After what year was the " Z " number known as a physical characteristic of atoms ?
In particular , only two years before Rutherford in 1911 had postulated that Z for gold atoms might be about half of its atomic weight , and only shortly afterward , Antonius van den Broek had made the bold suggestion that Z was not half of the atomic weight for elements , but instead was exactly equal to the element 's atomic number , or place in the periodic table .