1839

The Last Element in Nature

Marguerite Perey (1909–1975)

Science may be an endless frontier, but some parts do reach their limits. Francium was the last element to be found in nature. After that, all the new ones were synthesized through nuclear reactions. French physicist Marguerite Perey, who isolated francium from actinium while she was working in Marie Curie’s lab, became the last human being who would isolate a new element from ore samples, as scientists had been doing for centuries. Thus, the classical age of element discovery came to an end.

It was an appropriate element to finish with, because it proved to be one of the most difficult. While Perey was purifying actinium from uranium ore in the lab, she noticed uncharacteristic radiation coming from the sample, and her further investigations led to the isolation of the new, highly radioactive element (Perey eventually died from cancer as a result of radiation exposure). In fact, it’s no exaggeration to say that francium barely exists at all. Every one of its isotopes is radioactive—to be more accurate, they’re all wildly radioactive. The longest-lived one has a half-life of twenty-two minutes, and the only reason it can be found at all is that it’s continually being produced from the radioactive decay of actinium-227. A sample of uranium or thorium ore has a few atoms of francium scattered throughout it, but good luck detecting them before they disappear.

Chemists had suspected since the late 1800s that there was a metal beyond cesium, and the periodic table indicated that this must indeed be the case. (Before Perey’s discovery, there had been false alarms for years.) The alkali metal column gets more violently reactive as you go down it, and there is every reason to think that the chemistry of francium is similarly dramatic. But no one knows. Producing and gathering enough francium atoms for a reaction takes serious equipment, and there’s a limit to what any equipment can do. Francium’s radioactivity is so fierce that it’s probably impossible to produce even a speck large enough to see with the naked eye. It would blow itself into vapor from its own heat of radioactive decay.

Francium’s electronic configuration. The single electron in the outermost shell would make it very chemically reactive—if its nucleus would let it last long enough for anyone to notice.