Final answer:
Mendeleev placed tellurium before iodine based on the recurring chemical properties despite tellurium having a higher atomic weight; it was later justified when atomic number became the primary ordering principle for the periodic table.
Step-by-step explanation:
The reason why Mendeleev put tellurium (Te) before iodine (I), despite its higher atomic weight, was to maintain the consistency of chemical properties within a group. Mendeleev's periodic law emphasized the recurrence of similar properties with increasing atomic number, which wasn't known at the time. He recognized that iodine shared more chemical characteristics with halogens (like fluorine, chlorine, and bromine) than with elements it would have been placed near if ordered strictly by atomic weight. When atomic numbers were later used as the primary ordering principle, thanks to Moseley’s work, the periodic table's structure was fully justified, showing that Mendeleev's decision was correct despite being based on the wrong assumption that atomic masses had been incorrectly measured.