Dmitri Mendeleev and the periodic table of 1869

Who was Dmitri Mendeleev?

Dmitri Ivanovich Mendeleev was born in 1834 in Tobolsk, Siberia, the youngest child of a large family. He studied in St. Petersburg, did research in Heidelberg, Germany, and in 1860 attended the Karlsruhe Congress, where chemists debated how to measure atomic weights reliably. By the late 1860s he was a professor of chemistry at the University of St. Petersburg.

A textbook problem

In the late 1860s Mendeleev was writing a new textbook, Principles of Chemistry. He needed a logical order in which to present the elements, of which just over 60 were known. He gathered their atomic weights and chemical properties, such as the formulas of their oxides and the number of hydrogen or chlorine atoms they combine with, and looked for a pattern that would tie them all together.

How he built the table

In early 1869 Mendeleev arranged the elements in order of increasing atomic weight and saw that similar properties returned at regular intervals. He set the elements out in a grid so that families such as the alkali metals and the halogens fell into line. His paper was presented to the Russian Chemical Society in March 1869, read on his behalf by his colleague Nikolai Menshutkin, and a summary soon appeared in German, which carried the idea across Europe.

Mendeleev stated what he called the periodic law: the properties of the elements depend periodically on their atomic weights. By 1871 he had published an improved table in which the families ran down eight vertical groups, a layout much closer to the one used today.

Chemistry before weight order

Mendeleev trusted chemical behavior more than the numbers. Where strict weight order would have put an element in the wrong family, he moved it, placing tellurium ahead of the slightly lighter iodine so that iodine could join the other halogens. He also insisted that several accepted atomic weights were simply wrong. He doubled the value for uranium from about 120 to 240, raised indium’s from about 76 to 113 and lowered beryllium’s from about 14 to 9. Later measurements showed he was right about all three, although tellurium really is heavier than iodine, a puzzle that was not solved until 1913.

Gaps and eka-elements

The boldest feature of the table was its empty spaces. Mendeleev left gaps where no known element fit and, in 1871, described three of the missing elements in detail. He named them by adding the Sanskrit prefix eka-, meaning “one,” to the element above each gap in his table: eka-aluminium sat one place below aluminum, eka-boron below boron and eka-silicon below silicon. For each one he estimated the atomic weight, the density and the formulas and properties of its compounds.

Predictions confirmed

In 1875 the French chemist Paul-Émile Lecoq de Boisbaudran discovered gallium. Its atomic weight, about 69.7, was close to the 68 Mendeleev had given eka-aluminium. Lecoq first measured gallium’s density as 4.7 grams per cubic centimeter, but Mendeleev wrote that it should be close to his predicted 5.9; with a purer sample, Lecoq obtained 5.9.

In 1879 the Swedish chemist Lars Fredrik Nilson discovered scandium, and his colleague Per Teodor Cleve pointed out that it matched eka-boron, which Mendeleev had expected to have an atomic weight of 44 (scandium’s is about 45). In 1886 Clemens Winkler found germanium, a close fit for eka-silicon: Mendeleev had predicted an atomic weight of 72 and a density of 5.5, and germanium’s are 72.6 and 5.3.

Not all of his forecasts came true, and he did not foresee a whole family, the noble gases. But these three successes convinced most chemists that the periodic law was real.

Lothar Meyer’s parallel table

Mendeleev was not the only chemist on this trail. The German chemist Lothar Meyer drafted a similar table in 1868, although it was not published until after his death, and in 1870 he published a periodic table along with a graph of atomic volume against atomic weight that rose and fell in regular waves. Meyer was more cautious, however, and did not publish detailed predictions of missing elements. Mendeleev’s confident forecasts are a large part of why his name is the one most closely linked to the table.

Recognition and mendelevium

In 1882 the Royal Society in London awarded its Davy Medal to Mendeleev and Meyer, and in 1905 Mendeleev received the society’s oldest award, the Copley Medal. He was nominated for the Nobel Prize in Chemistry but never received it, and he died in St. Petersburg in 1907.

In 1955 a team at the University of California, Berkeley, including Albert Ghiorso and Glenn Seaborg, made element 101 by bombarding einsteinium with alpha particles and named it mendelevium in his honor. It was a notable tribute: an element discovered in the United States, named after a Russian scientist in the middle of the Cold War.