The example of Iodine and tellurium where iodine which had lesser atomic mass was placed before tellurium proved that order of the Periodic table based on atomic mass was not always correct.
<h3>Why did Mendeleev swap Iodine and tellurium in his Periodic Table so that they were not in order of atomic weight?</h3>
The periodic table as designed by Mendeleev was arranged such that the elements were arranged in order of increasing atomic mass.
However, in this case of tellurium and iodine, Mendeleev placed tellurium before iodine even though it had a greater atomic mass than iodine. This was because Mendeleev observed that elements with similar chemical properties were occurring in a regular pattern in the periodic table and iodine has similar chemical properties to bromine and chlorine before it.
Therefore, the case of Iodine and tellurium proved that arranging elements in the periodic table in order of increasing atomic mass was not always correct.
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