The History of the Periodic Table: From Triads to Element 118

How the periodic table was built, from Döbereiner's triads and Newlands' octaves to Mendeleev's gaps, Moseley's atomic number and the naming of element 118.

The periodic table on your classroom wall looks permanent and inevitable. It was not. It was assembled over about a century and a half by chemists who disagreed, guessed wrong, and occasionally had to rearrange the whole thing when new evidence arrived. The story of how it came together is also the story of how we learned what an atom actually is.

Before the table: sorting the elements

Ancient people knew gold, silver, copper, iron, tin, lead, mercury, sulfur and carbon without knowing they were elements. The modern idea of an element as a substance that cannot be broken down into anything simpler was set out clearly by Antoine Lavoisier in France in 1789, when he published a list of thirty-three substances he believed to be elementary.

In the early 1800s John Dalton in Manchester proposed that each element is made of its own kind of atom with a characteristic weight. That gave chemists a number to attach to each element, the relative atomic weight, and numbers can be put in order. As the list of known elements grew past sixty, chemists began to notice that certain elements behaved suspiciously like one another.

Triads and octaves

The first serious attempt to explain the resemblances came from the German chemist Johann Döbereiner. In 1829 he pointed out that several elements fell into groups of three with a striking pattern: the middle one had properties, and an atomic weight, roughly halfway between the other two. Calcium, strontium and barium formed one such triad, chlorine, bromine and iodine another, lithium, sodium and potassium a third. Döbereiner's triads were real, and we still recognise every one of them as neighbours in a group. But he could not extend the idea to the elements that did not fit, and most did not.

The English chemist John Newlands did better, in a sense, and was treated worse. In 1865 he arranged the elements by atomic weight and noticed that properties repeated every eighth element, much as notes repeat every octave on a piano. He called it the law of octaves. He was right about the repetition for the lighter elements, but his scheme forced some very unlike elements into the same box further down the list, and when he presented it to the Chemical Society of London he was mocked and the paper was not published.

Around the same time the German chemist Lothar Meyer was building tables of his own, and his full version appeared in 1870, a year after Mendeleev's. The two men later shared a Royal Society medal for the discovery. But it is Mendeleev's name that stuck, for a specific reason.

Mendeleev and the courage to leave gaps

Dmitri Mendeleev was a professor in St Petersburg writing a chemistry textbook. In early 1869, trying to find a sensible order in which to present the elements, he wrote each one on a card with its atomic weight and properties and shuffled the cards until a pattern appeared. The table he presented to the Russian Chemical Society that year listed the elements in order of increasing atomic weight, arranged so that elements with similar properties fell into the same column.

Two things set Mendeleev apart from everyone who had tried before.

First, when the atomic weight order and the chemical properties disagreed, he trusted the chemistry. He placed tellurium before iodine even though tellurium was heavier, because iodine plainly belonged with the halogens. He also declared several accepted atomic weights simply wrong, and later measurements proved him right about beryllium among others.

Second, and this was the genius move, when no known element fitted a position, he left it empty. Where Newlands had squeezed elements together, Mendeleev declared that the blanks were undiscovered elements and predicted their properties from the neighbours above, below and beside them. He gave three of them provisional names: eka-boron, eka-aluminium and eka-silicon, using the Sanskrit word for "one" to mean "one place below".

The predictions were tested within his lifetime.

• Gallium (eka-aluminium) was discovered in 1875 by the French chemist Paul-Émile Lecoq de Boisbaudran. When its measured density initially came out lower than Mendeleev's forecast, he suggested the sample was impure. A purer sample gave the predicted value.
• Scandium (eka-boron) was discovered in 1879 by Lars Fredrik Nilson in Sweden.
• Germanium (eka-silicon) was discovered in 1886 by Clemens Winkler in Germany. Its atomic weight, density and the properties of its oxide and chloride all fell close to Mendeleev's numbers.

Three correct predictions of elements nobody had seen turned the periodic table from a tidy summary into a scientific law, and chemists treated it as one from then on.

Mendeleev's table still had problems. It had no room for the noble gases, because none had been discovered; when William Ramsay and his colleagues found argon, helium, neon, krypton and xenon in the 1890s, an entire new column had to be added. The rare earth elements, today's lanthanides, refused to fit neatly anywhere. And there were those awkward pairs like tellurium and iodine where the atomic weight order was simply wrong. Something other than weight was doing the ordering, and nobody yet knew what.

Moseley and the atomic number

The answer came from physics. In 1913 a young English physicist, Henry Moseley, working first in Manchester and then Oxford, fired electrons at metal targets and measured the X-rays they gave off. He found that the frequency of the X-rays increased in perfectly regular steps from one element to the next, and that the steps followed the position in the periodic table, not the atomic weight. The quantity that increased by exactly one each time was the positive charge on the nucleus, which we now call the atomic number and know to be the number of protons.

Moseley's work did three things at once. It explained why tellurium comes before iodine: tellurium has 52 protons and iodine 53, whatever their weights. It showed precisely how many elements were still missing between hydrogen and uranium, because any skipped number was a gap. And it gave chemists a way to check whether a claimed new element was genuine. Moseley was killed at Gallipoli in 1915, aged 27.

Seaborg rearranges the bottom of the table

During the Second World War, the American chemist Glenn Seaborg and his colleagues were making new elements heavier than uranium, including plutonium, and finding that their chemistry did not behave as expected. The table at the time placed thorium, protactinium and uranium beneath the transition metals hafnium, tantalum and tungsten, and predicted that the next elements would behave like rhenium and osmium. They did not.

Seaborg's insight, proposed in 1944 and published soon after the war, was that the elements from actinium onwards were filling an inner f sub-shell, just as the lanthanides do a row above. They should therefore be pulled out of the main body and set beneath the lanthanides as a second detached row, the actinides. Senior colleagues warned him it would ruin his reputation. Instead it correctly predicted the chemistry of every heavier element made since. Seaborg went on to share a Nobel Prize, and element 106, seaborgium, was named after him in 1997 while he was still alive.

The modern table and the last four names

Today the table is looked after by the International Union of Pure and Applied Chemistry, IUPAC, which decides when a new element has been convincingly discovered and what it should be called. The groups are now numbered 1 to 18 in a single system, replacing the older Roman numerals that caused confusion between American and European textbooks.

Every element beyond uranium has been made in a laboratory by smashing lighter nuclei together, and the heaviest exist for only fractions of a second before decaying. In December 2015 IUPAC announced that the discoveries of elements 113, 115, 117 and 118 had been verified, completing the seventh period. The names were formally approved in November 2016.

• Nihonium (Nh, 113), from Nihon, a Japanese name for Japan, honouring the team at the RIKEN institute.
• Moscovium (Mc, 115), after the Moscow region, home of the Joint Institute for Nuclear Research at Dubna.
• Tennessine (Ts, 117), after the state of Tennessee, recognising Oak Ridge National Laboratory and its university partners. The -ine ending places it with the halogens.
• Oganesson (Og, 118), after the Russian physicist Yuri Oganessian, who led the Dubna heavy-element programme. It is only the second element named after a living person, seaborgium being the first, and the -on ending marks it as a noble gas, at least on paper.

Whether the table is finished is an open question. Several laboratories are attempting to make elements 119 and 120, which would begin an eighth period, and theorists disagree about how far it can go before nuclei become too unstable to exist at all.

Try it on Periodic Bubbles: pop gallium and germanium in the bubbles and compare them with aluminium and silicon directly above. You are looking at the elements that made Mendeleev's reputation.

Key takeaways

• Döbereiner's triads (1829) and Newlands' octaves (1865) spotted real patterns but could not cover all the elements.
• Mendeleev's 1869 table succeeded because he trusted chemical properties over atomic weight and left gaps for undiscovered elements.
• Gallium, scandium and germanium were found within seventeen years, matching his predictions and making the table a predictive law.
• Moseley showed in 1913 that atomic number, not weight, is the true ordering principle.
• Seaborg's actinide concept in the 1940s created the two detached rows at the bottom of the modern table.
• The seventh period was completed with the naming of nihonium, moscovium, tennessine and oganesson in 2016.