History of the Periodic Table: From Triads to a Complete Seventh Period

The periodic table on a classroom wall is not the invention of a single person. Chemists spent much of the nineteenth century hunting for order among the elements, twentieth-century physics explained why that order exists, and the last gaps in the seventh row were only filled in the twenty-first century. This guide follows the story through its key moments.

Background: more elements, better atomic weights

At the end of the eighteenth century Antoine Lavoisier set out the modern idea of an element as a substance that cannot be broken down further. In the early nineteenth century John Dalton's atomic theory spread, and chemists came to accept that each element has a characteristic atomic weight. New techniques, first electrolysis and later spectroscopy, added elements at a rapid pace. By the 1860s more than sixty were known, and the obvious question was how to organize them.

The international chemistry congress held in Karlsruhe in 1860 was an important step. It helped spread a consistent system of atomic weights based on Avogadro's hypothesis, and only then did lining the elements up by atomic weight become a meaningful exercise.

Döbereiner's triads (1817–1829)

From around 1817 the German chemist Johann Wolfgang Döbereiner noticed that when three chemically similar elements are grouped together, the atomic weight of the middle one is close to the average of the other two. By 1829 he had described several such groups, which he called triads.

• calcium, strontium, barium
• chlorine, bromine, iodine
• lithium, sodium, potassium

The average of the atomic weights of lithium and potassium, for instance, comes close to that of sodium. The triads did not cover every element, but they were the first clear sign that chemical properties and atomic weights are numerically linked. All three triads still sit together in the modern table, in groups 2, 17 and 1.

Newlands and the law of octaves (1864–1866)

Beginning in 1864 the English chemist John Newlands published a series of papers showing that if the known elements are arranged by atomic weight, similar properties tend to reappear at every eighth element. He compared this to the musical scale, where the eighth note repeats the first, and called it the law of octaves.

The pattern worked well for the lighter elements but broke down after calcium. Newlands also left no room for undiscovered elements and sometimes forced two elements into one position. When he presented the idea to the Chemical Society in London in 1866 the reception was cool; one member reportedly asked whether arranging the elements alphabetically might reveal a similar pattern. Recognition came much later, when the Royal Society awarded him the Davy Medal in 1887.

Mendeleev's periodic table (1869)

In March 1869 the Russian chemist Dmitri Mendeleev published a table that arranged the elements by atomic weight while placing chemically similar elements in the same family. At about the same time the German chemist Lothar Meyer produced a very similar table based on the periodicity of physical properties such as atomic volume. The Royal Society awarded both men the Davy Medal in 1882.

Two bold choices made Mendeleev's table stand out.

1. He left gaps. Where the properties did not fit, he refused to force the next known element into place and left an empty slot for one that had not yet been discovered.
2. He predicted what would fill them. Around 1870 he described several missing elements in detail, including their atomic weights, densities and the formulas of their oxides.

He named these unknown elements with the Sanskrit prefix "eka-", meaning "one", as in "one place beyond" a known element.

The gallium episode is the most dramatic. Lecoq de Boisbaudran's first measurement of gallium's density did not match Mendeleev's prediction of about 5.9 g/cm³. Mendeleev suggested he measure again, and once the sample was purified further the result came out close to the predicted value. Germanium also matched the forecast atomic weight, density and oxide properties well. Successes like these turned the table from a filing system into a statement about the laws of nature.

Mendeleev also reversed a few pairs whose atomic weights seemed to be in the wrong order. Tellurium is heavier than iodine, but iodine clearly belongs with the halogens, so he put tellurium first. He assumed the atomic weight measurements were wrong. The real explanation would come decades later.

A new family: the noble gases (1894–1898)

In 1894 Lord Rayleigh and William Ramsay isolated argon from air. It reacted with nothing, so it fitted none of the existing groups. Ramsay went on to identify helium on Earth, and in 1898 he and his colleagues found neon, krypton and xenon. The result was a whole new column, today's group 18. A discovery that looked as if it might break the table ended up reinforcing its regularity.

Moseley and the atomic number (1913)

The decisive change came with the discovery that atoms are built from a nucleus and electrons. In 1913 the young English physicist Henry Moseley measured the frequencies of the characteristic X-rays emitted by a series of elements when bombarded with electrons. He found that the square root of the frequency increased in equal steps from one element to the next, and showed that the step corresponds to the charge of the nucleus: the atomic number.

From then on the table was ordered by atomic number rather than atomic weight. Awkward pairs such as tellurium and iodine, argon and potassium, and cobalt and nickel fell naturally into place. Moseley's work also made missing atomic numbers unmistakable, marking gaps at 43, 61, 72, 75, 85 and 87 that were filled over the following decades. Moseley was killed at Gallipoli in 1915 during the First World War, at the age of twenty-seven.

Seaborg's actinide concept (1945)

In 1940 neptunium and plutonium were produced artificially, opening the era of elements beyond uranium. At the time thorium, protactinium and uranium were thought of as an extension of the d-block transition metals, and the new elements were expected to follow them. Attempts to find elements 95 and 96 on that basis kept failing.

In 1945 the American chemist Glenn Seaborg proposed that the elements from actinium onward form a separate series that fills the 5f orbitals, just as the lanthanides fill the 4f. This actinide concept went against the advice of some colleagues, but experiments designed around it led to the identification of americium and curium, and the second row beneath the table took its modern form. Seaborg shared the 1951 Nobel Prize in Chemistry, and element 106, seaborgium, became a rare example of an element named after a living person.

Completing the seventh period (2016)

From the late twentieth century, laboratories in the United States, Russia, Germany and Japan competed to make superheavy elements by colliding nuclei in particle accelerators. New elements were often confirmed from a handful of atoms, sometimes a single one, by tracking their decay chains.

At the end of 2015 a joint working party of IUPAC and IUPAP (the International Union of Pure and Applied Physics) recognized the discoveries of elements 113, 115, 117 and 118. Their names were finalized in November 2016.

• 113, nihonium (Nh): discovered in Japan, from the Japanese name for the country
• 115, moscovium (Mc): after the Moscow region
• 117, tennessine (Ts): after the US state of Tennessee
• 118, oganesson (Og): after the Russian physicist Yuri Oganessian

With that, all 32 places of period 7 were filled and the table reached its current form of 118 elements with no gaps. In 2019 the United Nations marked the 150th anniversary of Mendeleev's table with the International Year of the Periodic Table.

What comes next

Efforts to create elements 119 and 120 continue. Success would open an eighth period, and theory suggests a new block filling g orbitals could eventually appear. Just as Mendeleev's gaps pointed the way to new discoveries, today's table still carries its next open questions.