Patterns and order have always fascinated humans. Even in ancient times, people constructed massive stone monuments aligned with significant points in the solar cycle. In the 19th century, chemists observed similarities among elements and attempted to explain them.
Dmitri Mendeleev played a crucial role in the search for order among elements when he published the first draft of his periodic table over a century ago. In 2019, the global community of chemists celebrates this significant anniversary. Mendeleev’s table, like Stonehenge, reflects the regularities found in nature, which were initially mysterious. But how did Mendeleev construct this monumental achievement?
Table of Contents
Early Life of Dmitri Mendeleev
Dmitri grew up in Siberia, far from the western world. His hometown, Tobolsk, was closer to Beijing than Paris, and his path to scientific prominence was not easy. Born in 1834, Dmitri was the youngest of over a dozen Mendeleev siblings. Unfortunately, due to his father Ivan’s ill-health, a high school teacher, he had to retire soon after Dmitri’s birth. To support the family, Dmitri’s mother, Maria, took over the management of a semi-derelict glassworks previously run by her brother.
The family relied on this enterprise until tragedy struck in 1848, and the glassworks burnt down. Ivan passed away, and in 1849, Maria took her two youngest children to Moscow in hopes of securing Dmitri’s entrance into a university with the help of her brother. When the plan failed, they moved to St. Petersburg. In 1850, Dmitri reluctantly enrolled in the college where his father had trained as a teacher. A lecturer named Alexander Voskresensky, who had studied under Justus Liebig in Germany, nurtured Dmitri’s interest in chemistry.
Dmitri graduated in 1855 and published his dissertation on isomorphism and the relationship between physical form and chemical composition in a mining journal. He continued writing articles for scientific and technical periodicals, but financial stability eluded him. Both his mother and sister had passed away, and he himself battled with what was originally diagnosed as tuberculosis. However, a year of teaching in the Crimea improved his health, and a new doctor dismissed the earlier diagnosis with certainty.
In autumn 1856, Mendeleev successfully defended his master’s thesis on the relationship between the specific volumes of substances and their crystallographic and chemical properties. Shortly after, the University of St. Petersburg granted him a license as a chemistry tutor, providing access to their laboratory. In 1859, he received state funding for two years of advanced study abroad.
Establishing a Remarkable Career
At Heidelberg University in Germany, Mendeleev conducted research on various topics, including surface tension, capillarity, and evaporation. His interest in intermolecular forces persisted throughout his career. In 1860, he attended the Karlsruhe conference, where Italian chemist Stanislao Cannizzaro delivered a groundbreaking paper on atomic weights, now known as relative atomic masses. This presentation marked a significant step towards the periodic system. Previously, assigning atomic weights to elements had caused considerable dispute.
Some chemists argued that atomic weights were irrelevant or denied the existence of atoms altogether. Others proposed a system based on an atomic weight of eight for oxygen, assuming that the formula for water was HO instead of H2O. However, at the Karlsruhe conference, Cannizzaro used the ideas of his fellow countryman, Amadeo Avogadro, to support the H2O water formula and set an atomic weight of 16 for oxygen. In the 1860s, opinion began to shift in his favor, which was fortunate for Mendeleev since the regularities pointing towards the periodic table would have been less apparent in the older system.
Mendeleev returned to St. Petersburg in 1861, resuming teaching at the university while lecturing at the city’s Technological Institute. He also published an organic chemistry textbook and several articles for a technical encyclopedia. Moreover, he traveled extensively to apply scientific discoveries to Russia’s economic development, including a visit to the Baku oil fields, which initiated his long-term involvement in the emerging petrochemical industry.
In 1865, Mendeleev’s doctoral thesis on solution theory was accepted, and two years later, the university appointed him as a professor of general chemistry. He was tasked with lecturing on inorganic chemistry because there was a lack of satisfactory Russian textbooks on the subject. This challenged him to develop an orderly pattern for arranging the chemical elements. Although several attempts had been made by others, such as Leopold Gmelin in Germany, Jean Baptiste Dumas in France, and John Newlands in England, none had achieved significant success. Mendeleev, aware of these efforts, approached the task with his unique perspective.
The Unveiling of Mendeleev’s Table
The breakthrough happened in early 1869 as Mendeleev prepared for an industrial tour to investigate and improve cheese-making techniques. At the same time, while completing the first volume of his textbook, he struggled to establish a framework for the second volume. He later described this process:
“So I began to write down the elements with their atomic weights and typical properties – like atomic weights and analogous elements on separate cards. This soon convinced me that the properties of the elements are in periodic dependence upon their atomic weights…” – Dmitri Mendeleev, Principles of Chemistry, 1905 (emphasis added)
Mendeleev arranged his “cards” in columns and rows, similar to a game of solitaire or patience, which he enjoyed playing during his railway journeys. The vertical columns listed the known elements in increasing order of atomic weight, starting a new column whenever it allowed similar elements to be placed in the same horizontal row.
While some groups of elements, like the alkali metals and halogens, were clearly related, many others, such as the rare earth elements (lanthanides), posed challenges no matter how they were arranged. However, unlike his predecessors, Mendeleev refused to give up in the face of anomalies.
If an element seemed out of place in his table, he was willing to adjust its atomic weight to ensure compatibility with its neighboring elements. For example, he proposed that beryllium oxide’s formula was BeO instead of the accepted Be2O3. By lowering beryllium’s atomic weight, he was able to place it with magnesium rather than aluminum.
On March 6, 1869, Mendeleev presented the first rough sketch of his table to the Russian Chemical Society, an organization he had recently helped establish. Later that year, the society’s journal published a more refined version, which was also translated into German. While it received little attention outside of Russia, Mendeleev persisted, continuing to add more “cards” to his table.
Bridging the Gaps
The revised diagram Mendeleev published in 1871 is more recognizable to us today. To complete it, he made further assumptions. For instance, he lowered the atomic weight of tellurium to make iodine the heavier of the two. This allowed him to place iodine with the halogens and tellurium with sulfur and selenium. At that time, such adjustments fell within the realm of experimental error. Mendeleev couldn’t have anticipated that atomic number, rather than atomic weight, would become the organizing principle of the table or that mass spectrometry would eventually reveal the explanation for these and other anomalies.
Mendeleev also enhanced the cohesion of his table by leaving gaps for yet-to-be-discovered elements, completing the pattern he envisioned. He not only predicted their chemical properties but also assigned them notional values for physical properties like specific gravity and melting point.
The first element he predicted, gallium, was spectroscopically identified by French chemist Paul Lecoq de Boisbaudran in 1875. After enough gallium became available for testing, it matched all of Mendeleev’s predictions, except for its specific gravity, which initially seemed to be 4.7. However, after Mendeleev recommended re-measuring, it was confirmed to be 5.9, virtually identical to his prediction.
The discovery of scandium in 1879 and germanium in 1885, both exhibiting the properties Mendeleev had foreseen, convinced more chemists that his table, despite its remaining anomalies, was too valuable to ignore. Meanwhile, other researchers, such as Lothar Meyer in Germany, also identified periodic variations in the physical properties of the elements. Mendeleev once remarked, “Although I have had my doubts about some obscure points, yet I have never once doubted the universality of this law because it could not possibly be the result of chance” [Mendeleev, op cit].
While Mendeleev was correct about the overarching principle of periodicity, he was not infallible as a predictor. Several other elements he proposed were never found. He also argued until the end of his life that ether, an undetectable component in the then-accepted theories of light and electromagnetism, was indeed a chemical element, despite his inability to isolate it in the laboratory. He suggested it might be the lightest noble gas with an atomic weight of 0.17.
Later Years and Legacy
Mendeleev lived an unconventional life outside of science. He only cut his hair and trimmed his beard once a year, even declining to change this custom when meeting the Czar. His domestic arrangements were also irregular. In 1862, he married Feosva Lescheva at the suggestion of an elder sister who believed it was time for him to settle down. The couple had two children, but as their mutual unhappiness grew, they agreed to separate. They alternated between living in Dmitri’s townhouse and his country retreat.
Some years later, Mendeleev fell in love with Anna Popov, a 17-year-old art student. When Anna’s parents sent her away to continue her studies in Rome, Dmitri followed her. In 1881, at the age of 47, he proposed marriage. Although Dmitri and Feosva were still awaiting their seven-year interval, a requirement of the Russian Orthodox Church for subsequent marriages, they found a priest willing to perform the ceremony prematurely for a substantial fee. Despite their technically bigamous situation, Dmitri and Anna lived happily together and raised four children.
Mendeleev was also politically outspoken, identifying as a liberal and resigning from his professorship in 1890 to distance himself from the government’s harsh suppression of student protests. While his gesture earned applause from his students, it provoked hostility from official circles. Nevertheless, Sergius Witte, the minister of finance from 1892, recognized the value of Mendeleev’s contributions and appointed him as the head of the government’s bureau of weights and measures in 1893. From this position, Mendeleev continued to apply scientific knowledge to aid Russia’s economic development.
In 1905, the Royal Society in London awarded Mendeleev the Copley Medal, following his previous receipt of the Davy Medal in 1882. Although nominated for the Nobel prize in 1906, the awards committee ruled that his discovery was not recent enough for consideration, likely influenced by the Swedish physical chemist Svante Arrhenius, who had clashed with Mendeleev in the past.
Nearly half a century after his death in 1907, Mendeleev joined an even more exclusive club. In 1955, physicists at the University of California, Berkeley, bombarded element 99 (einsteinium) with alpha particles to produce traces of element 101. Officially confirmed as “mendelevium,” this new element immortalized his name in the very table he created. As our understanding of sub-atomic structures and quantum energy exchanges deepened, eventually explaining the table’s layout in detail beyond Mendeleev’s foresight, his achievement remained monumental.
Before Mendeleev, others had suggested the possibility of arranging the known elements in a meaningful pattern. They observed significant connections but failed to form a definitive picture. However, Mendeleev believed that the chemical elements must be viewed as a collective whole. Armed with this conviction, he reimagined the positions of known elements and left gaps for undiscovered ones, creating a table that brought coherence to the field of chemistry. While some predictions missed the mark, Mendeleev’s hits established his table as the foundation of our understanding of the elements, securing his reputation as one of the founders of modern chemistry.
For further reading on the subject:
- W. H. Brock, The Fontana History of Chemistry, Fontana Press, 1993
- M. Fontani, M. Costa, and M. V. Orna, The Lost Elements: The Periodic Table’s Shadow Side, Oxford University Press, 2015
- E. R. Scerri, The Periodic Table: Its Story and Its Significance, Oxford University Press, 2006
Article written by Mike Sutton, a science historian based in Newcastle, UK.