Here's why:

* Atomic Mass: At the time, Mendeleev arranged the elements by increasing atomic mass. Iodine had a slightly lower atomic mass than tellurium, which seemed to place iodine before tellurium.

* Chemical Properties: However, Mendeleev recognized that iodine's chemical properties were more similar to the halogens (Group 17) than to oxygen and sulfur (Group 16), where tellurium belonged.

By swapping their positions, Mendeleev ensured that elements with similar chemical properties were grouped together, making his periodic table more accurate.

This decision was later validated when the concept of atomic number was introduced, which actually defines an element's position on the periodic table. Iodine has a higher atomic number than tellurium, confirming Mendeleev's intuition.