History of the Periodic Table Timeline

Helium was first reported in 1868; the report was based on the new technique of spectroscopy, as some spectral lines in light emitted by the Sun did not match those of any of the known elements.

Image source: Helium

In 1894, British chemist William Ramsay and British physicist Lord Rayleigh isolated argon from air and determined that it was a new element, the first of the noble gases to be isolated on Earth.

Image source: Argon

In 1896, Ramsay tested a report of American chemist William Francis Hillebrand, who had found a stream of an unreactive gas from a sample of uraninite, leading Ramsay toward the discovery of helium on Earth.

Image source: William Ramsay

Using fractional distillation to separate the components of air, Ramsay discovered several more inert gases in 1898: metargon, krypton, neon, and xenon; spectroscopic analysis showed metargon was argon contaminated by a carbon-based impurity. Ramsay suggested a gas with atomic weight 20 between helium and argon, which proved to be neon.

In 1898, when only helium, argon, and krypton were definitively known, William Crookes suggested these elements be placed in a single column between the hydrogen group and the fluorine group, an early attempt to accommodate the inert gases in the periodic table.

Image source: William Crookes

In 1900, at the Prussian Academy of Sciences, Ramsay and Mendeleev discussed the new inert gases and their location in the periodic table; Ramsay proposed that these elements be put in a new group between halogens and alkali metals, to which Mendeleev agreed.

In 1902, Mendeleev wrote that the inert gases should be put in a new group 0, consistent with what Ramsay suggested to him and crediting Errera as the first to suggest the idea. Mendeleev added these elements to the table as group 0 without disturbing the basic concept of the periodic table.

In 1905, Swiss chemist Alfred Werner resolved the dead zone of Mendeleev's table. Although Mendeleev knew of lanthanum, cerium, and erbium, they were previously unaccounted for because their total number and exact order were not known, and Mendeleev could not fit them in his table by 1901.

Image source: Alfred Werner

In 1907, thorium and radiothorium, products of radioactive decay, were found to be physically different but chemically identical; this led Frederick Soddy to propose in 1910 that they were the same element with different atomic weights. In 1913, Soddy and Kazimierz Fajans published that many radioactive substances shared the same place in the periodic table.

Image source: Isotope

In 1869, Russian chemist Dmitri Mendeleev arranged 63 elements by increasing atomic weight in several columns, noting recurring chemical properties across them. He argued there were seven types of highest oxides, though his 1869 list misplaced seven then-known elements, including indium and thorium.

Image source: Dmitri Mendeleev

In a paper dated December 1869 which appeared early in 1870, Julius Lothar Meyer published a new periodic table of 55 elements, in which the series of periods are ended by an element of the alkaline earth metal group. Meyer had also revised his table in 1868, but this was published as a draft only after his death.

Image source: Lothar Meyer

In early 1870, Mendeleev decided that the atomic weights of indium, thorium, and the rare-earth metals must be wrong and that the rare-earth metals should be trivalent, increasing their predicted weights by half. He abandoned attempts to incorporate the rare-earth metals in late 1871.

Mendeleev continued to improve his ordering: in 1870, it gained a tabular shape with each column given its own highest oxide, and in 1871, he further developed it and formulated what he termed the 'law of periodicity'.

Image source: Periodic trends

In 1870, Mendeleev first tried to characterize the yet undiscovered elements, giving detailed predictions for three elements he termed eka-boron, eka-aluminium, and eka-silicium, and in 1871 he expanded his predictions further. His 1871 table left many more spaces for undiscovered elements.

Image source: Mendeleev's predicted elements

Mendeleev's eka-aluminium was discovered in 1875 and became known as gallium, providing a striking confirmation of his predictions and of the periodic law.

Image source: Gallium

Mendeleev's predicted eka-boron was discovered in 1879 and named scandium, further validating the predictive power of the periodic law.

Image source: Scandium

In 1881, Mendeleev and Meyer had an argument via an exchange of articles in the British journal Chemical News over priority of the periodic table. Mendeleev claimed priority, arguing that the creator of a scientific idea is the one who perceives its real aspect and illustrates it convincingly.

In 1882, the Royal Society in London awarded the Davy Medal to both Mendeleev and Meyer for their work to classify the elements; although two of Mendeleev's predicted elements had been discovered by then, his predictions were not mentioned in the prize rationale.

Image source: Davy Medal

Mendeleev's predicted eka-silicium was discovered in 1886 and named germanium, completing the confirmation of his three most famous predictions of new elements.

Image source: Germanium

In 1889, Mendeleev noted at the Faraday Lecture to the Royal Institution in London that he had not expected to live long enough 'to mention their discovery to the Chemical Society of Great Britain as a confirmation of the exactitude and generality of the periodic law'.

By 1890, Mendeleev's periodic table had been universally recognized as a piece of basic chemical knowledge, cementing its place at the foundation of the science.

Image source: History of the periodic table

Following Charles Glover Barkla's discovery of characteristic X-rays emitted from metals in 1906, Henry Moseley considered a possible correlation between X-ray emissions and physical properties of elements, providing experimental support for ordering elements by atomic number.

Image source: Henry Moseley

In 1913, amateur Dutch physicist Antonius van den Broek was the first to propose that the atomic number (nuclear charge) determined the placement of elements in the periodic table, building on the discovery of the atomic nucleus in the Geiger–Marsden experiments conducted between 1908 and 1913.

Image source: Antonius van den Broek

The Bohr model was developed beginning 1913, championing the idea of electron configurations that determine chemical properties. Bohr was influenced by Walther Kossel, who in 1916 was the first to establish an important connection between the quantum atom and the periodic table.

Image source: Bohr model

In 1914, Swedish physicist Johannes Rydberg noticed that the atomic numbers of the noble gases equal doubled sums of squares of simple numbers: 2, 10, 18, 36, 54, and 86, revealing a mathematical pattern in the structure of the periodic table.

Image source: Johannes Rydberg

Among the notable works that established the importance of the periodicity of eight was the valence bond theory, published in 1916 by American chemist Gilbert N. Lewis, followed by Irving Langmuir's octet theory of chemical bonding in 1919, in which Langmuir postulated 'cells' (atomic orbitals) arranged in 'equidistant layers' (electron shells).

Image source: Gilbert N. Lewis

In 1921, British chemist Charles Rugeley Bury made the next major step toward the modern theory by suggesting that eight and eighteen electrons in a shell form stable configurations. Bury is also credited with the first use of the term 'transition metal' in 1921.

Bohr's research led Austrian physicist Wolfgang Pauli to investigate the length of periods in the periodic table in 1924, work that contributed to the Pauli exclusion principle and the quantum mechanical explanation of the table's structure.

Image source: Wolfgang Pauli

Circa 1925, the periodic table changed by shifting some Reihen (series) to the right, into an extra set of columns (groups), reflecting the growing quantum mechanical understanding of electron shells and orbitals during the Old Quantum Theory period (1913 to 1925).

In 1940, neptunium and plutonium were the first transuranic elements to be discovered; they were placed in sequence beneath rhenium and osmium, respectively, opening the era of synthetic superheavy elements.

Image source: Neptunium

During his Manhattan Project research in 1943, American chemist Glenn T. Seaborg developed the actinide concept. In 1945, against the advice of colleagues, he proposed a significant change to Mendeleev's table: the actinide series, placing the heavy radioactive elements in a row below the lanthanides.

Image source: Glenn T. Seaborg

Following its acceptance, the actinide concept proved pivotal in the groundwork for discoveries of heavier elements, such as berkelium in 1949, demonstrating the power of the revised periodic table to guide new element discovery.

Image source: Berkelium

Burkhard Fricke in 1971 and Pekka Pyykkö in 2010 used computer modeling to calculate the positions of elements up to Z = 172, and found that the positions of several elements were different from those predicted by Seaborg, extending periodic table theory into the relativistic regime.

The discovery of tennessine in 2010 filled the last remaining gap in the seventh period of the periodic table, completing the current known structure of the table.