Periodic Table Published

In 1869, the Russian chemist Dmitri Mendeleev published a table that arranged the known chemical elements by atomic weight and grouped them by recurring chemical properties. It was not the first attempt to find order among the elements, but it became the most influential, because it left deliberate gaps and used them to predict that new elements would be found. The periodic table remains the organizing framework of chemistry.

By the mid-nineteenth century, chemists had identified around sixty elements and had begun to agree on methods for measuring atomic weights, helped by the Karlsruhe Congress of 1860. With better data, patterns became harder to ignore. Johann Döbereiner had noted "triads" of similar elements decades earlier, and in the early 1860s Alexandre-Émile Béguyer de Chancourtois and John Newlands proposed arrangements in which properties seemed to repeat at intervals. Newlands's "law of octaves" was met with skepticism, partly because it forced elements into a rigid pattern.

Mendeleev, then a professor at St Petersburg, was working on a textbook, Principles of Chemistry, and sought a logical way to present the elements. He ordered them by increasing atomic weight and noticed that similar properties recurred periodically. His table was presented to the Russian Chemical Society in early 1869 and soon appeared in print. Crucially, he was willing to break the strict order when chemical behavior demanded it, and he left blank spaces where he believed undiscovered elements belonged. In a few cases he also suggested that accepted atomic weights were wrong. Around the same time, the German chemist Lothar Meyer was developing a similar scheme, and historians generally credit both men, though Mendeleev's bolder predictions made his version more famous.

Those predictions were tested within a couple of decades. Mendeleev described the expected properties of elements he called eka-aluminium, eka-boron and eka-silicon. Gallium, discovered in 1875, scandium, in 1879, and germanium, in 1886, closely matched his descriptions. These successes persuaded many skeptics that the table reflected something real about nature rather than a convenient filing system.

The table's deeper explanation came only later. Discoveries such as the noble gases in the 1890s were absorbed into it as a new column. In the early twentieth century, Henry Moseley's work on X-ray spectra showed that elements are better ordered by atomic number, the number of protons in the nucleus, which resolved the few places where atomic weight had led Mendeleev astray. Quantum theory then explained the periodicity through the arrangement of electrons.

Today the table has well over a hundred entries, including synthetic elements created in laboratories. Its basic logic, however, still traces back to 1869, when a gap in a chart became a scientific forecast.