Fun With Alkali Metals

Speaking of science television shows (as I’ve been in a number of recent posts), have you seen Brainiac, over in the UK? It came up in conversation recently* and so I went to YouTube to have a look:

[Update: Ugh. They faked the final two explosions. Have a look here. Thanks Tristram Brelstaff for pointing this out. See my comment additions below in the explanation also.]

“The dog’s nuts of the periodic table…” Must remember to use that in my physics 100 class next year….

Ok… it’s definitely fun for the kids! The science? When they calm down, you get to tell them about why this extreme reaction happens. Here’s a rough explanation:

Start with reminding them about atoms being arranged with a positively charged nucleus in the central core, with negatively charged electrons outside, and how different atoms have different arrangements of electrons. Different chemical properties result from different arrangements. Similar chemical properties come from similar arrangements. It is useful to think of the electrons as arranged in layers or shells that get filled up as the atoms get more complicated. Why do you get chemical reactions between the elements to make compounds? Essentially, the reason is that the new compound has lower energy than the separated parts. Specifically, the alkali metals all have a single electron in their outermost shell. That lone electron does not take much effort (energy) to remove, and so they can combine with other elements rather readily – they’re highly reactive.

With water (a molecule of which is two hydrogens and one oxygen) they do this by combining with a hydrogen and an oxygen (to form an “alkali metal hydroxide”) and leave also a molecule of hydrogen. So you have a sudden release of hydrogen – explosion. (The energy given up from the move to the lower energy state conveniently ignites the hydrogen too – presumably upon contact with the oxygen in the air – you see this with the flames produced with sodium and potassium in contact with a little water.) So you’ve got fast reaction speed releasing lots of hydrogen (maybe with some ignition too, depending upon the situation) – all you need for a bang! (I think the big bath explosions in the video have no ignition going on – it’s all happening underwater where there’s no free oxygen – must be all just the rapid hydrogen production producing the fun…[Update: Ha. They faked those explosions because they could not get the big dramatic explosions they wanted for TV. Sad. Now I wonder if they’d have got a nice bang if they kept the water shallow, or supported the metal so that the reaction takes place nearer the surface so that the weight of the water would be less. There’d also be the greater possibility of the hydrogen reacting with the oxygen of the air with the help of the reaction’s heat for ignition….. Then they’d get a bang, I’d expect!])

That’s all well and good. So why do the alkali metals have a more and more violent reaction as you move to larger and larger atoms? lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). It all depends upon how readily that outer electron can get stripped off. As the atom gets bigger, that outer electron is further away from the positive nuclear core it’s bound to – less energy needed to give it up – so the more rapid and violent the explosion**.

Hmmm…. I think I’ll have a look at a few more Brianiac shows later on, and maybe comment a bit more. Any favourites?

-cvj

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*Thanks Nick Warner!
**Like I said, this is rough. It might be that for the larger alkali metal atoms, more energy is given up in forming the new compound too. Might that also add to the explosion’s severity? And by how much? I don’t know.

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