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Francium sits at the bottom of group 1, under caesium. Chemists knew from Mendeleev's table that something belonged there, yet decades of claims came to nothing. The person who finally found it was not a famous professor but a young technician named Marguerite Perey. This post covers how she found element 87, the decay chain behind it with a worked example, and what her way of working can teach you.
Who was Marguerite Perey?
A 2022 history in the Bulletin for the History of Chemistry gives her life as 1909 to 1975. She was born in Villemomble, a small town east of Paris. In June 1929 she received a state diploma in chemistry, and Marie Curie then hired her to work at the Institut du Radium. Her job was to purify actinium, a rare radioactive element. By 1934 she had a sample of 5 mg of lanthanum oxide containing 0.053 mg of actinium-227.
How did she find element 87?
In 1938, the same history explains, Perey noticed a penetrating beta activity that grew over a period of about 21 minutes. It matched none of the known members of the actinium decay series. She repeated the work with different equipment and got the same result. When she separated all of actinium's known descendants, none of them showed this activity. The only explanation left was a new element made by actinium itself.
By the displacement rule, an alpha decay moves an element two places back in the periodic table. Actinium is element 89, so its alpha product had to be element 87. Perey called the new isotope actinium K. Her note, "Sur un élément 87, dérivé de l'actinium", was presented to the French Academy of Sciences on 9 January 1939 with her as the only author. The same year she and M. Lecoin reported its beta spectrum in Nature. She later found that about 1.2% of actinium atoms decay this way.
The Royal Society of Chemistry's francium page explains why the search had been so hard. Most actinium decays by beta emission, and only about 1% takes the alpha route to element 87. The element also vanishes quickly: the RSC lists a half-life of 22 minutes for francium-223.
Worked example: following Perey's decay chain
Step 1: the alpha branch. Actinium-227 has Z = 89 and A = 227. Alpha decay removes 2 protons and 4 nucleons, giving Z = 87 and A = 223. That is francium-223.
Step 2: what francium becomes. Francium-223 is a beta emitter. Beta decay raises Z by 1 and leaves A unchanged, giving Z = 88 and A = 223, an isotope of radium.
Step 3: how many francium atoms? Take a sample where 10,000 actinium atoms decay (sample numbers for practice). About 1.2% take the alpha route, so 10,000 × 0.012 = 120 francium atoms form.
Step 4: how fast they disappear. With a half-life of 22 minutes, 66 minutes is 66 ÷ 22 = 3 half-lives. The fraction left is (1/2)³ = 1/8, so 120 ÷ 8 = 15 atoms remain.
If half-life calculations still feel slow, our guide to radioactive decay and half-life has more practice.
What mistakes do students make with decay chains like this?
Moving the wrong way in the table. Alpha decay lowers Z by 2; beta-minus decay raises it by 1. Write both changes before naming the product.
Changing the mass number in beta decay. A stays the same, because a neutron turns into a proton.
Applying the branch percentage after the half-life. First find how many atoms form, then let them decay.
Why was her discovery doubted?
The 2022 history describes open resistance. Jean Perrin, a physics Nobel laureate, publicly downplayed her result and backed an earlier claim by his former students that element 87 was stable. Those stable-element claims were refuted in the 1940s. The RSC notes that she was accepted as the rightful discoverer after the Second World War.
She defended her doctoral thesis on element 87 on 21 March 1946. During the defence she suggested the name "catium", because the element should form a cation so readily. Irène Joliot-Curie felt it would suggest a cat, so Perey chose francium, after her country.
How does francium fit the group 1 trend?
The RSC page adds a twist. You might expect francium to lose its outer electron most easily of all group 1 metals. The measured first ionisation energy is 393 kJ mol⁻¹, about 17 kJ mol⁻¹ more than caesium's, because relativistic effects pull its electrons slightly closer to the heavy nucleus. So caesium, not francium, forms a cation most easily.
What happened to Perey later?
In 1949 she took the chair of nuclear chemistry at the University of Strasbourg. On 12 March 1962 the French Academy of Sciences elected her by 48 votes to 13, the first woman it had ever elected, though as a corresponding member rather than a full member. Years of radiation exposure damaged her health, and she died on 13 May 1975. The 2022 history records at least five Nobel nominations in chemistry.
What can a student take from how Perey worked?
Treat an odd result as data. The 21-minute activity looked like an error until she checked it.
Rule out the known causes first. Her proof came from testing every known decay product, not from one striking reading.
Repeat under new conditions. Getting the same result with different equipment made her case strong.
Keep going when people doubt you. She finished her studies while still working, and her thesis settled the question.
Some ideas in nuclear physics are easier to see than to read. The Fermi Simulation Lab has interactive simulations built for ideas like those.
Try it now: write the decay chain from actinium-227 to radium-223 from memory, then work out how many of 200 francium atoms remain after 44 minutes.
