History: Mendeleev and the gaps

In 1869 about two thirds of the natural elements were known. Mendeleev left spaces for the rest.

Imagine sorting a deck of cards where a few cards are missing and nobody has told you which ones. You could still lay out the suits and the numbers, and the gaps would show you exactly what was absent. That is close to what the Russian chemist Dmitri Mendeleev did with the elements in 1869. He was writing a chemistry textbook, and he put each known element on a card with its atomic weight and properties, then moved the cards around until a pattern appeared.

The 1869 table

On 6 March 1869 Mendeleev's paper on the relation between the properties of the elements and their atomic weights was read to the Russian Chemical Society in St Petersburg. Mendeleev was away, and his colleague Nikolai Menshutkin presented it. The table put the elements in order of atomic weight and placed elements with similar chemistry in the same family. The German chemist Lothar Meyer published a very similar arrangement independently in 1870, and in 1882 the Royal Society of London gave its Davy Medal to both men.

What set Mendeleev apart was that he trusted the pattern more than the list of known elements. When the next element by weight did not fit the family below which it would fall, he left a blank and assumed an element was missing. He also swapped a few elements out of weight order when their chemistry demanded it. Tellurium is heavier than iodine, but iodine clearly belongs with chlorine and bromine, so he put tellurium first. Later work showed that the true order is by atomic number, and on that count he was right.

Predictions that came true

Mendeleev described the missing elements in detail, especially in 1871, using the prefix eka, Sanskrit for "one", to mean "one place beyond". Eka-aluminium should have an atomic weight near 68 and a density of 5.9 g/cm³. In 1875 Paul-Émile Lecoq de Boisbaudran found gallium, atomic weight 69.7 and density 5.91 g/cm³. Eka-boron, near 44, turned out to be scandium, found by Lars Fredrik Nilson in 1879. Eka-silicon was predicted at about 72 with a density of 5.5 g/cm³. Clemens Winkler found germanium in 1886: 72.6 and 5.32 g/cm³.

The germanium result in particular persuaded chemists that the table was more than a convenient filing system: it could predict. Not every prediction worked. Mendeleev had no place for the noble gases, which nobody suspected until argon was found in 1894, and he did not foresee the long run of lanthanides. The explanation for why the pattern exists came only in the twentieth century. In 1913 Henry Moseley showed with X-rays that each element has a whole-number atomic number, and the arrangement of electrons in shells explained why properties repeat.

The table is now complete to the end of period 7. In 2016 IUPAC approved the last four names: nihonium (113), moscovium (115), tennessine (117) and oganesson (118). Nihonium was made at the RIKEN institute in Japan, the first element discovered in an Asian country. These elements exist for fractions of a second, but each one was placed in its column before it was made, the same way Mendeleev placed gallium. Switch the model to "Height: Year of discovery" and the tallest tiles fill the bottom row.