Alpha decay | Physics | Khan Academy

Why doesn't our periodic table go on forever? Why don't we have, for example, elements with 300 protons? So, say, a TH000 protons. Well, the short answer is because the heavier the elements, the more unstable they become. For example, elements about atomic number 83 don’t even have a stable isotope. All their isotopes are radioisotopes, which means sooner or later these will decay.

If you consider even heavier elements, well, they will decay almost instantly, and there's no chance that they exist in nature. But why are they unstable in the first place, and how do they undergo decay? How do we use this to create smoke detectors? Let's find out.

So, let's start by asking ourselves: Why do heavy nuclei become unstable in the first place? Well, the answer is because there are two forces of nature at play over here that are acting against each other. The first one is the good old Coulomb's repulsion, the electric force. All the positive protons are repelling against each other; this is what tries to blow the nucleus apart.

But then what keeps the nucleus together in the first place? Well, it turns out there is a force even stronger than that: the strong nuclear force. This force is an attractive force, and that's what keeps the nucleus together. It does so because the strong nuclear force has two advantages over the electric force. The first one is that it is the strongest force of nature, so that's amazing.

But there's another one. You see, the Coulomb's repulsion, the electric repulsion, is only between the protons because it's only between charged particles; neutrons do not participate. Right? However, when it comes to the strong nuclear force, both protons and neutrons participate. So they're all involved in the strong nuclear attraction.

This is why a lot of stable nuclei exist in our nature; because the strong nuclear attraction just overpowers the electric repulsion, or the Coulomb's repulsion. But compared to the electric force, the strong nuclear attraction has one disadvantage, and that is it is a short-ranged force.

What does that mean? Well, if you concentrate on just, say, one proton, then it is being repelled by all the other protons in this nucleus, regardless of how far the other protons are because electric forces are long-ranged. It doesn't matter how far they are; the electric force works. Therefore, it's being repelled by all the other protons.

But the nuclear force is short-ranged. This means even though it is super strong and everything, only the few protons and neutrons in its vicinity, the ones in its neighborhood, are the only ones that can attract it. The ones that are far away? Well, they can’t reach it; the nuclear forces are short-ranged.

Now, for lighter nuclei, this is not a problem because it is very small, so most of the protons and neutrons are within the nuclear range. But what happens as the nucleus gets heavier? Well, just imagine an extreme case; if there were lots and lots of protons and neutrons, all the protons will repel it. So, the electric force becomes incredibly large on this proton, but not all the particles are going to attract it. Only the ones in the neighborhood are going to attract it.

This means if you have too many particles and the nucleus is too big, eventually the electric force will overpower our strong nuclear force, and that's what will make it unstable. So, now hopefully you can see how or why, as the nucleus becomes heavier, it becomes bigger and bigger. Because of the short-range nature of the nuclear force, eventually the electric force can win out, and that's what makes these things unstable.

So, if you now imagine elements with thousands of protons, there's no chance that they will stay put. The electric force will just blow it apart. It'll just break instantly. But what about the heavy elements that we do have in the periodic table? Well, over here, the strong nuclear force can sort of hold on to them but not forever.

They have found a way to become more stable. How? Well, they just spit out a helium nucleus, and we call this the alpha decay. We call it the alpha decay because when we first discovered it, we didn't know which particle this was, and we just called it the alpha particle. But later on, we realized that it's just a helium nucleus with two protons and two neutrons.

Okay, this might raise a lot of questions. Now, the first question could be: How does this make things stable? Well, you can see the daughter nucleus now has fewer particles in it, so it's slightly smaller than the parent nucleus. If it's smaller, it's that much easier for the strong nuclear force to hold on to it, and therefore it becomes more stable than the parent nucleus.

Now, that doesn’t mean that this is completely stable; this might still be a radioisotope, and it might further undergo more alpha decay, which is totally possible. Okay, but then a follow-up question that comes to my mind is: Why a helium nucleus? Why not anything else? Why does it spit out precisely this?

Well, the short answer is that because helium nucleus is incredibly, incredibly stable, and so it's just more energetically favored, and therefore that's what happens. Okay, another question we could have is: When things become stable, the energy decreases, right? This should now have less energy compared to the parent nucleus. That's what it means to be stable. But if that's the case, where did the energy go?

Well, that energy comes out as the kinetic energy of these particles. The alpha particle will take up most of the kinetic energy, but the daughter nucleus will also have some recoil as well. And now guess what? These alpha particles can go and hit other atoms, make them jiggle, causing heat. This is how radioactive heating works.

And, fun fact folks, we believe this is majorly what keeps the Earth's core pretty hot. I mean, there are some other reasons, but we think this is the major one. And another fun fact: This is where most of the helium on our planet comes from, from the radioactive decay of elements found inside the Earth.

Anyways, let's now familiarize ourselves with this and take a couple of examples. One example could be uranium-92. It turns out it will undergo an alpha decay. The question for us is: What happens after the alpha decay? Can we predict what the daughter isotope would look like?

Well, let's see. Here's how I like to think about it. I know if it's an alpha decay, then a helium nucleus comes out; therefore, my daughter nucleus must have two less protons in it. It started with 92 protons, so two less protons, which means that the daughter nucleus should have 90 protons in it. Similarly, my daughter nucleus will now have four particles less in total.

It started with 238, now it has four particles less, which means it should now have 234 particles in it. That should be its new mass number, so my new daughter nucleus should look like this. But what is it? I don't remember which it is. It's not uranium anymore because it is 90, and I don't remember.

You don't have to remember; that's why we have the periodic table. If I just look at the periodic table, I see 90234 here, so this is thorium. Thorium! So look, we get a new element altogether! And if you look at the periodic table, you can see we've gone from uranium to thorium, so that means we hop two to the left in the periodic table. That kind of makes sense because you're losing two protons, so you hop two elements to the left.

Okay, why don't you try one in this example? Let's say there is some parent nucleus that undergoes an alpha decay and gives you neptunium-93-237. Can you pause and predict what the parent nucleus would be, its atomic number and the mass number, when you pause and try?

Alright. Again, it's an alpha decay, so I know a helium nucleus must have been thrown out. Therefore, I can now predict: well, there is 93 in the daughter, two came out, so the parent had to have 93 + 2, which means 95 protons in it. Similarly, four particles came out, so 237 is in the daughter nucleus. That means the parent must have had 237 + 4, which equals 241 total number of particles in it.

Therefore, my parent nucleus would be having an atomic number of 95, which is over here. Am stands for americium. And so there you have it; that's my parent nucleus. And again, you can see if you look at the periodic table, after the alpha decay we've gone two elements to the left of the periodic table.

So you see, all we have to do is keep track of the total number of protons and neutrons. They stay the same because nothing is changing. The total number of protons and neutrons here will be the same as the total number of protons and neutrons here. If you keep track of that, we'll be able to predict what our daughter nucleus would be or what our parent nucleus would be.

Now, there is a reason why I took this example, because this is the radioisotope used in smoke detectors. But how does alpha decay help us detect smoke? Let's see. The heart of these radioactive smoke detectors will contain two plates connected to a battery, so that they have a positive and a negative charge.

You have the americium source at the bottom. Now, the americium is going to give out a lot of alpha particles. But so far, did you notice something about the alpha particles? They have two protons in them, but they don't have any electrons because they came from the nucleus, which means alpha particles have a plus two charge on them.

Since they're moving with a high speed, they can now knock off a lot of electrons from the atmospheric particles over here, from the oxygen, nitrogen, and all the other atoms. That’s why we'll now end up with a sea of electrons and positive ions. Now, of course, the helium nuclei will eventually pick up electrons and become neutral atoms, but eventually they leave behind a lot of positive and negative charges.

As a result, this whole thing becomes a conductor now, and therefore, there will be a continuous current running in the circuit. Now this process, where the helium nucleus is able to knock off electrons, is what we call ionization, and that's why we call helium to be highly ionizing radiation.

But anyways, what happens when we have smoke? Well, it disrupts this process; it doesn't allow the helium to ionize a lot of these atoms anymore because of the smoke particles in between that significantly reduces the current, and that is sensed by a sensor in the circuit, and the alarm goes off. This is how alpha decay can be used to detect smoke.

I find this absolutely mind-boggling. But you might be concerned, thinking that we have a radioactive element in our house. Now, isn’t that dangerous? Well, in general, radioactive elements are dangerous. But here's the thing about alpha particles: sure, they are extremely ionizing; however, they don't go very far.

They can be easily stopped, say by just a piece of paper, because they're so bulky; therefore, although they have a high ionization power, they have very low penetration power. That's why none of the alpha particles will even leave the casing of your smoke detector, and so you don't have to worry about anything.

But what if you decide to now open up the smoke detector to have a closer look at the americium? Well, now that can be dangerous because you might have some neptunium-237 lying around somewhere over here, floating around. You might ingest it. Neptunium is also radioactive, and therefore now you have radioactive elements inside your body that can be dangerous. So don’t do that.