Francium (Fr)

Marguerite Perey joined Marie Curie's laboratory in 1929 as a nineteen-year-old technician with a technical diploma and no university degree. Her job was purifying actinium, one of the most tedious separations in radiochemistry, and she became very good at it — good enough that by 1939 she was doing the work independently and could tell when a sample was behaving oddly.

The oddity was a beta activity with a half-life of about twenty minutes that had no business being in a purified actinium fraction. Actinium-227 decays almost entirely by beta emission to thorium-227. But a little over one per cent of the time it takes the other route and emits an alpha particle instead, and the product of that minority branch is element 87. Perey had found the last element ever discovered in a natural sample.

Catium, and the name it did not get

She called the isotope actinium-K at first. When it came to naming the element she proposed catium, on the reasoning that it forms the most readily produced cation of any element. Irène Joliot-Curie objected that English speakers would think of cats, and Perey settled instead on francium, after her country. It became the second element named for France after gallium, and the last element to be named after a discovery in nature rather than in an accelerator. IUPAC accepted it in 1949; the symbol was briefly Fa before settling as Fr.

Perey went on to a doctorate in 1946 and a chair at Strasbourg, and in 1962 became the first woman elected a corresponding member of the French Académie des sciences — a partial membership that Marie Curie herself had been denied outright. She died in 1975 after a long illness attributed to a working lifetime among radioactive materials.

Russium, virginium, moldavium

Element 87 had been claimed at least three times before Perey, and each claimant named it after somewhere they were from.

• D. K. Dobroserdov reported it in potassium samples in 1925 and proposed russium. What he had measured was almost certainly the beta activity of potassium-40.
• Fred Allison announced it in 1930 as virginium, using the same magneto-optic apparatus behind his equally fictitious claim on element 85.
• Moldavium, put forward by Horia Hulubei and Yvette Cauchois in 1936, rested on X-ray lines that a later analysis assigned to something else entirely.

All three were searching for a long-lived element 87 in ordinary materials. The heaviest alkali metal has no long-lived isotope at all, so there was nothing to find in the places they were looking.

Twenty-two minutes

Francium-223 is the longest-lived isotope of the element, and it lasts 22 minutes. Everything else is shorter. There is no francium old enough to have survived from the formation of the solar system, so every francium atom on Earth right now was made within the last hour or two by the decay of actinium in uranium ore.

Because production and decay are in equilibrium, the total planetary inventory is fixed and tiny. The standard estimate puts the planet's whole holding at no more than about thirty grams, scattered as isolated atoms through billions of tonnes of rock. It has never been chemically isolated in a weighable amount, and the physics forbids it rather than the funding.

The melting point in reference tables — a little above room temperature — was never taken with an instrument. It is a curve drawn through lithium, sodium, potassium, rubidium and cesium and continued one square past the last real data point. Nobody has ever assembled enough francium in one place to melt, and nobody is going to.

Not the most reactive metal after all

The textbook expectation is straightforward. Reactivity increases down group 1 as the valence electron sits further from the nucleus and is more easily lost, so francium at the bottom should be the most violently reactive metal in existence.

It is not, and the reason is relativity. In an atom this heavy, the electron spends part of its time close enough to a nucleus of 87 protons to reach relativistic speeds. The resulting mass increase contracts the 7s orbital and pulls it closer in, binding that outer electron more tightly than the trend predicts. Francium's first ionisation energy comes out around 393 kJ/mol — 4.073 eV, the figure on the card above — measurably higher than cesium's 376.

So it is cesium, not francium, that gives up its outer electron most easily, and francium's chemistry — as far as it has been studied on the scale of individual atoms in solution — sits slightly back from the extreme its position promises.

The reference books have not followed. The Pauling electronegativity carried for francium almost everywhere, including in the card above, is 0.7 — below cesium's 0.79, and so the wrong way round. That figure is not a measurement of anything. Pauling assigned it in the 1930s by reading down the column, at a point when no property of the element had been determined, and no one has revisited it since; the ionisation energies, which are measured, point the other way. Both numbers appear on this site because both are what the literature contains, and the one that came out of an experiment is the one to believe. Francium remains extraordinarily reactive by any ordinary standard. It is simply not the record-holder its address suggests.

Why physicists want more of it

Francium's only genuine use is as a probe of the weak nuclear force, and the reason is the same heaviness that spoils its reactivity.

The weak interaction mixes atomic states of opposite parity by a tiny amount, producing effects that violate the mirror symmetry electromagnetism obeys. The size of that violation grows roughly as the cube of the atomic number. The benchmark measurement in atomic physics was made on cesium in the 1990s and remains one of the most precise low-energy tests of the Standard Model. Francium's effect is around eighteen times larger, which means a francium measurement of the same fractional precision would be a far sharper test — and could reveal the nuclear anapole moment, a property of the nucleus that arises from weak interactions between the protons and neutrons inside it.

Doing that requires holding francium atoms still. Groups at Stony Brook, and later the FrPNC collaboration at TRIUMF in Vancouver, have made francium by fusing an accelerated beam with a gold target and caught the atoms in a magneto-optical trap, holding on the order of a hundred thousand of them at a time in a suspended cloud a millimetre across. That is about as much francium as anyone has ever held in one place at once, and it weighs something like four hundredths of a femtogram — a cloud glowing faintly in a vacuum chamber, replenished continuously because it is decaying while you look at it.

A brief and abandoned medical idea

In the 1950s Perey and others investigated whether francium might concentrate in tumours and serve as a diagnostic marker. The work went nowhere, and the reason is arithmetic rather than biology: an element whose entire world supply is a few tens of grams spread through the crust, and which halves in quantity every twenty-two minutes, cannot be manufactured, shipped or administered. Even astatine, scarcer still, at least has a usable seven-hour isotope that a cyclotron can make to order in the morning and a hospital can use the same afternoon. Francium has nothing of the kind, and no prospect of acquiring one. Whatever it is ever used for will happen within a few metres of the accelerator that produced it.