Before discovering protons, neutrons and electrons, scientists tried to classify the elements by arranging them in order of their atomic weights. The atomic weight of an element is equivalent to what we now call its relative atomic mass.
Early periodic tables were incomplete, since many elements were unknown. Also, some elements were placed in groups with elements that were not similar to them.
Dmitri Mendeleev was a Russian chemist. He wrote chemistry books and was looking for ways to organise the known elements. He published his first periodic table of the elements in 1869. In it, he arranged the elements in order of increasing atomic weights. He also took into account the properties of the elements and their compounds. This meant that his table:
Mendeleev left gaps for elements not known at the time. By looking at the properties of the elements next to a gap, he predicted the properties of undiscovered elements. For example, Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap next to silicon. Another scientist later discovered the missing element, which was named germanium. Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.
Iodine has a lower atomic weight than tellurium. So iodine should be placed before tellurium in Mendeleev's periodic table. 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.
Mendeleev didn't know about isotopes, but their existence explains pair reversals. The positions of iodine and tellurium were reversed in Mendeleev's table because:
The high relative abundance of these tellurium isotopes gives tellurium the greater relative atomic mass. The atomic number of tellurium is 52 and the atomic number of iodine is 53, so these elements are in the correct order in the modern periodic table.