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  • Francium is the heaviest of the alkali metals.

  • At least until element 119 is synthesized.

  • It also has the biggest atom of any element

  • in the whole periodic table. Now some of you may have seen on YouTube,

  • there are videos that pretend to be huge explosions of francium

  • And I'm never quite sure whether I should be really pleased about this,

  • because it shows that people are using the periodic table in the right way,

  • and deducing that francium should be really very reactive

  • Or whether I should be tearing out my hair.

  • Because the videos are fake, and they are chemically wrong

  • But most of all they miss the really exciting part about francium.

  • Which is the really moving story about Marguerite Perey

  • the woman who discovered francium at the beginning of the 20th century

  • it was obvious that they ought to be an element at the bottom of the

  • Alkali metals and so lots of people were looking for it the American professor Fred Allison

  • in Alabama, the one who thought he'd discovered element 85,

  • also thought that he discovered element

  • 87 but it turned out that his

  • Method that he was using which is quite complicated, so I won't explain it now was fundamentally flawed

  • It was wrong, and he wasn't measuring anything

  • Except something psychological in his mind. It was realized fairly early on

  • That element 87 could be made from element 89 actinium

  • by emission of an alpha particle an alpha particle is two protons and two neutrons,

  • so if you take two away from 89 you get 87

  • and the problem was that actinium

  • is quite a rare element and was very difficult to get hold of a sample it had been discovered by Andre Debierne

  • Around about 1900 he was working in the same Institute as Marie Curie

  • and in 1929 a young woman

  • Marguerite Perey went to the lab and

  • Started as a lab assistant with Marie Curie, and she was put to purifying

  • actinium. You would begin with literally tons of impure ore.

  • Probably of uranium, and then you would gradually

  • Boil it up dissolve it

  • Extract material until you ended up with just a few milligrams of the material you wanted.

  • The other problem was that nobody realized how dangerous radioactivity was.

  • So they were all working in the lab with very few precautions the upshot of this was that

  • Margaret Perey became a real expert in handling actinium

  • and separating it four years after she started Madame Curie died. She started working simultaneously

  • for Andre Debierne, the professor who had discovered

  • actinium and also Marie Curie's daughter

  • Irène Curie Joliot you for some completely unknown reason these two people didn't realize they were both

  • directing the work of the same person the aim was to get the purest possible sample of actinium

  • and then measured the radioactivity coming from it the way the actinium decays

  • is entirely due to the physics of the nucleus the reason for purifying it is to get rid of all the other radioactive

  • Impurities which might send out signals that would completely mask the effect you were looking for so what

  • Marguerite Perey

  • observed was when she took a really pure

  • solution of actinium and measured the radioactivity very rapidly for the first two hours the

  • radioactivity of the solution increased and then it came approximately constant, and so she deduced that what was happening

  • was that the

  • Actinium was decaying to form a new element

  • which itself was decaying and giving out much stronger radiation the reason you saw this buildup was because

  • material went into solution

  • And you got to a stage where it was being formed at the same rate as at which it was decaying

  • She also discovered that this radioactivity could be precipitated by

  • adding caesium salt and then precipitating the caesium salt as a

  • perchlorate caesium and

  • Element 87 should have similar chemical properties

  • So you expect them to precipitate together whence it precipitated you could show that the precipitate was radioactive

  • Rather than the solution and the half-life was about 21 minutes

  • So you had to work really fast

  • The upshot was that she had discovered element 87

  • In 1939 she published her paper on element 87 derived from Actinium

  • Now there was a really exciting and interesting situation

  • Marguerite was a lab assistant. She had no chemical qualifications

  • apart from basic training

  • But she had enough material for a doctorate

  • Possibly even for a big prize

  • But she couldn't submit this material for a doctorate

  • Because she didn't have a chemistry degree what happened was that she was allowed to go and study

  • During the Second World War to get the necessary

  • certificates to allow us to submit

  • her main discovery for doctoral thesis now the interesting thing about this paper

  • is that there is only one author?

  • Marguerite Pepe

  • And there's the name here of Jean Pepin the person who actually submitted this paper to the journal on her behalf

  • What's interesting that neither of the two professors have their names on the papers and

  • apparently after the discovery had been made

  • They discovered that

  • Marguerite was as it were double-timing them and working with both of them and neither of them could agree

  • whose name should go on the paper

  • And eventually they decided that she should publish just alone it was probably a misunderstanding

  • and perhaps a reflection of the way that lab assistants were treated

  • And nobody actually asked her who she was working for they were both ordering her about now in

  • 1946 when she submitted her thesis

  • she was asked what name she wanted for the element

  • And she suggested that she should call it catium,

  • c-a-t-i-u-m,

  • because she believed that being at the bottom of the periodic table

  • it would form cation a positive ion more easily than any other element

  • However, Madame Curie's daughter

  • who spoke English, said she thought it was a bad name because English speakers would think it was something to do with cats,

  • the furry animals, therefore it wasn't appropriate for name for an element so instead the second choice was

  • francium named after France originally

  • with the symbol, FA

  • But then this was changed to Fr

  • In fact Seaborg wrote to Margaery purée to ask which was the correct symbol

  • And she answered FL.

  • After her doctorate She had quite an

  • impressive career. She was made a professor of chemistry

  • And she was also elected as the first woman

  • to the L'Académie des Sciences,

  • the French Academy of Sciences the first woman since the 17th century, so it was really quite a trend

  • But sadly, because of her exposure to all these radioactive elements

  • as she got older, she developed chronic radiation sickness and problems

  • So towards the end of her life when she was really keen to do more research into francium her health

  • deteriorated and she died quite young in 1975

  • But what's exciting is that research is continuing in francium. People now can synthesize francium

  • in accelerators by the method of taking a light atom and a heavy one in this case the light item is oxygen,

  • atomic number eight, and gold atomic number 79.

  • Eight plus seventy nine makes eighty seven.

  • So, for example there has been an interesting paper that was published in 2005

  • on the relative rates of extraction of francium and cesium from solution.

  • And even if you have just a tiny amount of francium because it's radioactive,

  • you can follow the process very easily,

  • because modern radiation detectors can detect

  • even a few atoms. I think from the point of view of periodic videos the most interesting fact is that people have measured the

  • ionization energy of francium, that's the energy required to remove

  • one electron to make francium plus.

  • This is the reaction that occurs when you put a metal in water. It shows just how reactive the ion is.

  • Most surprisingly the amount of energy to remove the electron from francium is slightly greater than

  • required to remove it from cesium.

  • So cesium is actually a bit more reactive than francium. The difference is not very great.

  • In principle, this is quite surprising because you would think that the bigger the atom,

  • the further the electron will be away and therefore the easier it is to remove it.

  • And we've seen this in our videos when we've compared the reactivity of lithium.

  • Which is really small with sodium potassium.

  • Rubidium and cesium as the atoms get bigger the reason why francium doesn't behave as people expect

  • is due to what I call the relativistic effects, and we've talked about this on our video about mercury.

  • Is that as the atom gets bigger,

  • the electrons are circulating faster and faster at speeds that are a fraction of the speed of light,

  • which affects the mass of the electron,

  • and therefore makes the atom a bit smaller than one might expect.

  • Therefore the electron is a little closer to the nucleus and is bound a bit more strongly than it might be otherwise,

  • if there were not these effects. The atom is still much bigger than caesium

  • but it's not quite as big as you would expect and we're talking about quite a small energy difference between cesium

  • and francium but it's real, so this brings me back to the fake videos.

  • Francium would not react with water more violently than caesium.

  • So, you shouldn't be surprised that I want to tear my hair out.

  • Perhaps it's understandable even I was wrong on one of our videos.

  • I said that the video of the century would be francium and water

  • It would be the damp squib of the century.

  • No one will ever see francium reacting with water because the so few atoms and in fact the world record

  • for getting francium atoms in one place and one time is

  • 300,000 atoms. That might sound like a lot,

  • but it is an unimaginably small speck of francium, and you wouldn't see anything if you drop that into water

  • At the moment the caesium you can see the lovely gold color it and she believed me

  • melting

  • Look at it. Yeah. It's very warm here today. and, um, the cesium's melted, so we're just going to try and

  • freeze it again.

Francium is the heaviest of the alkali metals.

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B1 element curie cesium radioactivity discovered atom

Francium - Periodic Table of Videos

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    林宜悉 posted on 2020/03/27
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Keywords

chronic

US /ˈkrɑnɪk/

UK /'krɒnɪk/

  • adjective
  • Always or often doing something, e.g. lying
  • (of a person) having a bad habit that is difficult to change.
  • (of an illness) persisting for a long time or constantly recurring.
  • (Of disease) over a long time; serious
material

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UK /məˈtɪəriəl/

  • noun
  • Cloth; fabric
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  • other
  • Fabric or cloth.
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realize

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UK /'ri:əlaɪz/

  • verb
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expect

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measure

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UK /ˈmeʒə(r)/

  • noun
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research

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UK /rɪ'sɜ:tʃ/

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method

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  • (Organized and planned) way of doing something
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flaw

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UK /flɔ:/

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direct

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UK /də'rekt/

  • verb
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require

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UK /rɪ'kwaɪə(r)/

  • other
  • To order or demand something
  • To demand something
  • To need something; to make something necessary.
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  • verb
  • To demand that someone does something
  • To need to do something, to make it necessary