Page "Atomic number" Paragraph 7

A simple numbering based on periodic table position was never entirely satisfactory.

Besides iodine and tellurium, later several other pairs of elements ( such as argon and potassium, cobalt and nickel ) were known to have nearly identical or reversed atomic weights, leaving their placement in the periodic table by chemical properties to be in violation of known physical properties.

Another problem was that the gradual identification of more and more chemically similar and indistinguishable lanthanides, which were of an uncertain number, led to inconsistency and uncertainty in the numbering of all elements at least from lutetium ( element 71 ) onwards ( hafnium was not known at this time ).