Only about 30 grams of francium exist on Earth at once

Only about 30 grams of francium, roughly one ounce, is estimated to exist in Earth’s entire crust at any moment, according to Los Alamos National Laboratory. All of it is francium-223, which appears fleetingly in uranium minerals as actinium decays and has a half-life of just 22 minutes, so nature destroys it about as fast as it makes it.

Is francium the rarest natural element?

No. Francium is often called that, but Los Alamos gives the title to astatine, element 85, with only about 25 grams on Earth at any given time. Both are trace by-products of radioactive decay that vanish almost as soon as they form.

A second assumption also fails. As the heaviest alkali metal, francium is often expected to be the one most eager to shed its outer electron. Measurements say otherwise: removing that electron takes 393 kJ/mol, about 17 kJ/mol more than for caesium, which keeps the crown.

Who discovered francium, and why that name?

Marguerite Perey, once Marie Curie’s lab assistant, identified element 87 at the Curie Institute in Paris in 1939 while purifying actinium-227, about 1% of which decays by a side route into francium. At her PhD exam she proposed “catium”, for its supposed ease in forming cations, until an examiner joked that English speakers might think it was named after a cat. She chose francium, after France. It was the last element discovered in nature rather than made in a lab, and in 1962 Perey became the first woman elected to the French Academy of Sciences.

No one has ever prepared a weighable amount of francium. Physicists study it a cloud at a time: Canada’s TRIUMF laboratory has laser-cooled and trapped up to 250,000 francium atoms to probe the weak nuclear force.