Discovery of the periodic table

The periodic table is an arrangement of the chemical elements by increasing atomic number, laid out so that elements with similar chemical behavior line up in columns. The Russian chemist Dmitri Mendeleev published the first widely influential version in 1869, and he used the gaps in it to predict elements that nobody had yet found.

Background

Chemists spent decades looking for order among the elements before the table appeared. In 1789 Antoine Lavoisier published a list of 33 “simple substances,” which still included heat and light but treated an element as the end point of chemical decomposition. In 1805 John Dalton added relative atomic weights, with hydrogen set at one unit, which gave later chemists numbers they could compare.

In 1829 Wolfgang Döbereiner noticed triads, groups of three elements in which the middle one had a weight and a reactivity close to the average of the other two. Sodium, which falls between lithium and potassium, was his example. Between 1862 and 1867 several scientists built early tables, among them Alexandre-Émile Béguyer de Chancourtois, John Newlands, William Odling and Gustavus Heinrichs. None of these systems drew much attention when it appeared.

The discovery

Mendeleev was born in Siberia in 1834 and worked in St. Petersburg. He presented his periodic classification on 6 March 1869, arranging the known elements so that those with comparable properties sat in the same column. That date is given in a historical summary on the IUPAC website, and other accounts describe the first published table as dating from 1869 as well.

His table ordered the elements mainly by atomic weight, but it left deliberate blanks where no known element seemed to fit. He predicted that four missing elements would one day be found and gave them provisional names: eka-aluminum, eka-silicon, eka-boron and eka-manganese. The strength of the idea was that it made testable claims about substances no one had seen.

How it was confirmed

Within fifteen years, other chemists found the first three predicted elements and named them gallium, scandium and germanium. The match between prediction and discovery did much to establish Mendeleev’s reputation. Today gallium is a familiar entry in the table in its own right.

The fourth prediction took longer. Element 43 was produced artificially in a particle accelerator at the University of California, Berkeley, and sent to Palermo, Sicily, where Emilio Segrè and his coworkers identified it in 1937. It was later named technetium.

A physical explanation for the pattern arrived in the early twentieth century. The discoveries of X-rays, radioactivity and the electron between 1895 and 1897 showed that atoms have internal structure. In 1913 Niels Bohr explained why certain elements share a column: their atoms have similar arrangements of electrons in shells around the nucleus.

Between 1913 and 1914 Henry Moseley showed experimentally that elements are better ordered by an ordinal number, later called the atomic number, than by atomic weight. His method also revealed how many elements remained undiscovered between hydrogen, number 1, and uranium, number 92.

Structure of the modern table

The standard table has 118 elements arranged in seven periods, which are the horizontal rows, and 18 groups, which are the vertical columns. The periods correspond to the electron shells, labeled K through Q. Four blocks, called s, p, d and f, describe which kind of orbital is being filled across the table.

IUPAC defines the numbered groups 1 to 18 and the collective names used for them. A commonly used scheme recognizes ten families of elements, among them the alkali metals, the alkaline earth metals, the lanthanoids, the actinoids, the transition metals, the metalloids, the halogens and the noble gases. These family names are conventions rather than universal definitions, and sources differ slightly on where the lines fall.

Only two elements are liquid at room temperature: bromine, which melts at minus 7.2 degrees Celsius, and mercury, which melts at minus 38.8 degrees Celsius. Elements in the same column tend to share a pattern of chemical behavior, which is why a chemist can estimate the properties of an unfamiliar element from its neighbors. Oxygen, for example, heads a column of elements with related bonding habits.

Impact and legacy

The table is now the standard reference chart of chemistry. A 2019 history from the American Chemical Society described it as complete for the first time, with seven full periods and 118 elements. The most recently confirmed members are nihonium, moscovium, tennessine and oganesson.

The table has also been revised since 1869. Glenn Seaborg proposed placing the actinides as part of the f-block, a layout that similar proposals by Alfred Werner and Charles Janet had anticipated. Researchers are still trying to make elements 119 and 120, which would begin an eighth period.

Today the International Union of Pure and Applied Chemistry maintains the official table. It sets the criteria for accepting that a new element has been discovered, defines the temporary and final naming rules, assigns the numbered groups 1 to 18 and reviews standard atomic weights. Its most recent release, dated 4 May 2022, carries the atomic weights from the 2021 table. Each weight is an interval or a rounded value because the isotope mix of natural samples varies slightly. UNESCO proclaimed 2019 the International Year of the Periodic Table to mark 150 years since Mendeleev’s first version.

Questions remain about presentation. Chemists still debate which elements belong in group 3 and whether any layout is better than the conventional one. Some have suggested Janet’s left-step table as a candidate.

Disputes over credit

Mendeleev was not the only person to see the pattern. The science writer Philip Ball noted in Chemistry World that the German chemist Julius Lothar Meyer wrote down a very similar system a year earlier, with the familiar groupings such as the halogens and alkali metals. Meyer did not publish it, and it came to light only after his death in 1895.

Ball also raises the question of whether the table was a discovery or an invention. Most accounts credit Mendeleev because his 1869 table was published, extended and tested by predictions that came true.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 5, 2026Researched and drafted with AI assistanceReport an issue