How did Mendeleev's table predict undiscovered elements?

Mendeleev arranged the known elements by increasing atomic weight and noticed that elements with similar properties turned up at regular intervals. Where an element seemed to be missing from the pattern, he left a gap and said the gap belonged to an element not yet discovered. He then estimated that element's properties from the elements around it. [1] [2]

Mendeleev published his first periodic table in 1869. According to BBC Bitesize, he wrote element names on cards and ordered them from lightest to heaviest. He then grouped them by chemical and physical properties. The result was a table with similar elements lined up in groups and with some gaps in it. [3] [2]

Strictly following atomic weight would sometimes have put an element in a column of unlike elements. Rather than force it in, he left a gap. In a related move, he swapped iodine and tellurium so that iodine lined up with chlorine and bromine, even though iodine has the lower atomic weight. [4] [2]

To predict a missing element, Mendeleev looked at the elements on either side of a gap and above and below it. One predicted element he called eka-silicon, which would sit below silicon. He expected it to have a relative atomic mass of about 72 and to form an oxide with the formula XO2. [4] [2]

Lothar Meyer in Germany independently recognised a periodic relationship in 1870 and also ordered the elements by atomic mass. OpenStax notes that Mendeleev went a step further by using his table to predict unknown elements resembling aluminum and silicon. [5]

Gallium was discovered in 1875 and matched Mendeleev's prediction for one gap. Germanium followed in 1886, and its properties matched those predicted for eka-silicon very closely. These successes raised scientists' confidence in the table and helped it gain acceptance. [4] [5]

BBC Bitesize says there were about 64 or 65 known elements in Mendeleev's time, and that he predicted four more. The sources give slightly different counts for the known elements. Mendeleev and Meyer also had a long dispute over priority, though OpenStax says Mendeleev's contributions are now more widely recognised. [3] [5]

By the twentieth century it became clear that the periodic relationship depends on atomic number rather than atomic mass, and modern tables are ordered that way. Some gaps lasted a long time. Mendeleev had predicted elements below manganese, and Ida Tacke and Walter Noddack found rhenium, one of the last stable elements to be discovered. [5]