Periodic Table

How the periodic table survived a war to secure chemistry’s future

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A century ago, the discovery of hafnium confirmed the validity of the periodic table — but only thanks to scientists who stood up for evidence at a time of global turmoil.

Close-up of Hafnium on the periodic table of elements
Hafnium is a transition metal named after the Latin name for Copenhagen (Hafnia), where the element was discovered.

Hafnium isn’t a particularly remarkable element. It’s not your explosive sodium, shimmering mercury or stinky sulfur. It’s a greyish metal and is commonly used as a neutron absorber in the control rods of nuclear power plants and nuclear submarines, and as an insulator in computer chips. But hafnium’s discovery, which was reported in Nature a century ago this week1, was of disproportionate importance. The element was identified by two scientists working in Copenhagen: Dutch physicist Dirk Coster and Hungarian chemist Georg von Hevesy. The find secured not only the periodic table’s legacy but also the future of chemistry. Hafnium also came to represent a hard-won victory against those determined to undermine evidence-based discovery.

Dmitri Mendeleev’s periodic table of elements, created in 1869, emerged from the realization that chemical elements such as oxygen and hydrogen share certain relationships. Mendeleev’s contribution, and that of the German chemist Julius Lothar Meyer, working independently, provided an order for the elements, along with criteria for classifying them into neat groups. Remarkably, both Mendeleev’s and Lothar Meyer’s schemes were based on elements’ subatomic structure — several decades before the discovery of electrons and protons.

When Mendeleev devised the periodic table’s rough form, he started with 63 known elements. To make the table work, he had to leave gaps where as-yet undiscovered elements might be placed. These elements soon began to turn up. For example, the predicted ‘element 68’, gallium, was identified a few years later, in 1875. By 1914, just seven gaps remained.

A breakthrough occurred in 1913, when Henry Moseley, a British physicist, showed that elements could be arranged by their atomic number, or their number of protons. Moseley’s work provided both a more accurate ‘gap map’ and a method for identifying elements from the spectra produced by exposing candidate elements to X-rays.The battle behind the periodic table’s latest additions

But the discovery (and naming) of element 72, hafnium, was anything but straightforward. The French chemist Georges Urbain originally proposed, in 1911, that element 72 belonged among the periodic table’s rare earth elements, and named it celtium. But around a decade later, the Danish physicist Niels Bohr — who used quantum theory to develop a model of the atom in which electrons orbit the nucleus — predicted that element 72 would be among the transition metals, and closer to zirconium (element 40). This was finally confirmed by Coster and von Hevesy — both working at Bohr’s lab in Copenhagen — who searched zirconium minerals for the element2. The duo named their discovery hafnium, after the Latin name for Copenhagen. They obtained the X-ray spectra in December 1922 and their paper1 followed in January 1923.

But this was far from the end of the controversy, because Urbain stubbornly refused to give up, even though he had already had warning that the material he called celtium did not conform to the criteria for element 72. In 1914, Moseley and Urbain had collaborated on an unpublished X-ray study that failed to show that celtium was element 72. Urbain explained this away, saying that the X-ray method was simply not sensitive enough3 — an assessment that the New Zealand-born physicist Ernest Rutherford, writing in Nature, agreed with4. Urbain also suggested that the Copenhagen team were trying to take credit for his work5; in their response, Coster and von Hevesy refused to personalize the dispute and argued on the basis of their results6.

Debate continued, with scientists from the Netherlands, Germany and Scandinavia on the side of the Copenhagen team, while those from France and the United Kingdom (who were backing a boycott of German science in the wake of the First World War) took the counter position. Hafnium was accepted by the International Union of Pure and Applied Chemistry only in 1930, a few years after the boycott formally ended. In France, celtium continued to occupy the place of element 72 among the rare earth elements until the early 1940s2.

One hundred years on from hafnium’s discovery, the periodic table remains both robust and relevant, even at a time when reams of data on an element can be accessed at the click of a mouse. The table offers, at a glance, a reference to how an element might behave in a chemical reaction, and clues to its similarity to other elements in a group.

So far, there are 118 confirmed elements, with the addition of four superheavy synthetic elements in 2015. But a time will surely come when there will be no new elements to make and the periodic table will reach its limits. As yet there is no consensus on when this is likely to happen or how big the last element will be, but chemists say its atomic number could exceed 170.

When that time comes, the periodic table will still remain, a map guiding scientists through the vastness of chemical space — all the molecules that have ever formed, and all those yet to be discovered, whether on Earth or elsewhere in the Universe. It’s a tribute to the enduring values of international scientific cooperation and the steadfastness of researchers that an unremarkable transition metal, discovered 100 years ago in the aftermath of one of the world’s greatest conflicts, made the periodic table what it is today.

Nature

Four new elements officially added to periodic table

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All were named for a scientist, region or country important to the field. Video provided by NewsyNewslook

The options could have included mythological characters or minerals, but the International Union of Pure and Applied Chemistry settled on names based on the locations where elements were discovered or researchers who helped find them.

The elements are:

• 113 Nihonium (Nh), named by the RIKEN Nishina Center for Accelerator-Based Science in Japan. Nihon means Japan or literally “the Land of Rising Sun,” in Japanese. The element is the first discovered in and named after an Asian country.

• 115 Moscovium (Mc), chosen to honor the Russian region and the Joint Institute for Nuclear Research, where experiments were conducted.

• 117 Tennessine (Ts), which recognized the state for contributions from Oak Ridge National Laboratory, Vanderbilt University and University of Tennessee at Knoxville.

• 118 Oganesson (Og), which honored Yuri Oganessian for pioneering research on super-heavy elements.

“The names of the new elements reflect the realities of our present time,” said Natalia Tarasova, president of the scientific group, listing places from three continents and the pivotal role of researchers such as Oganessian.

The endings – “ium” and “ine” and “on” – relate to where the elements are seated around the table.

The scientific group proposed the names in May and sought public comment for five months. The names were finalized Wednesday. No other requests for names are pending, so the period table is complete – for now.

“Overall, it was a real pleasure to realize that so many people are interested in the naming of the new elements, including high-school students, making essays about possible names and telling how proud they were to have been able to participate in the discussions,” said Jan Reedijk, president of the group’s Inorganic Chemistry Division. “For now, we can all cherish our periodic table completed down to the seventh row.”

Four elements earn permanent seats on the periodic table

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The seventh row of the periodic table is officially full.

periodic table

On December 30, the International Union of Pure and Applied Chemistry announced that a Russian-U.S. collaboration had attained sufficient evidence to claim the discovery of elements 115, 117 and 118. IUPAC awarded credit for the discovery of element 113 to scientists at RIKEN in Wako, Japan (SN Online: 9/27/12). Both groups synthesized the elements by slamming lighter nuclei into each other and tracking the decay of the radioactive superheavy elements that followed.

Researchers at the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California, which are among the institutions credited with elements 115, 117 and 118, had also laid claim to element 113 after experiments in 2004 (SN: 2/7/04, p. 84) and 2007. But garnering recognition for the three other elements softened the blow, says Dawn Shaughnessy, who leads the experimental nuclear and radiochemistry group at Livermore. “I’m personally very happy with IUPAC’s decision,” she says.

Published reports on the newly recognized elements will appear in early 2016, says IUPAC executive director Lynn Soby. Official recognition of the elements means that their discoverers earn the right to suggest names and symbols. Element 113 will be the first element discovered and named by researchers in Asia.

Why do we value gold?

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Mankind’s attitude to gold is bizarre. Chemically, it is uninteresting – it barely reacts with any other element. Yet, of all the 118 elements in the periodic table, gold is the one we humans have always tended to choose to use as currency. Why?

Why not osmium or chromium, or helium, say – or maybe seaborgium?

I’m not the first to ask the question, but I like to think I’m asking it in one of the most compelling locations possible – the extraordinary exhibition of pre-Columbian gold artefacts at the British Museum?

That’s where I meet Andrea Sella, a professor of chemistry at University College London, beside an exquisite breastplate of pure beaten gold.

He pulls out a copy of the periodic table.

Periodic table

“Some elements are pretty easy to dismiss,” he tells me, gesturing to the right-hand side of the table.

“Here you’ve got the noble gases and the halogens. A gas is never going to be much good as a currency. It isn’t really going to be practical to carry around little phials of gas is it?

Gold – key facts

Gold - symbol, atomic number and weight
  • Symbol: Au (from Latin aurum)
  • Atomic number: 79
  • Weight: 196.97
  • One of the “noble” metals that do not oxidise under ordinary conditions
  • Used in jewellery, electronics, aerospace and medicine
  • Most gold in the earth’s crust is thought to derive from meteors
  • Biggest producers: China, Australia, US, Russia
  • “And then there’s the fact that they are colourless. How on earth would you know what it is?”

The two liquid elements (at everyday temperature and pressure) – mercury and bromine – would be impractical too. Both are also poisonous – not a good quality in something you plan to use as money. Similarly, we can cross out arsenic and several others.

Sella now turns his attention to the left-hand side of the table.

“We can rule out most of the elements here as well,” he says confidently.

“The alkaline metals and earths are just too reactive. Many people will remember from school dropping sodium or potassium into a dish of water. It fizzes around and goes pop – an explosive currency just isn’t a good idea.”

A similar argument applies to another whole class of elements, the radioactive ones: you don’t want your cash to give you cancer.

Out go thorium, uranium and plutonium, along with a whole bestiary of synthetically-created elements – rutherfordium, seaborgium, ununpentium, einsteinium – which only ever exist momentarily as part of a lab experiment, before radioactively decomposing.

Then there’s the group called “rare earths”, most of which are actually less rare than gold.

Unfortunately, they are chemically hard to distinguish from each other, so you would never know what you had in your pocket.

This leaves us with the middle area of the periodic table, the “transition” and “post-transition” metals.

Elementary Business

Plant, balloons and aluminium can

This group of 49 elements includes some familiar names – iron, aluminium, copper, lead, silver.

But examine them in detail and you realise almost all have serious drawbacks.

We’ve got some very tough and durable elements on the left-hand side – titanium and zirconium, for example.

The problem is they are very hard to smelt. You need to get your furnace up into the region of 1,000C before you can begin to extract these metals from their ores. That kind of specialist equipment wasn’t available to ancient man.

Aluminium is also hard to extract, and it’s just too flimsy for coinage. Most of the others in the group aren’t stable – they corrode if exposed to water or oxidise in the air.

Take iron. In theory it looks quite a good prospect for currency. It is attractive and polishes up to a lovely sheen. The problem is rust: unless you keep it completely dry it is liable to corrode away.

“A self-debasing currency is clearly not a good idea,” says Sella.

We can rule out lead and copper on the same basis. Both are liable to corrosion. Societies have made both into money but the currencies did not last, literally.

So, what’s left?

Why is gold golden?

2,000-year-old golden funerary mask from Colombia

Gold’s golden colour has been a mystery until very recently, says Andrea Sella.

The secret lies in its atomic structure. “Quantum mechanics alone doesn’t explain it,” he says.

“When you get to gold you find the atom is so heavy and the electrons move so fast that you now have to include Einstein’s theory of relativity into the mathematics.

“It is only when you fold together quantum mechanics with relativity that suddenly you understand it.”

Unlike other metals, which in their pure form reflect light straight back, electrons in the gold “slosh around a little,” Sella says, with the result that gold “absorbs a bit of the blue spectrum light, giving the light that is reflected back its distinctive golden colour”.

Of the 118 elements we are now down to just eight contenders: platinum, palladium, rhodium, iridium, osmium and ruthenium, along with the old familiars, gold and silver.

These are known as the noble metals, “noble” because they stand apart, barely reacting with the other elements.

They are also all pretty rare, another important criterion for a currency.

Even if iron didn’t rust, it wouldn’t make a good basis for money because there’s just too much of it around. You would end up having to carry some very big coins about.

With all the noble metals except silver and gold, you have the opposite problem. They are so rare that you would have to cast some very tiny coins, which you might easily lose.

They are also very hard to extract. The melting point of platinum is 1,768C.

That leaves just two elements – silver and gold.

Both are scarce but not impossibly rare. Both also have a relatively low melting point, and are therefore easy to turn into coins, ingots or jewellery.

Silver tarnishes – it reacts with minute amounts of sulphur in the air. That’s why we place particular value on gold.

It turns out then, that the reason gold is precious is precisely that it is so chemically uninteresting.

Gold’s relative inertness means you can create an elaborate golden jaguar and be confident that 1,000 years later it can be found in a museum display case in central London, still in pristine condition.

So what does this process of elemental elimination tell us about what makes a good currency?

First off, it doesn’t have to have any intrinsic value. A currency only has value because we, as a society, decide that it does.

“Start Quote

That’s the other secret of gold’s success as a currency – gold is unbelievably beautiful”

Andrea Sella

As we’ve seen, it also needs to be stable, portable and non-toxic. And it needs to be fairly rare – you might be surprised just how little gold there is in the world.

If you were to collect together every earring, every gold sovereign, the tiny traces gold in every computer chip, every pre-Columbian statuette, every wedding ring and melt it down, it’s guesstimated that you’d be left with just one 20-metre cube, or thereabouts.

But scarcity and stability aren’t the whole story. Gold has one other quality that makes it the stand-out contender for currency in the periodic table. Gold is… golden.

All the other metals in the periodic table are silvery-coloured except for copper – and as we’ve already seen, copper corrodes, turning green when exposed to moist air. That makes gold very distinctive.

“That’s the other secret of gold’s success as a currency,” says Sella. “Gold is unbelievably beautiful.”

But how come no-one actually uses gold as a currency any more?

Chart showing gold price adjusted for inflation

The seminal moment came in 1973, when Richard Nixon decided to sever the US dollar’s tie to gold.

Since then, every major currency has been backed by no more than legal “fiat” – the law of the land says you must accept it as payment.

Nixon made his decision for the simple reason that the US was running out of the necessary gold to back all the dollars it had printed.

Find out more

In Elementary Business, BBC World Service’s Business Daily goes back to basics and examines key chemical elements – and asks what they mean for businesses and the global economy.

  • And here lies the problem with gold. Its supply bears no relation to the needs of the economy. The supply of gold depends on what can be mined.

In the 16th Century, the discovery of South America and its vast gold deposits led to an enormous fall in the value of gold – and therefore an enormous increase in the price of everything else.

Since then, the problem has typically been the opposite – the supply of gold has been too rigid. For example, many countries escaped the Great Depression in the 1930s by unhitching their currencies from the Gold Standard. Doing so freed them up to print more money and reflate their economies.

The demand for gold can vary wildly – and with a fixed supply, that can lead to equally wild swings in its price.

Most recently for example, the price has gone from $260 per troy ounce in 2001, to peak at $1,921.15 in September 2011, before falling back to $1,230 currently.

That is hardly the behaviour of a stable store of value.

So, to paraphrase Churchill, out of all the elements, gold makes the worst possible currency.

Apart from all the others.

Study says new element 115, ununpentium, does exist.

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One more element may soon be added to the Periodic Table. On September 10, 2013, an international team of scientists working at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany reported that they have acquired new evidence supporting the existence of element 115. The new evidence will be reviewed by the International Union of Pure and Applied Chemists (IUPAC), and if confirmed, element 115 will likely be given a new name and added to the Periodic Table of Elements. Its temporary name, which is being used as a placeholder, is ununpentium.

Element 115 is one of a number of superheavy elements—elements with an atomic number greater than 104—that are so short-lived, they can’t be detected in nature. Scientists can, however, synthesize these elements in a laboratory by smashing atoms together.

element115-particlecollision-580

In 2004, scientists from the United States and Russia first reported the discovery of element 115. Unfortunately, the evidence from that research and a few more studies that followed was not enough to confirm the existence of a new element.

Now, scientists are developing new techniques to detect the presence of superheavy elements. In an experiment conducted at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany, scientists successfully bombarded a thin layer of americium (atomic number 95) with calcium (atomic number 20) to produce ununpentium (atomic number 115). Ununpentium was observed with a new type of detector system that measured the photons that were released from the reaction. The unique photon energy profile for ununpentium can be thought of as the element’s fingerprint, the scientists say.

ununpentium-wikimediacommons-580

Dirk Rudolph, lead author of the new study and Professor at the Division of Nuclear Physics at Lund University in Sweden, commented on the findings in a press release. He said:

This can be regarded as one of the most important experiments in the field in recent years, because at last it is clear that even the heaviest elements’ fingerprints can be taken. The result gives high confidence to previous reports. It also lays the basis for future measurements of this kind.

Presently, there are 114 elements in the Periodic Table of Elements. Two new elements, flerovium (atomic number 114) and livermorium (atomic number 116), were added to the Periodic Table in 2012. While elements 113 and 118 are also thought to exist, their presence has not yet been confirmed.

The next step for element 115 will be for the IUPAC to review all of the evidence to date and make a decision as to whether more experiments are needed or if the current evidence is sufficient to support the discovery of a new element. If the latter occurs, the scientists who first discovered element 115 will be asked to formally submit a new name for the element. Then, the new name will be released for scientific review and public comment. If approved, the element along with its new name will be added to the Periodic Table of Elements. Element 115 is currently called ununpentium, which is just a placeholder until its formal name is established.

The research was supported by ENSAR (European Nuclear Science and Applications Research), the Royal Physiographic Society in Lund, the Swedish Research Council, the German Federal Ministry of Education and Research, the US Department of Energy and the UK Science and Technology Facilities Council.

Bottom line: On September 10, 2013, an international team of scientists working at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany reported that they have acquired new evidence that supports the existence of element 115 (ununpentium). The research was published on September 10, 2013 in the journal Physical Review Letters. After the IUPAC reviews and confirms the evidence, element 115 will likely be given a new name and added to the Periodic Table of Elements.