Periodic table is sequenced based on the atomic number of an element which is the number of Protons in its nucleus. Tellurium has 52 and Iodine has 53 protons and hence appear in that order. Atomic number (protons) is more significant in determining the properties of an element than its weight. Atomic number and Atomic weight are not the same. Atomic number - number of protons Atomic weight - relative atomic mass of an element (protons+ neutrons) In case of Tellurium, it has a higher atomic weight than Iodine. And also Atomic weight is not actually its Weight (defined by the Gravitational force), but a number denoting the average of the number of protons and neutrons of an element from different sources (isotopes). Atomic weight has no units. So there is some confusion in using the naming convention.