Why did Mendeleev place tellurium before iodine in his periodic table of the elements? - Leaving Cert Chemistry - Question a - 2017

Mendeleev placed tellurium before iodine because he prioritized the chemical properties of the elements over their atomic weights. When organizing the periodic table, he aimed to group elements with similar chemical properties to ensure a periodic recurrence of these properties.

In his arrangement, tellurium and iodine had similar chemical characteristics, but listing elements strictly by atomic mass resulted in an incorrect placement. Mendeleev believed that tellurium's atomic mass had been inaccurately measured, which justified its position before iodine in the table.