On a February day in 1869, so the well-worn story goes, the Russian chemist Dmitri Mendeleev was struggling to organise the known chemical elements and, exhausted, fell asleep at his desk. He is supposed to have dreamed of a table in which all the elements fell neatly into place, and to have woken and written it down. The dream is probably embellished — Mendeleev himself gave differing accounts, and he had wrestled with the problem for a long time — but the achievement was real. He had produced the first widely useful periodic table of the elements, and its genius lay not only in what it contained but in the holes he deliberately left in it.
Mendeleev's insight was that if you arranged the elements in order of increasing atomic weight, their chemical properties recurred at regular intervals — periodically. Elements with similar behaviour, such as the reactive metals lithium, sodium and potassium, lined up in the same column. This was the "periodic law." A German chemist, Julius Lothar Meyer, arrived at a very similar arrangement independently and at almost the same time, and the two are sometimes credited jointly. But Mendeleev went a decisive step further, and that boldness is why his name is the one attached to the table.
Betting on what wasn't there
When the pattern demanded that a certain kind of element should exist but none was known, Mendeleev did not fudge his table to close the gap. Instead he left an empty square and predicted that an element with specific properties would one day be discovered to fill it. He even gave the unknown elements provisional names using the Sanskrit prefix "eka," meaning "one": eka-aluminium, eka-boron and eka-silicon, sitting respectively below aluminium, boron and silicon in his grid. For each he predicted an atomic weight, a density, and how it would combine with other elements.
It was a scientific gamble of the highest order, and it paid off spectacularly. In 1875 the French chemist Paul-Emile Lecoq de Boisbaudran discovered a new element he named gallium — Mendeleev's eka-aluminium. There is a famous wrinkle: Boisbaudran first measured gallium's density as lower than Mendeleev had predicted, whereupon Mendeleev, from Russia, effectively told him to check his sample again. On repurifying the metal, Boisbaudran found the density was higher than his first reading and matched Mendeleev's figure almost exactly. In 1879 the Swedish chemist Lars Fredrik Nilson found scandium, the predicted eka-boron. And in 1886 the German chemist Clemens Winkler discovered germanium, the eka-silicon, whose properties fit Mendeleev's forecast so closely that the periodic law was placed beyond serious doubt.
What the table couldn't yet see
Mendeleev's table was not perfect, and its imperfections are instructive. He had ordered the elements by atomic weight, but a handful of pairs — such as tellurium and iodine — seemed to sit in the wrong order if that rule were followed strictly. Mendeleev trusted the chemistry over the weights and placed them by their properties, and he turned out to be right, though the underlying reason had to wait. In 1913 the young English physicist Henry Moseley showed that the true organising principle is not atomic weight but atomic number — the number of protons in the nucleus — which resolved the awkward pairs cleanly.
There was also an entire family of elements that no one suspected in 1869: the noble gases. Helium, neon, argon and their relatives were discovered in the 1890s, and because they were so unreactive they had left no chemical fingerprints for earlier chemists to notice. Rather than shattering the table, they slotted in as a brand-new column at its edge, a striking confirmation of how robust the underlying pattern was.
It is worth stressing how unusual Mendeleev's approach was for its time. A pattern that merely accommodates the facts already known is common enough in science; a pattern that sticks its neck out and forecasts specific facts not yet observed is far rarer and far more powerful, because it can be decisively tested. When gallium, scandium and germanium duly appeared with the weights and densities he had specified, they did more than fill three gaps — they demonstrated that the periodic law captured something genuinely true about the structure of matter, rather than being a convenient filing system. That is the difference between a classification and a theory.
Mendeleev died in 1907, having become one of the most celebrated scientists of his age. In a fitting tribute, element 101, a synthetic radioactive metal created in a laboratory in 1955, was named mendelevium in his honour. The periodic table that hangs in every chemistry classroom is, in a sense, a monument to a piece of scientific nerve: the willingness to point at an empty square and say, with confidence, that something must be there.
Quiz nuggets
- Dmitri Mendeleev published his periodic table in 1869, arranging elements by increasing atomic weight.
- He left gaps for undiscovered elements, naming them eka-aluminium, eka-boron and eka-silicon.
- These predictions were confirmed by the discovery of gallium (1875), scandium (1879) and germanium (1886).
- Henry Moseley showed in 1913 that elements are properly ordered by atomic number rather than atomic weight.
- Element 101, mendelevium, was named after Mendeleev following its synthesis in 1955.