There is about an ounce of francium in the Earth’s crust. It’s the most unstable of the first 101 elements. It lasts only 45 seconds longer than an episode of Archer. So how did anyone discover it?
Francium isn’t rare because it’s impossible to make. It occurs naturally wherever there are large stores of uranium. The reason why no one had any idea that francium existed is because it doesn’t stick around. Francium-223 is francium’s most stable isotope and it has a half-life of 22 minutes. As a result, there’s only about an ounce of natural francium on Earth at any given moment.
How does anyone discover something that just barely exists? In this case, it took a sharp-eyed 26-year-old physicist who decided to investigate an unusual phenomenon that she’d read about. Marguerite Perey was only 19 when she went to work as a chemistry technician at the Radium Institute as an assistant to Marie Curie. (In a Working Girl-like twist, Perey assumed that the soft-spoken older woman who interviewed her was a secretary, only gradually realizing over the course of the interview that the woman was Curie herself.) While at the institute, she studied actinium, an element that was discovered back in 1899 and was a product of uranium.
Actinium gave off radiation as beta particles (electrons or positrons), but some American scientists noted that it gave off more energy than they had expected. This piqued Perey’s interest, and in 1935 she started digging. It was a long, slow process, but Perey discovered that while mostly actinium gave off beta particles, about one percent of the time actinium-227 emitted alpha particles. Alpha particles are two protons and two neutrons—helium nuclei—and by leaving an atom, they change it into a new and different element. Perey learned that actinium-227, which had 89 protons, had suddenly become an element that had 87 protons. The fact that that element didn’t stick around long complicated her research, but in 1939 she announced that she had discovered element 87. Possibly taking her cue from Marie Curie, who had named polonium after her homeland, Perey called the element francium.
Perey, who had had to give up her dream of becoming a doctor when her family fell on hard times, was granted a doctorate and a research team of her own. By the time she was 50, however, she suffered one of the drawbacks of being a pioneer in early physics. Like many physicists of that era, she was exposed to massive amounts of radiation—so much that by the 1960s, she would set off radiation counters in labs. Instead of spending her last decade and a half studying the element she had discovered, she split much of her time between medical treatment and instructing her students on how to protect themselves from the radiation exposure she had suffered.