Question

In Mendeleev's periodic table, why does tellurium with atomic mass =127.6 appear before iodine having atomic mass 126.9?

Answer 1

Iodine has a lower relative atomic mass than tellurium. So iodine should be placed before tellurium in Mendeleev's tables.

However, iodine has similar chemical properties to chlorine and bromine (halogens). To make iodine line up with chlorine and bromine in his periodic table, Mendeleev swapped the positions of iodine and tellurium.