Dmitri Mendeleev (1834-1907) was a Russian chemist. In the 19th century, he was one of several scientists who were 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 relative atomic mass. He then considered the properties of the elements and their compounds. Mendeleev found that this allowed him to arrange elements with similar properties below each other into groups. However, to make this work, he had to:
Mendeleev’s 1869 table showed groups of elements in horizontal rows, but from 1871 he showed groups of elements in vertical columns.
Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties and physical 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 next to silicon. The element germanium was discovered later. Its properties were found to be similar to the predicted ones, which supported the ideas behind Mendeleev’s periodic table.
Iodine has a lower relative atomic mass than tellurium. 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.