Mendeleev put the elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table. read more
Iodine's atomic number is 53 whereas tellurium's is 52. Atomic number is basically the number of protons that an element has. Therefore, as iodine has more protons than tellurium, it appears later in the table. read more
By placing iodine after tellurium it will fall in the halogens where it is better suited based on its chemistry. Even Mendeleev did that. read more
The “atomic mass” of tellurium is higher than that of iodine as displayed on modern periodic tables, but Mendeleev placed Tellurium 1st due to the way atomic masses are calculated, based on a weighted average of all of an element’s common isotopes. read more