This is what you're making. It's the nucleus of a Uranium-235 atom with a loose neutron lurking in the wings, just waiting to blow it all to smithereens. How do I know it's Uranium? 92 protons. In our model they're green... I would have liked them to be pink -- something about Pink Protons just appeals to the ear, but there was a misjudgement in someone's estimate of how many mini marshmallows might be eaten before we started counting out the ones needed for the model, and well... we were stuck. That or we'd have to go back to the grocery store and I already mentioned how much I appreciated that this project only took one trip! So my advice? Buy two bags... maybe three.So back to the protons. Here's the big thing about protons. They are the one single thing that tells you the most about what you've got. If it has 92 protons then it IS, absolutely and positively, Uranium. If it's 93? Not Uranium. 93 is Neptunium. 94 is Plutonium and I have to admit here that I only just now realized that they're named after the planets... That might have been a useful hint back in 6th grade when I had to memorize the periodic table, but at my age it's probably only good for bar bets. But I digress.
The point is, if you had a molecule of Oxygen (8 protons) and you somehow managed to remove a proton, you would then quite suddenly have Nitrogen (7 protons) with all of its different properties. So 92 protons is Uranium. Now why Uranium-235? The 235 means that there are 235 total thingies in that nucleus. And by thingies of course I mean protons and neutrons (since electrons are all floating around outside the nucleus). So that means 235 mini marshmallows. This is why you should always look up the details before wheeling your shopping cart up the candy aisle and saying "hey, we'll just pick up some marshmallows and have a nice little spur-of-the-moment lesson on nuclear fission - I'm sure one bag is enough!" Two bags. Trust me.
So if there are 235 thingies, and 92 of them are protons, that leaves 143 neutrons. Now you see the pink problem. It turns out there are more pink mini marshmallows in every bag I've opened so far. I don't know if that's on purpose or not, random (we don't buy that many -- it's a really small sample size!), regional, or what, but not only are there more pinks, there are orange ones too, which look an awful lot like the pinks and you can fake it a little bit in that direction if you're running a little short. I personally think they're short of greens because the greens taste vile, but then I don't think I'm really their target demographic anyway. In any case. For our model we used green for protons and pink (or orange... just a couple) for neutrons.
Pile up all those marshmallows on a plate. Then get your hands a little wet and start squishing. You don't want a homogenous mess, you just want them damp enough to stick together, but maintain their identies as protons and neutrons. Once you have it reasonably stuck, take the spare neutron. This is the one the nuclear reactor fires at the uranium...
Let your enthusiastic kiddo do the firing (if your house is anything like ours this part is way more fun than anything else, even the part where they're allowed to eat a few subatomic particles). The uranium splits, more or less in half, and what comes out? More neutrons! Now if you had a whole class of kids you could illustrate a chain reaction. One neutron blows up one nucleus, which then releases let's say three neutrons, which if they're lucky run into three more nuclei, which split themselves and go on to release more neutrons, and soon you have a few dozen sugared-up children running around the lab flinging marshmallows at each other. Now you know what a nuclear bomb looks like.One thing you have to mention, although I'm sure it gets lost in the fun of blowing things up, is that most things don't do this. In fact most Uranium doesn't do this, because most Uranium is actually not Uranium-235... it's Uranium-238. More neutrons (still 92 protons though!!) Uranium-238 doesn't go around splitting in half and flinging neutrons at its classmates. It's much more likely to just lose a piece here and there... a couple few parts here and there, even entire Helium neutrons (2 protons with 2 neutrons), which are called alpha particles, and electrons and positrons, which are beta particles, and which can make a proton into a neutron or vice versa. Of course if you're changing the number of protons (which each of these bits falling off is doing) then you don't end up with Uranium in the end. Actually it goes through a lot of different steps before it ends up as Lead. This is the whole half-life thing that radioactive elements go through. They lose pieces at a reasonably predictable rate, turning into other things on the way to something stable. What we were doing with the marshmallows is nuclear fission. Start with Uranium, end with two molecules of something completely different and about half the size.
That's a rather big difference.... and why you can still get old red Fiestaware with the Uranium glaze. Now I'm not saying I'd eat off them myself (although my grandmother did for years and lived well into her nineties), but your vintage salt and pepper shakers aren't going to blow each other up in the china cabinet. They will however lose enough particles to keep your geiger counter clicking.
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