Discovered in 1939 at the Curie Institute by a former assistant of Marie Curie (1939)
Marguerite Perey, working on the decay of actinium-227
Science · Chemistry
Francium
1939
Chemistry — alkali metals
Francium is element 87, the heaviest alkali metal and the last element to be discovered in nature rather than made in a laboratory. Mendeleev had predicted it as "eka-caesium", and several chemists claimed it wrongly — as virginium, moldavium, alkalinium and russium — before Marguerite Perey, working at the Curie Institute in Paris in 1939, identified it in the decay chain of actinium-227 and named it after her country. Its longest-lived isotope, francium-223, has a half-life of only twenty-two minutes, so no visible quantity has ever existed; it is the second-rarest naturally occurring element, after astatine.
How to spot them
- ▸Element 87, the last element to be found in nature rather than synthesised
- ▸Named after a country by the woman who discovered it, in 1939
- ▸Its longest-lived isotope survives about twenty-two minutes
- ▸Mendeleev predicted it as 'eka-caesium'; it was once called actinium K
- ▸The heaviest alkali metal, never seen in visible quantity
Key facts
Element 87, named after the discoverer's home country
She first proposed 'catium', but it sounded too much like 'cat' in English
Mendeleev's predicted 'eka-caesium', and briefly called actinium K
Named for the natural decay chain in which its 223 isotope appears
The last element to be discovered in nature rather than made artificially
Everything found since — from neptunium and plutonium onwards — was synthesised first
Its most stable isotope has a half-life of just 22 minutes
It is the most unstable of the first hundred-odd elements
The heaviest of the alkali metals, and the second-rarest naturally occurring element
Only astatine is rarer; at most a trace exists in the Earth's crust at any moment
Key ideas
- It occurs naturally only as a fleeting branch of the actinium decay series, produced as fast as it decays away, so it can never accumulate
- Because no weighable sample exists, most of its chemistry is inferred from its place in Group 1 and measured on a few hundred thousand atoms held in magneto-optical traps
Quick-fire facts
- •Atomic number 87, symbol Fr; discovered by Marguerite Perey at the Curie Institute in Paris in January 1939
- •Perey had been Marie Curie's personal laboratory assistant from 1929; in 1962 she became the first woman elected to the French Academy of Sciences, as a corresponding member
- •Francium-223 has a half-life of 21.8 minutes; it arises as a minor alpha-decay branch of actinium-227
- •The last element to be discovered in nature; all elements found later were made in laboratories
- •It is the second-rarest naturally occurring element after astatine, and the most unstable of the first hundred elements
- •Earlier false claims to element 87 included virginium (Fred Allison, 1930), moldavium, alkalinium and russium
- •Mendeleev's periodic table predicted it as 'eka-caesium', the missing member below caesium in Group 1
- •The largest sample ever assembled was around 300,000 atoms, laser-trapped at Stony Brook in 1996; it has never been seen with the naked eye
- •It has no commercial use; trapped francium is studied for precision tests of the electroweak interaction
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Sources
- American Physical Society, APS News — The 83rd Anniversary of the Discovery of Francium
- Los Alamos National Laboratory — Francium — Periodic Table of Elements