The periodic system was formulated in the 1860s by analysing order and similarities among chemical elements as provided by the chemical space of those times , that is the reported chemicals up to the 1860s . Order was provided by atomic weights, which were determined by finding the smallest common combining weight of a large set of compounds containing a reference element. Similarity was mainly determined on the basis of common empirical and molecular formulae, for instance the formation of halides with equal stoichiometric coefficients .