How Mendeleev Created the Periodic Table

Dmitri Mendeleev built the periodic table the way you might organize a hopelessly messy drawer: by laying everything out where he could see it. While writing a chemistry textbook in 1869, he wrote each known element's name, weight, and chemical habits on its own card, then shuffled the deck in order of increasing atomic weight until a pattern surfaced — chemical properties repeating at regular intervals. Then he did the audacious part: he left empty spaces where the pattern demanded an element nobody had ever found, and described those missing elements anyway.

Why was a chemist playing cards with the elements?

Mendeleev wasn't trying to become immortal; he was trying to finish a book. In 1867 he had been appointed to the chair of inorganic chemistry at St Petersburg University, and he found no textbook good enough for his students — so he wrote his own, Principles of Chemistry, published in two volumes between 1868 and 1870. The trouble with writing a chemistry textbook, he discovered, is that chemistry in the 1860s was a heap of disconnected facts. Each element had its own personality, and nobody had found the plot connecting them. A meeting nine years earlier had quietly loaded the gun. In 1860, Mendeleev attended the first international chemistry congress at Karlsruhe with fellow Russian Alexander Borodin — better remembered today as a composer. The congress exposed an embarrassing mess: chemists couldn't even agree on atomic weights. Was oxygen 8 or 16? Was water H2O or OH? Out of that confusion, the Italian chemist Stanislao Cannizzaro produced the most accurate list of atomic weights then known, drawing on the neglected work of Amedeo Avogadro. The Royal Society of Chemistry's account of Mendeleev's life calls Cannizzaro's list "essential to Mendeleev in recognising periodicity." Without trustworthy weights, there was no pattern to find. So Mendeleev began collecting. For years he gathered every scrap of data about every element — its weight, its compounds, its quirks — and wrote it all down on separate cards. The cards were his database, and like any database, they were useless until they were sorted.

How did shuffling cards reveal the pattern?

The decisive day was 17 February 1869 by the Julian calendar Russia then used — 1 March in the Gregorian calendar the rest of Europe followed. Mendeleev sat down with his cards and began arranging them the way Russians arranged cards for patience, the solitary card game: laying them out, picking them up, trying new orders. Somewhere in the shuffling, he later recalled, the pattern suddenly resolved. Listed by increasing atomic weight, the elements' properties repeated — lithium, sodium, and potassium behaving like siblings; then a new family beginning, then another.

He first called his arrangement the Periodic System; an improved version in 1871 became the Periodic Table. The first public announcement came about a month later, and fittingly for the legend that would grow around him, Mendeleev wasn't even there: he was ill, so a colleague, Professor Menschutkin, read the paper to the recently formed Russian physicochemical society. A summary appeared soon after in a German chemistry periodical, and outside Russia, almost nobody noticed. What they were ignoring was, by the account of the Royal Society of Chemistry, superior to every earlier attempt in two ways. Mendeleev wasn't the first chemist to try ordering the elements — others had noticed repeating properties before him — but his predecessors had mostly treated the gaps and mismatches as problems to tidy away. Mendeleev treated them as information.

Why did he leave empty spaces — and why did that make him famous?

The first superiority was corrective. Where an element's accepted atomic weight clashed with its chemistry, Mendeleev trusted the chemistry. Beryllium, for instance, had been given an atomic weight of 13.5 on the assumption of the wrong valency; Mendeleev saw no room for it between carbon and nitrogen, but with the correct valency it slotted perfectly above magnesium. He made similar repairs to uranium and indium — effectively telling the experimentalists of Europe that their measurements were wrong, and being right. The second superiority was prophetic, and it was the gamble of his career. Where his pattern demanded an element between two known ones, he left a blank — and then, in print, described the missing element's atomic weight, density, melting behavior, and the formulas of its compounds. He named these ghosts with the Sanskrit prefixes eka, dvi, and tri — one, two, three — a nod, via his Sanskritist friend Otto von Böhtlingk, to the ancient Indian grammarian Panini. "Eka-aluminium" meant the unknown element one row below aluminium. In 1875 the French chemist Emile Lecoq de Boisbaudran, using the new technique of spectroscopy, isolated a new metal and called it gallium. When its details reached Mendeleev, he recognized his eka-aluminium — and announced that the Frenchman had measured its density incorrectly. This, the Royal Society of Chemistry notes, struck some as bordering on arrogance. Then the gallium sample was purified, the density remeasured, and Mendeleev was vindicated. Within a decade, two more of his ghosts materialized: scandium (his eka-boron), found by Nilson in 1881, and germanium (his eka-silicon), found by Winkler in 1886. Chemists stopped scoffing and started using the table as a treasure map.

Did the periodic table really come to him in a dream?

Here the legend outruns the evidence, and Mendeleev himself fed it. He told a colleague that he had seen the complete arrangement in a dream — the elements falling into place as required — and had written it down immediately on waking, with only one later correction needed. It's a wonderful story, and it has been repeated in a thousand classrooms. But the staff at his memorial museum in St Petersburg favor a different scene. According to the museum, the first sketches of the table were made not in a burst of nocturnal inspiration but on the back of a letter from one of his family's properties — a cheese factory. (Mendeleev, whose civil-service career would later include stints as a government inspector of cheese and vodka, apparently never strayed far from dairy correspondence.) The museum even keeps a copy of that letter as proof. Both stories can be true in the way that matters. A dream — or a letter, or a missed train — might have supplied the final click, but the click only worked because of the years of data on the cards. Mendeleev had been collecting, weighing, and comparing since the early 1860s. The vision, whenever it arrived, was the punctuation mark on a very long sentence, not the sentence itself. And his true originality wasn't the flash of arrangement anyway: other chemists of the 1860s, including Lothar Meyer — with whom Mendeleev would share the Royal Society's Davy Medal in 1882 — were circling similar patterns. What set Mendeleev apart was the nerve to publish the blanks.

Is his table still the one on classroom walls?

In one important respect, no — and Mendeleev would probably have been delighted by the reason. We now know that elements are ordered by atomic number (the count of protons) rather than atomic weight, a refinement that came with the discovery of the atom's inner architecture — the story told in how scientists finally mapped the atom's structure. Remarkably, the two orderings disagree in only three places among the stable elements (argon–potassium, cobalt–nickel, tellurium–iodine). In the tellurium–iodine case, Mendeleev had faced exactly the conflict and placed them by chemistry anyway, insisting one of the accepted weights had to be wrong. The world caught up to him slowly and then all at once. By the 1880s he was lecturing to packed halls across Europe; the Nobel chemistry prize nearly went to him in 1906, the committee having recommended him for the periodic system. When he died in St Petersburg in 1907, his students carried a giant periodic table aloft at his state funeral. A century and a half after the cards were shuffled, the United Nations declared 2019 the International Year of the Periodic Table — and the hunt for new elements that his blanks began, continued by researchers like Marie Curie in her isolation of radioactive elements and the era of accidental discoveries like X-rays, still hasn't stopped. A textbook nobody liked, a deck of cards, and the nerve to describe what wasn't there yet: that is how Mendeleev created the periodic table — not in a dream, but in the patient space just before one.