DMITRI Mendeleev had a problem. As a professor at the University of St Petersburg in Russia, he was supposed to teach chemistry to students, and to guide him in that task he had arranged a contract with a Russian publisher to write a two-volume textbook. By January 1869, he had completed the first volume, but it covered only eight of the 63 chemical elements then known. How was he going to cram the remaining 55 into volume two?
Clearly he couldn’t afford to take the same rambling stroll through the properties of the elements as he had in the first volume. He needed some system to organise the material. But was there any order to the building blocks of the physical world?
When we retell stories of scientists pondering great questions like this, they are often made to seem romantic. There is the period of struggle and confusion that ends when a lone genius sees the light, perhaps in a reverie or dream. Then everything falls into place, the paradigm shifts, and nothing is ever the same. How much more noble this sounds than a desperate attempt to meet a publisher’s deadline.
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Some accounts of how Mendeleev devised the periodic table try to make it fit that romantic template. They allege that the Russian, originally from the remote town of Tobolsk in Siberia, was obsessed with finding structure among the elements and laid them out written on cards, like a game of solitaire. He tried all sorts of arrangements without success, the story goes, until he fell asleep, exhausted, in his study in February 1869.
“I saw in a dream a table where all the elements fell into place as required,” he was later reported to have said. On waking, he hastily wrote down his vision, and two weeks later published his “Suggested System of the Elements”. At last, the steadily growing list of elemental substances from hydrogen to lead had a logic to it.
That logic, said Mendeleev, is essentially this: if you order the elements according to their atomic weight – how much a constant quantity of each element weighs relative to the lightest, hydrogen – their properties seem to repeat at regular intervals. You can therefore write the list of elements as a table, with columns of elements that share similar attributes.
This pattern is still the basis of modern periodic tables, but the tale of how Mendeleev discovered it crumbles under examination. Take the dream. “I don’t believe it,” says historian of science at Princeton University, an expert on Mendeleev’s life and work. “The sources are too iffy.” Mendeleev never made the dream claim himself; it came from a colleague 40 years later. Even then, he could easily have meant something like a daydream, says Gordin. Mendeleev himself emphasised that his discovery “was the product of insight and chemical knowledge”, he adds. But Gordin admits that “people seem to love this story, so I don’t think my historian’s fussiness is going to squelch it anytime soon”.
At any rate, there was never a perfect arrangement of the table that made sense of all the available knowledge. For one thing, Mendeleev’s ranking of the elements by atomic weight isn’t what we use today. Atomic weights were deduced by experiment: by breaking chemical compounds into their constituent elements and weighing how much of each they contained. But the more fundamental ranking comes from the atomic number of each element, the number of protons in nuclei of their atoms. In the middle of the 19th century, no one was sure if atoms were even real, and Mendeleev himself was sceptical.
Even the atomic weights were disputed. For example, oxygen and hydrogen combined to make water in a ratio of eight parts to one, but did this mean water molecules contained equal numbers of hydrogen and oxygen atoms, with the latter eight times heavier, or twice as many hydrogen atoms as oxygens, with the latter 16 times heavier? (It is actually the second of these, the molecular formula being H2O.)
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It was, in fact, growing support for the H2O formula of water that helped Mendeleev order the elements properly. But he needed to take some liberties to ensure that elements with similar chemical behaviour fell into the same group. For example, he decided to give beryllium oxide the formula BeO, rather than (as most people thought) Be2O3, to get beryllium into the same column as magnesium, with which it seemed to have similarities. Mendeleev would later be proved right about the formula. In this and other small ways, his table is an example of an idea asserting precedence over the available data, challenging the common view in science that if your hypothesis disagrees with the data then you must discard it, no matter how elegant it is.
And even if Mendeleev’s table was a master stroke, it wasn’t a total revelation. The German chemist Johann Wolfgang Döbereiner had done something similar, by grouping chemically alike elements into groups of three, which he called “triads”, in the 1820s. Others, including William Odling and John Newlands in the UK, Alexandre-émile Béguyer de Chancourtois in France, and Julius Lothar Meyer in Germany, sketched out arrangements of the elements gathered into families in the 1850s and early 1860s. Meyer virtually had the periodic table sorted by 1868, but he didn’t publish it until a year after Mendeleev.
“The steadily growing list of elemental substances finally had a logic to it”
This work was no secret, although Mendeleev insisted later that he had been unaware of it. “This seems a little odd,” says chemist at the University of California, Los Angeles, a leading expert on the periodic table. Disputes about who got there first ensued, making you wonder if the dream was just a convenient device.
Finally, the notion that, thanks to Mendeleev’s periodic table, the scales fell from the eyes of his peers doesn’t stack up. A few scientists, such as Russian chemist Julia Lermontova, did take note and tried to clarify the ordering of elements by improving methods of separation and characterisation. But there was no abrupt paradigm shift in chemistry. “At first, there wasn’t too much of a reaction to it,” says Gordin.
Curiosity about Mendeleev’s table only began to grow six or seven years after it was published, when the element gallium was discovered by French chemist Paul-émile Lecoq. It fitted the prediction made by Mendeleev of a heavier element below aluminium with atomic weight 68, for which he had left a space in his table, giving it the provisional name eka-aluminium. Ostensibly, Lecoq named it patriotically after the old Latin form of his country, Gallia. But it is widely suspected that the name was also a bit of sly self-advertising, as the Latin word gallus means cockerel – le coq in French.
Another of Mendeleev’s predicted elements, labelled eka-silicon, was discovered in 1886 and christened germanium. This capacity to make predictions was what distinguished Mendeleev’s table from earlier ones.
“Curiosity about the table grew as some of the gaps were filled”
Still, recognition was a long time coming. That may have been partly due to Mendeleev’s eccentric demeanour: his long hair, unruly beard and allegedly bad temper. The British chemist William Ramsay, having met him in London in 1884, described him as peculiar, “every hair of whose head acted in independence of every other”. But having conversed with him in broken German, Ramsay – who augmented the periodic table with a whole column of noble gases at the end of the century – found Mendeleev “a nice sort of fellow”.
Although the tales of Mendeleev’s invention of the periodic table can be more fiction than fact, that doesn’t detract from its significance. It was the most comprehensive ordering of the building blocks of matter and, unwittingly, it pointed the way to the underlying quantum rules that govern the composition and properties of atoms. It helped unite chemistry and physics, and revealed a deep aspect of nature’s design. Just don’t try to pretend that it arrived in a dream.
Check out the rest of our special on the 150th anniversary of the periodic table:Â
- Find out why the most beautiful table in science is worth celebrating
- How well do you know the periodic table? Try our quiz
- ł˘±đ˛ą°ů˛ÔĚý
- Try our special crossword
- Discover the favourite elements of some top scientists and share your own
- Journey inside the Russian factory making the heaviest atoms in the universe
