Mendeleev predicted the existence of undiscovered chemical elements, described their properties in remarkable detail before anyone had seen them, and even corrected the accepted atomic weights of elements that were already known. His 1869 periodic table was not just a list of what had been found; it was a map that pointed to what was missing. Three of those predictions, for elements he called eka-aluminum, eka-boron, and eka-silicon, became the most celebrated, but they are only part of the story.
How the Gaps Took Shape
By the middle of the nineteenth century, chemists had identified more than 60 elements. For each one they had measured properties like atomic weight, density, specific heat, and the number of bonds it formed. The result was an enormous heap of data with no clear organizing logic. Mendeleev noticed that when elements were arranged by increasing atomic weight, their chemical properties tended to repeat at regular intervals. Hydrogen-like behavior showed up, then lithium-like behavior, then something resembling sodium, and so on in a wave-like pattern.1Studies in History and Philosophy of Science Part A. Prediction and the periodic table
The key insight was what Mendeleev did when the pattern broke. If an element’s known properties did not fit the column where its atomic weight said it should go, he sometimes moved it to a different column, trusting the chemical resemblance over the weight measurement. And where no known element fit at all, he left the space empty. Those gaps were his predictions: not vague guesses, but specific claims that an element with particular properties was waiting to be found. He gave three of the most prominent gaps temporary names using the Sanskrit-derived prefix “eka-” (meaning “one beyond”) and published detailed forecasts of what each element’s atomic weight, density, melting behavior, and oxide formulas would turn out to be.1Studies in History and Philosophy of Science Part A. Prediction and the periodic table
The Three Predictions That Made History
Eka-Aluminum Becomes Gallium
Mendeleev placed a gap directly below aluminum in his table and predicted an element he called eka-aluminum. He forecast its approximate atomic weight, said it would be a soft metal with a low melting point, and described the formula of its oxide. In 1875, the French chemist Paul-Emile Lecoq de Boisbaudran discovered a new element in zinc ore using spectroscopy and named it gallium. When its properties were measured, they lined up closely with what Mendeleev had described years earlier.2Studies in History and Philosophy of Science Part A. Prediction and the periodic table
There is a famous wrinkle in this story. Lecoq de Boisbaudran initially reported gallium’s density at a value that differed from Mendeleev’s prediction. Mendeleev wrote to the French chemist and argued that the measurement had to be wrong because his periodic system demanded a different figure. Lecoq de Boisbaudran re-purified his sample, measured again, and found a density closer to what Mendeleev had predicted. For many scientists of the time, the notion that a theoretical table could correct a laboratory measurement was startling.
Eka-Boron Becomes Scandium
A gap below boron led Mendeleev to predict eka-boron, an element he expected to have a relatively low atomic weight, an oxide with a specific formula, and properties intermediate between those of boron and the heavier members of its group. Four years later, in 1879, the Swedish chemist Lars Fredrik Nilson isolated a new element from the mineral euxenite and named it scandium. Its properties matched Mendeleev’s description.1Studies in History and Philosophy of Science Part A. Prediction and the periodic table
Eka-Silicon Becomes Germanium
The third gap sat below silicon. Mendeleev’s eka-silicon was predicted to be a grayish metal with a density around 5.5, a specific type of oxide, and a chloride that would be a volatile liquid. In 1886, the German chemist Clemens Winkler found a new element in the mineral argyrodite and called it germanium. Its measured properties, from density to the boiling point of its chloride, fell close to what the periodic table had forecast.2Studies in History and Philosophy of Science Part A. Prediction and the periodic table
Germanium’s discovery, coming more than fifteen years after the original prediction, may have been the most persuasive of the three. By 1886, the periodic table was no longer brand new, and the scientific community could see a pattern: Mendeleev’s system kept being right about things that had not yet been tested.
Correcting Known Elements, Not Just Predicting New Ones
Predicting missing elements gets the most attention, but Mendeleev also used the table’s internal logic to challenge existing data. If an element’s reported atomic weight placed it in a column where its chemistry did not belong, Mendeleev argued that the weight was wrong rather than that the table was flawed. He proposed revised atomic weights for several elements so they would land in positions consistent with their known chemical behavior. In a number of cases, later measurements proved him right.
The boldest of these corrections involved tellurium and iodine. Tellurium has a slightly higher atomic weight than iodine, so a strict weight-based ordering would place tellurium after iodine. But chemically, tellurium clearly belongs in the same family as sulfur and selenium, while iodine belongs with chlorine and bromine. Mendeleev reversed the pair, insisting that chemical properties should override the raw weight sequence. At the time, this looked like a fudge to make the system work. It was only decades later, after Henry Moseley showed that the real ordering principle is atomic number rather than atomic weight, that Mendeleev’s reversal was fully vindicated. Tellurium has a lower atomic number than iodine despite its heavier weight, so the swap was correct all along.
Predictions That Missed the Mark
The popular version of this story can make it sound as though Mendeleev never got anything wrong. He did. Beyond the three celebrated successes, he predicted several other elements that turned out not to exist. Mendeleev foresaw elements he called eka-manganese, eka-cesium, and others occupying specific slots in his table. Some of those gaps were eventually filled by real elements but with properties that differed significantly from his forecasts. Others corresponded to no real element at all.3ChemTexts. Where Mendeleev was wrong: predicted elements that have never been found
He also made predictions about elements lighter than hydrogen and heavier than uranium, convinced that the periodic pattern must extend in both directions. He speculated at various points that the luminiferous ether, a substance physicists of his era believed filled all of space, might itself be an element lighter than hydrogen. That conjecture went nowhere. And some of his heavier-element predictions were simply too far ahead of the experimental capabilities of the time to be testable, though the general idea that heavier elements exist beyond what was then known was of course correct.4Russian Chemical Reviews. Superheavy elements in D I Mendeleev’s Periodic Table
The failures matter because they show that Mendeleev’s system was not simply a crystal ball. It was a pattern-recognition tool that worked brilliantly within certain limits and less well when pushed beyond them. Some of the wrong predictions came from treating the periodicity as more rigid than it actually is, or from trusting atomic weight data that was itself unreliable.
The Noble Gases Nobody Saw Coming
Perhaps the most dramatic test of the periodic table came not from a prediction Mendeleev got right, but from an entire family of elements he never predicted at all. In the 1890s, William Ramsay and Lord Rayleigh discovered argon, a gas that did not react with anything. Over the following years, Ramsay and his collaborators isolated helium, neon, krypton, and xenon. These five noble gases added a whole new column to the periodic table, a group with zero valency that no one, Mendeleev included, had anticipated.
This could have destroyed the table’s credibility. Instead, the noble gases fit neatly into the existing structure once a new column was added at the far right. The periodic pattern held; it just had one more column than anyone had realized. Mendeleev himself initially resisted the idea, doubting that completely inert elements could exist, but he eventually accepted the evidence and incorporated them. Historians of science have pointed out that the accommodation of the noble gases into the table, after their discovery, did as much to cement confidence in the periodic system as the prediction of gallium or germanium had done beforehand.2Studies in History and Philosophy of Science Part A. Prediction and the periodic table
Why the Table Eventually Needed Atomic Number
Mendeleev arranged the elements by atomic weight because that was the measurable quantity available in his day. The system worked impressively well, but the tellurium-iodine reversal and a few other anomalies showed that weight alone was not the whole story. The underlying principle that actually explains the periodic pattern, atomic number, was not identified until the early twentieth century.
Just over a hundred years ago, the English physicist Henry Moseley carried out a systematic series of experiments using X-ray spectroscopy. He showed that the frequencies of X-rays emitted by an element under bombardment were characteristic of that element and could be used to identify the charge on its atomic nucleus. That nuclear charge is the atomic number, and Moseley demonstrated that organizing elements by atomic number rather than atomic weight resolved every remaining anomaly in the table.5PubMed. Henry Moseley, X-ray spectroscopy and the periodic table
Moseley’s work also revealed exactly how many elements were still missing. Because atomic numbers are whole numbers without gaps, he could count the holes in the sequence and say with certainty which elements remained undiscovered. Where Mendeleev had relied on chemical intuition to guess where gaps existed, Moseley provided a definitive checklist. Several of the remaining missing elements, including hafnium, rhenium, and technetium, were discovered in subsequent decades partly because Moseley’s numbering told chemists precisely what to look for.
How Much Did the Predictions Actually Matter?
In textbooks, the discovery of gallium, scandium, and germanium is usually presented as the moment the scientific world accepted Mendeleev’s table. The story has a satisfying arc: bold prediction, dramatic confirmation, universal acclaim. Historians who have looked at the evidence more carefully paint a more complicated picture.
A detailed re-examination of how the periodic table gained acceptance found little support for the standard narrative that these predictive successes were outstandingly important in establishing Mendeleev’s system. Accommodations, meaning the table’s ability to organize and make sense of elements that were already known, played at least an equal role. The incorporation of argon and the other noble gases into the table without breaking its structure, for instance, was a powerful demonstration that the system captured something real about nature.2Studies in History and Philosophy of Science Part A. Prediction and the periodic table
This distinction between prediction and accommodation is a long-running debate in the philosophy of science. Some philosophers argue that correctly predicting something unknown should count more heavily in favor of a theory than merely fitting data that was already available when the theory was built. Others counter that both demonstrate the same underlying thing: the theory captures a genuine regularity. In Mendeleev’s case, the historical record suggests that contemporary chemists were influenced by both kinds of success roughly equally. The predictions made better headlines, but the accommodation of dozens of known elements into a coherent framework was what many working chemists found most useful day to day.6Studies in History and Philosophy of Science Part A. Use-novel predictions and Mendeleev’s periodic table: response to Scerri and Worrall (2001)
Other Chemists Who Made Similar Predictions
Mendeleev was not the only chemist to notice periodic patterns in the elements. The German chemist Lothar Meyer independently developed a periodic table around the same time, and earlier attempts by John Newlands (the “law of octaves”) and Alexandre-Emile Béguyer de Chancourtois (a helical arrangement he called the “telluric screw”) had also captured aspects of elemental periodicity. What set Mendeleev apart was not just his table but his willingness to make specific, falsifiable claims about undiscovered elements and to insist that the table’s pattern was more trustworthy than individual measurements. Meyer’s table, for instance, was in many respects just as accurate, but Meyer was more cautious and did not publish the same kind of bold predictions.
This matters because Mendeleev’s confidence in his own system is part of what made it scientifically productive. By publicly staking his reputation on elements that did not yet exist, he gave other chemists a clear target. The predictions were not just forecasts; they were research programs. When Lecoq de Boisbaudran found gallium, he was not specifically searching for eka-aluminum, but the immediate comparison to Mendeleev’s published predictions turned a single discovery into evidence for an entire theoretical framework.
How Mendeleev’s Approach Echoes in Modern Chemistry
The periodic table of 2024 contains 118 confirmed elements, and the logic Mendeleev used, predicting properties from an element’s position in the table, remains central to chemistry. When researchers at heavy-ion laboratories produce a handful of atoms of a new superheavy element, one of the first questions is whether its behavior matches what its position in the table would predict. Elements 113 through 118 were all confirmed in the twenty-first century, and in each case their placement followed from the same periodic pattern Mendeleev identified using pencil and paper.
That said, the heaviest elements are starting to push the table’s logic into new territory. Relativistic effects on electrons orbiting very large nuclei can alter chemical behavior in ways that make a superheavy element less like its lighter column-mates than the table would suggest. Oganesson (element 118), for example, sits in the noble gas column, but theoretical calculations suggest it may not behave like a typical noble gas at all. Whether the periodic table can continue to serve as a predictive tool for elements beyond 118, if they are ever synthesized, is an open question. Mendeleev’s fundamental insight, that the properties of elements follow a repeating pattern tied to an underlying numerical sequence, holds firm. How far that pattern can be extrapolated remains to be seen.