Why ordering the elements was hard
By the early 1800s, chemists knew a growing number of elements, and the list kept lengthening as new ones were isolated. Without an organizing idea, they were just a catalog. A pattern needed a reliable property to sort by, and the obvious candidate was atomic weight, the weight of an element’s atoms relative to some standard. But atomic weights were themselves uncertain for decades, because chemists disagreed about formulas such as how many atoms of oxygen combined with a given metal.
A major step came at the 1860 Karlsruhe congress, where Stanislao Cannizzaro argued for a consistent method of finding atomic weights, building on Avogadro’s earlier hypothesis. Many chemists left more willing to use a common system, which made comparison possible.
Early patterns
Before any full table, chemists noticed local patterns.
- Döbereiner’s triads (1829). Johann Wolfgang Döbereiner pointed out groups of three elements with similar properties, such as calcium, strontium and barium, where the middle element’s atomic weight was close to the average of the other two.
- De Chancourtois (1862). Alexandre-Émile Béguyer de Chancourtois arranged elements by atomic weight along a spiral wrapped around a cylinder, which he called a telluric screw, and saw similar elements line up. His paper was poorly illustrated, and it had little impact.
- Newlands (1864 and 1865). John Newlands in England listed elements by atomic weight and noticed that every eighth element seemed to repeat properties, which he compared to octaves in music. When he presented the idea to the Chemical Society in 1866, it was met with skepticism, and a remark comparing it to arranging elements alphabetically is often reported. He was later recognized, and the Royal Society awarded him the Davy Medal in 1887.
These attempts were not failures so much as partial pictures. The patterns were real, but none of them left room for undiscovered elements or dealt well with exceptions.
Mendeleev’s table
Dmitri Mendeleev, a Russian chemist teaching at St. Petersburg, was writing a textbook, Principles of Chemistry, and wanted to organize the elements for his students. In early 1869 he arranged them by atomic weight and by chemical behavior, and his table was presented to the Russian Chemical Society in March 1869. He published a periodic law stating that the properties of elements vary periodically with atomic weight. The popular story of the table coming to him in a dream is unreliable.
What set his version apart was boldness. Where the pattern demanded it, he left gaps for elements not yet known, and he reordered a few elements against the atomic weights of his day because their chemistry fit better elsewhere. He also predicted properties of the missing elements, calling them eka-aluminium, eka-boron and eka-silicon, using the Sanskrit-derived prefix eka for “one.”
Predictions confirmed
The table gained credibility when the gaps were filled.
- Gallium was discovered by Paul-Émile Lecoq de Boisbaudran in 1875, and it matched much of what Mendeleev had said about eka-aluminium.
- Scandium was found by Lars Fredrik Nilson in 1879 and corresponds to eka-boron.
- Germanium was isolated by Clemens Winkler in 1886 and matched eka-silicon closely.
Even here, accounts sometimes overstate how perfect the predictions were; a few of Mendeleev’s other predictions did not work out. But the successes persuaded many chemists that the periodic law was a real feature of nature.
Lothar Meyer and the priority argument
Julius Lothar Meyer in Germany developed a similar classification at about the same time. A table in his 1864 book arranged elements by valence, and in 1870 he published a version with a graph of atomic volume against atomic weight, showing the periodic rise and fall clearly. Meyer had reached important ideas independently, and some of his work was done before Mendeleev published. A dispute over credit followed, with each side accusing the other of borrowing. Mendeleev made firm predictions and was willing to change accepted weights, while Meyer was more cautious. In 1882, the Royal Society of London gave the Davy Medal jointly to both men.
Mendeleev is nonetheless the name most often attached to the table, and many historians consider that fair, as long as the contributions of others are acknowledged. He was also considered for a Nobel Prize in chemistry and did not receive one; he died in 1907, and the reasons for his omission are debated.
Revisions: noble gases and atomic number
Later discoveries changed the table’s structure. In the 1890s, William Ramsay and others isolated argon and then helium, neon, krypton and xenon, a whole family of unreactive gases that had no place in the original table. They were added as a new group, which the periodic law absorbed without breaking.
The deeper reason for the pattern was uncovered in the early 1900s. A few pairs of elements, such as tellurium and iodine, seemed out of order by atomic weight. In 1913, Henry Moseley measured X-ray spectra of elements and showed that a more fundamental number, the nuclear charge or atomic number, increased in regular steps. Ordering by atomic number removed the anomalies. Moseley was killed at Gallipoli in 1915, and some have wondered what else he might have achieved.
Why the table still matters
The modern table organizes more than a hundred elements by electron structure, which explains why elements in one column behave alike. It guides work in many fields, from stainless steel and aluminum smelting to silicon chips and lithium-ion batteries. It is also regularly extended, since new elements made in laboratories and in nuclear reactors have been added at the end. The story of the table is a good example of a discovery shared among many people over several decades, rather than one flash of insight.