Mendeleev placed tellurium before iodine even though tellurium appeared to have the higher atomic weight.
The apparent reversal reflected Mendeleev’s priority: chemical properties and periodic position mattered more than a strictly increasing weight sequence. Tellurium belongs with oxygen, sulfur, and selenium, while iodine belongs with the halogens fluorine, chlorine, and bromine. Swapping them would have damaged those chemically meaningful groupings.
Mendeleev initially suspected that tellurium’s accepted atomic weight was wrong. Modern chemistry explains the order through atomic number: tellurium is element 52 and iodine is element 53. Their standard atomic weights are approximately 127.60 and 126.90, respectively, so the mass difference remains real, but atomic number—not atomic weight—determines their positions in today’s table.
This is a classic example of Mendeleev trusting periodic relationships before the nuclear basis of the periodic law was understood.