While developing his periodic table, Dmitri Mendeleev broke his own rule and placed the element tellurium before iodine in the group. He did this even though tellurium's atomic mass was known to be greater than iodine's. In his notes, Mendeleev stated that he expected tellurium to have a lower atomic mass than iodine. However, all measurements continue to show that tellurium has a greater atomic mass than iodine even though tellurium acts like it has a lower atomic mass. What is the most likely reason that Mendeleev placed tellurium before iodine? Mendeleev observed that tellurium has chemical properties like other elements in its group, and he did not know that neutrons cause the greater atomic mass. Iodine isotopes usually have more neutrons than tellurium isotopes, but only the protons affect the properties that Mendeleev could have observed. Mendeleev observed that tellurium and iodine both had very similar chemical properties, but he did not know that iodine atoms have more electrons. Iodine atoms always have more protons than tellurium atoms, but Mendeleev only weighed elements without observing any other properties.