After what year was the " Z " number known as a physical characteristic of atoms ?
The ordering of atoms in the periodic table did tend to be according to atomic " weights " , but there were a few famous " reversed " cases where the periodic table demanded that an element with a higher atomic weight ( such as cobalt at weight 58.9 ) nevertheless be placed at a lower position ( Z = 27 ) , before an element like nickel ( with a lower atomic weight of 58.7 ) , which the table demanded take the higher position at Z = 28 .