Page "Atomic number" Paragraph 6

Dmitri Mendeleev claimed he arranged his tables in order of atomic weight (" Atomgewicht ") However, in deference to the observed chemical properties, he violated his own rule and placed tellurium ( atomic weight 127. 6 ) ahead of iodine ( atomic weight 126. 9 ).

This placement is consistent with the modern practice of ordering the elements by proton number, Z, but this number was not known or suspected at the time.