Chemistry: The Nature of Radioactivity

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Nuclear chemistry is such a significant departure from what we're talked about up until now that it might make you nauseous. It might give you headaches. Feel free to shut off your computer at any point and come back to this. Because we're going to talk about something that really is so different it might make you ill.
We're going to now talk about chemistry that doesn't involve electrons. Up until now, redox chemistry - forming coordinate covalent bonds, forming covalent bonds, ionic bonds - all that stuff involved electrons. And now, we're going to totally disregard the electrons. We're going to not even include the electrons in our balanced reactions, because what we're going to study is the reactions of the nucleus - the nucleus that, up until now, we've thought of as sort of immutable. An atom is an atom. An element is an element. And what we're going to say is, maybe sometimes that's not true. Sometimes there are elements that become other elements, not because they change the number of electrons. If you add an electron to a fluorine, it becomes fluoride, but it's still got the same number of protons and neutrons, whereas, what we're saying is, you can turn fluorine into some entirely different element.
Here's an example of a reaction, and I'll just walk you through it. It says []goes to thorium-234, plus a helium nucleus. And, to make sure we understand what we're talking about here, you've seen this before, when we talked about isotopes. The letter - that's the atomic symbol, the thing you see in the periodic table. The number in the upper left-hand corner is called the mass number and it's the sum of the total number of neutrons and protons. And the reason why we can call it the mass number is because in atomic mass units, the proton and the neutron each weigh approximately 1. And so, that's just the sum of the number of protons and neutrons. We call protons and neutrons together nucleon. So we can say that there are 238 nucleons in the nucleus of a uranium atom. Now 92, that's the atomic number. That's the number of protons. And clearly, the number of protons uniquely determines the atomic symbol. So these two things are degenerate. They're unnecessary to have both. We can have 92 or we can have U. Conventionally, what you do is you leave off the 92. And so, in this lecture, I'm going to include the atomic numbers, but what we're going to see is, later on, I'll probably just leave them out and you'll have to go look them up or figure them out by glancing at a periodic table.
Okay, back to our reaction. This is an example of alpha decay. In an alpha decay, what happens is a particle gets spit out of the nucleus of the parent to give the daughter, plus another particle. And in this case, it's uranium-238 becoming thorium-234 plus a helium nucleus, where 2 protons and 2 neutrons - we have mass difference of 4 and a charge difference of 2. And that piece that came out - that's a helium nucleus. Now, if we think about the electrons - and we'll do it now and we're never going to do it again - if this started out as a neutral atom, then it had 92 electrons. Then this thorium is going to have 2 more electrons than it needs, so it will be a di-anion. And this thing, which was a helium nucleus, will have a 2 plus charge, so it needs 2 electrons. And so if gets those two electrons from the thorium, then everything would charge balance. So we're not even going to worry about the electrons, but they're there and can account for them. It's just not necessary, because we're not worried about the electrons. We're only worried about the nuclei. And given that we're not worried about the electrons - in fact, a piece of uranium metal is going to behave exactly same as a piece of uranium oxide, which is going to behave exactly the same as a gas sample of uranium hexafluoride. In other words, if uranium is a metal or a compound or whatever, its nucleus is going to behave exactly the same.
So this is the first example. They way to remember alpha is that's the biggest thing that comes out of the nucleus as a single well-identified particle, and that's a helium nucleus. Now, for beta decay, something else comes out of the nucleus - and this is going to make you sick - an electron. Now, that electron is not coming out of the core electron shell or the valence electron shell. It's actually coming out of the nucleus. Where does it come from? It comes, effectively, from the decay of a neutron into a proton and an electron. And now, this electron - what we're doing is, we're saying it has zero mass - so the mass number is zero - and its charge is minus 1. So now we're generalizing the idea of atomic number to represent charge of the nucleus. And in this case, the charge of not a nucleus, but the particle is minus 1. That's going to allow us to keep track in an accounting scheme and you'll see why.
So, the electron that comes out comes out when a neutron gets converted into a proton. That means that the number of protons - or the atomic number - changes by 1. So it goes from iodine-131 to xenon-131 - different atom, now 54 protons before 53 protons. And notice how these reactions balance. The mass number sums to the same. So 131 goes to 131 plus zero. And the charge, 53, goes to 54 plus minus 1. And so this electron, which wasn't a part of the nucleus originally but became part of the nucleus because of neutron decay, that's what we call a beta particle.
Now accompanying both alpha decay and beta decay and all these other kinds of emission, very often you get gamma radiation. And gamma radiation is electromagnetic radiation in exactly the same way - visible light, ultra-violet light, infrared, x-rays - except it's the most energetic. It's more energetic than x-rays and this accompanies a lot of electromagnetic radiation, but we don't have to worry about balancing reactions for this, because this is electromagnetic radiation. There's no particle, per se, and so we don't have nucleon numbers or anything like that.
Now the fourth type is called positron emission, and a positron - once again, it's going to make you sick - is an electron but with a positive charge. We've never encountered this before, although you may have heard the term "positron" in the context of Star Trek: The Next Generation - Data's positronic net. His brain is supposed to be this positronic net. Well, the positron is your first encounter with the idea of antimatter - something that is sort of out of our world - not really part of this world. And the reason is, as soon as it sees an electron - so here's a positron. It has the same mass as an electron, which is zero, but it has a positive charge. But as soon as it sees an electron, the two of them annihilate to form two gamma rays. So, a flash of light and they're gone - a lot of light, in the form of gamma rays - but just a lot of light. And this positron emission comes from a Proton turning into a neutron and kicking out a positron. And so if we think about fluorine-18 going to oxygen-18 - we had 9 protons, now we go to 8 protons. And what's left over is a particle that has no mass because we have a mass number of 18 in both the parent - the reactants are called the parents and the product is called the daughter. We have 18 mass number in the parent and the daughter, so the mass number of the particle that gets kicked out has to be a zero. And then the charge - 9 plus, 8 plus and 1 plus - so this is a balanced reaction for the decay of fluorine-18 to oxygen-18. And again, what happened inside the nucleus - and if this bothers you, we're going to come back to it again, so don't get overly worked up - is that a proton turns into a neutron and it kicks out a positron.
Now the last type that I want to talk about is something called electron capture. And the effect is the exactly the same - that a proton turns into a neutron - except what happens is the nucleus actually grabs onto an electron. In this case, the electron that it grabs onto actually does come out of the core. So this rubidium atom, or rubidium ion, grabs an electron from its own core. For instance, it has to be an s electron, but it could be 1s or 2s or 3s. Remember, s electrons have some probability of being found inside the nucleus. Well, what happens is that one of the protons in the rubidium grabs onto that electron and turns into a neutron. And when it turns into a neutron, the mass number stays exactly the same, so 81 plus zero is equal to 81. But 37 minus 1 becomes 36, so rubidium goes one earlier in the periodic table. There's one less proton, and so it turns from rubidium into krypton. And you can see that. All you have to do is add up the mass numbers and add up the charge and you can see that you have a balanced reaction.
So let's practice a few. Here have oxygen-15 going to nitrogen-15. And if we add across, what particle has to be left over? Well, the mass number has to be zero and the charge has to be 1, because 15 = 15 + 0 and 8 = 7 + 1. Now what particle has zero mass number and a charge of plus 1? Why it's our old friend the positron. While I'm on the subject, I'll give you a shorthand for the positron, which is , where beta minus is the electron, beta plus is the positron. Obviously, there's less information here, because you can't remember all of this. So we'll do this. We'll write this notation when we balance the reactions, but you'll know what I'm talking about when I write a beta plus.
Okay, let's do the next one. We have 40 - so this potassium-40. It turns out some fraction of your body - about 1 potassium in 10,000 - is a radioactive potassium. Radioactivity is all around us. You cannot have a radioactive-free world. You are radioactive because naturally occurring potassium consists of about 1 part in 10,000 potassium-40. All right, so it decays by beta decay. This is a beta - it's an electron that gets kicked out of the nucleus - and the product has to have a mass number of 40 and it has to have a charge of 20. So what has a mass number of 40 and a charge of 20? Look at the periodic table, find out what element is the 20^th element - has 20 protons - and it is calcium. So this is calcium-40, which is the product of the beta decay of potassium-40.
And finally, radon - radon is something I'm going to have more to talk about as well - it's a radioactive noble gas that collects in your basement, and it decays by alpha decay. So if it spit out a helium nucleus, what's going to be left over? The mass number goes from 222 minus 4, so it goes to 218. And the atomic number goes to 86 minus 2, which is 84. What is element 84? Well, it's going to be 2 elements to the left in the periodic table from radon and it is polonium. So when radon-222 decays by an alpha decay, the daughter is polonium-218.
Well, now you might ask how do we know that a particular nucleus is going to undergo a particular type of radioactive decay? In other words, how do we know that oxygen-15 is going to do a positron emission versus potassium doing a beta emission, or something like that? For that matter, how do we know that oxygen is going to emit a positron versus capture an electron? Well that's going to come out of something that's next, which is understanding why nuclei are stable and when they're not, what do they want to do to gain more stability? And that's in the next tutorial.
Nuclear Chemistry
Radioactivity
Nature of Radioactivity Page [3 of 3]