Mendeleev's Periodic Table (1869): Ordering the Chemical Elements

Mendeleev’s periodic table, drawn up in St Petersburg in March 1869, arranged all the chemical elements then known by atomic weight and chemical properties. The Russian chemist Dmitri Mendeleev noticed that similar properties recurred at regular intervals and used this “periodic law” to leave gaps for undiscovered elements, whose properties he predicted. When gallium, scandium and germanium were found to match his forecasts, the periodic table became the organising framework of chemistry.

Background: too many elements, no clear order

By the mid-nineteenth century chemists had identified about 63 elements, from familiar metals like iron and gold to newly discovered ones such as rubidium and caesium. Since Lavoisier’s chemical revolution of the late eighteenth century, an element had been defined as a substance that could not be broken down further, and John Dalton’s atomic theory had given each element a characteristic atomic weight.

But the list had no obvious structure. Atomic weights were often uncertain, because chemists disagreed about the formulas of compounds. A major step came at the international chemistry congress in Karlsruhe in September 1860, where the Italian chemist Stanislao Cannizzaro argued for a consistent method of calculating atomic weights. Among the delegates was the young Mendeleev.

Earlier attempts at a pattern

Several scientists had already noticed partial patterns. In 1829 the German chemist Johann Wolfgang Döbereiner described “triads”, groups of three similar elements such as calcium, strontium and barium, in which the middle element’s atomic weight was roughly the average of the other two.

In 1862 the French geologist Alexandre-Émile Béguyer de Chancourtois arranged the elements in a spiral around a cylinder, his “telluric screw”, but his paper was published without its diagram and was largely ignored. In 1865 the English chemist John Newlands proposed a “law of octaves”, noting that properties seemed to repeat every eighth element. Members of the Chemical Society in London mocked the idea in 1866, one asking whether he had tried arranging the elements alphabetically.

In Germany, Lothar Meyer was working along similar lines. He produced a partial table in 1864 and a fuller one, published in 1870, that closely resembled Mendeleev’s.

Mendeleev’s path to the table

Dmitri Ivanovich Mendeleev was born on 8 February 1834 (27 January Old Style) in Tobolsk, Siberia, the youngest of a large family. After his father’s death, his mother took him to St Petersburg, where he trained at the Main Pedagogical Institute. He studied in Heidelberg from 1859 to 1861 and attended the Karlsruhe congress.

By the late 1860s he was professor of chemistry at St Petersburg University and was writing a textbook, Osnovy khimii (Principles of Chemistry). He needed a logical order in which to present the elements, and he began comparing groups of similar elements, such as the halogens and the alkali metals, by their atomic weights.

On 1 March 1869 (17 February Old Style) he drafted a table titled “An Attempt at a System of Elements, Based on Their Atomic Weight and Chemical Affinity”. His surviving notes show him writing, crossing out and rearranging elements as he worked. The table was printed and sent to chemists in Russia and abroad.

The periodic law of 1869

Mendeleev’s paper “On the Relationship of the Properties of the Elements to Their Atomic Weights” was presented to the Russian Chemical Society on 18 March 1869 (6 March Old Style). Mendeleev was unable to attend, and his colleague Nikolai Menshutkin read it on his behalf. It was published in the society’s journal later that year, and a German summary spread the idea abroad.

The central claim was that when the elements are arranged by atomic weight, their properties show a clear periodicity: similar elements appear at regular intervals. In his table, elements with similar properties, such as lithium, sodium and potassium, fell into the same family.

Mendeleev made two bold decisions. First, he left gaps where the pattern required an element that was not yet known. Second, where the order of atomic weights conflicted with chemical properties, he trusted the properties. He placed tellurium before iodine, although tellurium’s measured atomic weight was higher, and he argued that some accepted atomic weights, such as those of beryllium, indium and uranium, were wrong.

Early versions of the table looked different from the familiar modern chart. In 1869 Mendeleev arranged related elements in horizontal rows, with atomic weights increasing down the columns; the arrangement with families in vertical columns came in his revised table of 1871. Some placements were still uncertain, and he marked several with question marks.

Bold predictions and their confirmation

In 1871 Mendeleev published a refined version of the table and detailed predictions for three missing elements. He named them using the Sanskrit prefix eka-, meaning “one”, as the element one place below a known one: eka-aluminium, eka-boron and eka-silicon.

The first test came in 1875, when the French chemist Paul-Émile Lecoq de Boisbaudran discovered a new metal in zinc ore and named it gallium. Mendeleev recognised it as eka-aluminium. Lecoq initially reported its density as about 4.7; Mendeleev, who had predicted about 5.9, suggested the sample was impure. Remeasurement gave about 5.9, and the episode made Mendeleev famous.

In 1879 the Swedish chemist Lars Fredrik Nilson discovered scandium, which matched eka-boron. In 1886 the German chemist Clemens Winkler isolated germanium, a close fit for eka-silicon: Mendeleev had predicted an atomic weight of about 72 and a density of about 5.5, and germanium’s values were about 72.6 and 5.35.

By the numbers

Key people

• Dmitri Mendeleev (1834–1907): Russian chemist who formulated the periodic law and predicted new elements.
• Lothar Meyer (1830–1895): German chemist who independently developed a similar table; he shared the Royal Society’s Davy Medal with Mendeleev in 1882.
• John Newlands (1837–1898): English chemist whose law of octaves anticipated periodicity; he received the Davy Medal in 1887.
• Paul-Émile Lecoq de Boisbaudran (1838–1912): French chemist who discovered gallium in 1875.
• Henry Moseley (1887–1915): English physicist who showed in 1913 that elements should be ordered by atomic number.

Challenges and the modern table

The table faced new tests. In 1894 Lord Rayleigh and William Ramsay discovered argon, and helium, neon, krypton and xenon soon followed. These unreactive noble gases had no place in Mendeleev’s scheme, and he was initially sceptical. They were eventually added as a new group, fitting the periodic pattern.

The deeper reason for periodicity became clear only with atomic physics. The discovery of the electron in 1897 and of the atomic nucleus in 1911 led to Henry Moseley’s X-ray studies of 1913, which showed that each element has a characteristic atomic number, equal to the charge of its nucleus. Ordering by atomic number resolved anomalies such as tellurium and iodine. Quantum theory later explained periodicity through the arrangement of electrons in shells.

Myths and facts

A well-known story says Mendeleev saw the complete table in a dream. It comes from a later recollection by a colleague, the geologist Alexander Inostrantsev, and is not supported by Mendeleev’s working notes, which show careful trial and error.

Another popular claim is that Mendeleev set the 40 percent standard for Russian vodka. His 1865 doctoral dissertation was indeed on mixtures of alcohol and water, but historians have found no evidence that he devised the vodka standard.

It is also sometimes said that Mendeleev alone discovered periodicity. In fact, several chemists glimpsed it; Mendeleev’s distinction lay in the completeness of his system and in his confident use of it to make testable predictions.

Legacy

Mendeleev became one of Russia’s most celebrated scientists, though he was never elected to the Russian Academy of Sciences. From 1893 he directed the Chief Bureau of Weights and Measures. He died in St Petersburg on 2 February 1907 (20 January Old Style). In 1955 element 101 was named mendelevium in his honour, and the United Nations declared 2019, the 150th anniversary of his table, the International Year of the Periodic Table.

The periodic table remains the single most important reference in chemistry. It guides the search for new materials and helped scientists understand the radioactive elements studied by Marie Curie and others.

Timeline

• 1829: Johann Döbereiner describes triads of similar elements.
• 1834, 8 February: Dmitri Mendeleev is born in Tobolsk, Siberia.
• 1860, September: The Karlsruhe congress debates consistent atomic weights.
• 1865: John Newlands proposes the law of octaves.
• 1869, 1 March: Mendeleev drafts his first periodic table.
• 1869, 18 March: His paper is presented to the Russian Chemical Society.
• 1871: Mendeleev publishes detailed predictions of missing elements.
• 1875: Gallium (eka-aluminium) is discovered.
• 1886: Germanium (eka-silicon) is discovered.
• 1913: Henry Moseley establishes ordering by atomic number.

Why it matters

Mendeleev’s periodic table turned a confusing list of substances into an ordered system with predictive power. Its success showed that chemistry follows deep natural laws, and it remains the map on which chemists and physicists locate every element, known or yet to be made.