Like many scientists working at the end of the 19th century, the Russian chemist Dmitri Mendeleev (1834-1907) was looking for ways to organise the known elements. Mendeleev published his first periodic table of the elements in 1869.
Mendeleev arranged the elements in order of increasing 'atomic weights' (nowadays, this would be called relative atomic mass). He also took into account the properties of the elements and their compounds. This meant that his table:
However, from their relative atomic mass, some pairs of elements next to each other were in the wrong order.
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.
Mendeleev left gaps in his table for elements not known at the time. By looking at the properties of the elements next to a gap, he could also predict the properties of these undiscovered elements. For example, Mendeleev predicted the existence of 'eka-silicon', which would fit into a gap below silicon. The element 'germanium' was discovered later. Its properties were found to be similar to the predicted ones and confirmed Mendeleev's periodic table.