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.