic weights.
ðə kənvˈɛnʃənəl sˈɪmbəl zˈiː kˈʌmz fɹʌmðə dʒˈɜːmən wˈɜːd nˈʌmbɚ, wˈɪtʃ, bᵻfˌoːɹ ðə mˈɑːdɚn sˈɪnθəsˌɪs ʌv aɪdˈiəz fɹʌm kˈɛmɪstɹi ænd fˈɪzɪks, mˈɪɹli dᵻnˈoʊɾᵻd ɐn ˈɛlɪmənts nuːmˈɛɹɪkəl plˈeɪs ɪnðə pˌiəɹɪˈɑːdɪk tˈeɪbəl, hˌuːz ˈɔːɹdɚ wʌz ðˈɛn ɐpɹˈɑːksɪmətli, bˌʌt nˌɑːt kəmplˈiːtli, kənsˈɪstənt wɪððɪ ˈɔːɹdɚɹ ʌvðɪ ˈɛlɪmənts baɪ ɐtˈɑːmɪk wˈeɪts.