The Modern Alchemist: Reacting Fluorine with Caesium

Behind the Scenes of the 2012 Christmas Lectures

The two most reactive elements in the Periodic Table.

In preparation for the 2012 Christmas Lectures Dr Peter Wothers heads off to the University of Leicester to conduct an extraordinary experiment - reacting the most reactive metal in the periodic table (Caesium) with the most reactive non-metal (Fluorine).

Due to the extreme reactivity of the two elements, Fluorine expert Professor Eric Hope is on hand to enable the experiment to be conducted safely in a unique set of apparatus.

We believe this is the first time the reaction has been caught on camera.

After this succesful test, Professor Hope transported some Fluorine to the Ri to conduct the same experiment as part of Lecture Two of the 2012 Christmas Lectures.




Dr Peter Wothers
University of Leicester

The Royal Institution / Tom Hewitson

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So I've come up to the University of Leicester, and I've brought with me a tube here containing Caesium. Caesium is found in the far corner here. And it is the most reactive metal in the entire periodic table, even more reactive than Francium. This is a radioactive element and it decays. But actually, Caesium is more reactive than this. It's the most reactive thing we can find.

Now, why have I come up to Leicester? I brought this because I want to see how it reacts with the most reactive, non-metal in the entire periodic table. And that's the element Fluorine. And we don't think this has ever been shown before, so we're not entirely sure how it's going to go. So I'm gonna go and see.

Having decided that I wanted to do this crazy reaction, I needed to find just the right person to handle the Fluorine. And this is Professor Eric Hope here from the University of Leicester. So, thank you very much for helping me out with this.

You're welcome.

And what was your first reaction, by the way, when I said I want to try Caesium with Fluorine?

I thought you were totally and utterly mad.

Uh huh. That's probably most people's reaction. Now we've designed this apparatus with quite a lot of thought. One of the problems here is that we want to do this at the Ri. And transporting Fluorine is a little bit tricky, is it?

Certainly. It reacts vigorously, as we will see, with virtually any element in the periodic table.

Eric's come up with this fantastic idea of just filling the coils in the apparatus, this plastic tubing that has been sensitised to fluorine so it's not going to react with it.

Basically, what we have in here is Fluorine, but at the same pressures there is in the atmosphere around it that nothing's going to leak in, nothing's going to leak out. So even if there was a major event, then the Fluorine would not be released from the apparatus.

To show the extreme reactivity of Fluorine, we're going to set fire to something. We all know how hard it is to start a barbecue using this stuff, this is charcoal, trying to get it to burn with the Oxygen from the air. I'd be easy, though, if the atmosphere contained Fluorine.

Is this your one or is this my one?

No that was yours, I think.

OK, was it?


You see.

OK, should we see the reaction then?

Let's have a go.

Wow. This demonstrates the extreme reactivity of Fluorine gas. This was a tiny quantity of Fluorine being played onto the surface of the charcoal there. And as soon as the two came into contact, there was this very violent reaction, generated a lot of heat, as you saw the flames there.

But the charcoal isn't terribly reactive. Caesium is much more reactive. So I think we should try this reaction now then.

This is my Caesium. Have you seen Caesium before?


There's quite a lot of it, isn't there? Quite beautiful.

Beautiful, yes.

And so dangerous.

So this is a combination between the two most reactive elements in the periodic table. We've got Fluorine, the most reactive non-metal, and Caesium, the most reactive metal.

Let's push them through again.


Yeah, it looks good. All right, we are nearly ready. I'll lower this.

Yep, and I'll turn.

You look after your pipes.

Incredibly beautiful reaction there between Caesium and Fluorine. This intense light. And I don't think many people have seen that before.

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