  Unveiling numerical trends among either atomic or equivalent weights that
somehow preserved resemblances among elements was frequent in the 1860s.
Standing out from the crowd, Meyer and Mendeleev went beyond numerical
relationships, certainly motivated by a pedagogical aim. Both were after a
system synthesizing the chemical knowledge of their times in an appealing way
to be presented to chemistry students. Is it still the periodic system aiming
at that? Is it really a map of the current chemistry landscape? Solving these
questions entails addressing others such as: what is the periodic system? If it
is about chemical elements, do we really know what a chemical element is? Is
the system unique? How was and how is currently built up? Is it actually used
to conduct chemical research? A suitable tool for chemical predictions? Or is
it just a mnemotechnic for fancy trends? Does it have a limit, or multiple
ones, instead? Let us try to address these questions and let us begin by
analysing the concept of chemical element.
