The periodic table of Mendeleef, and the physical periodicity
typified by Lothar Meyer's atomic volume curve, were of immense
value to the development of chemistry from the mid-nineteenth to
early in the present century, despite the fact that the quantity chosen
to show periodicity, the atomic weight, was not ideal. Indeed,
Mendeleef had to deliberately transpose certain elements from their
correct order of atomic weight to make them Hf into what were the
obviously correct places in his table; argon and potassium, atomic
weights 39.9 and 39.1 respectively, were reversed, as were iodine and
tellurium, atomic weights 126.9 and 127.5. This rearrangement was
later fully justified by the discovery of isotopes. Mendeleef s table
gave a means of recognising relationships between the elements but
gave no fundamental reasons for these relationships.