Why are Tellurium and Iodine the wrong way round on the periodic table?

Since Mendeleev's table was based on atomic weight, some things didn't match perfectly. For example, tellurium and iodine caused Mendeleev some problems. The atomic mass of tellurium was greater than iodine according to the best estimates of that time, but they didn't fit the groups that Mendeleev devised. In fact, they should be switched to follow the groups he devised. The reason for this discrepancy is the fact that atomic number (the number of protons in an atom), not atomic weight, determines the order of the elements in the table, the basis of the modern periodic table.

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Iodine has only one common isotope - iodine-127. Tellurium has eight different isotopes - the commonest being tellurium-126 (19%), tellurium-128 (32%) and tellurium-130 (34%). The relative atomic mass of tellurium is therefore higher than that of iodine. So iodine should be placed before tellurium in Mendeleev's tables. However, iodine has similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swapped the positions of iodine and tellurium.

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