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Course: AP®︎/College Physics 2 > Unit 8
Lesson 3: Nuclear physicsAlpha decay
During alpha decay, a large, unstable parent nucleus becomes a smaller daughter nucleus. It does this by emitting an alpha particle, a clump of two protons and two neutrons (a He-4 nucleus). The nucleus's atomic number decreases by two, and its mass number decreases by four. The alpha particle is high-energy ionizing radiation—it travels at high speed because it carries away the majority of the energy lost by the nucleus during the decay. Created by Mahesh Shenoy.
Want to join the conversation?
- Wow! Who came up with the smoke detector? I've had smoke detectors in my house for as long as i've lived and never wondered how they work.(8 votes)
- My question is kind-of related, "What type of decay do Alkaline Metals have (as they are radioactive)?"(3 votes)
- Some alkaline earth metal isotopes are stable and do not decay. Others are radioactive. The type of decay depends on the isotope.
For example, Ra-223 is a radioisotope of radium that undergoes alpha decay. Ra-225 is another radioisotope of radium, but it undergoes beta-minus decay.(6 votes)
- Mahesh described the reason behind the instability of large nuclei as the electrostatic force becoming prevalent over the strong nuclear force. But why can't we (apparently) have a nucleus with low number of protons compared to neutrons? Say, a hydrogen with 20 neutrons. It seems the answer would be the same as to why some isotopes are less stable than the others?(3 votes)
- What if the nucleus shot out more alpha decay and more until it was little like other alpha decay, would it make it super stable than it was before or would it have to keep some to stabilize itself?(3 votes)
- why is it called an alpha decay it's helium being spit out it's the 2nd element and alpha is the 1st Greek letter(1 vote)
Video transcript
- [Instructor] Why doesn't our
periodic table go on forever? Why don't we have, for example,
elements with 300 protons, or say, 1,000 protons? Well, the short answer is
because heavier the elements, the more unstable they become. For example, elements
above atomic number 83, they don't even have a stable isotope. All their isotopes are radio isotopes, which means sooner or
later these will decay. And 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? And 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, which are acting against each other. The first one is the
good 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. And this force is an attractive force, and that's what keeps
the nucleus together. And it does so because
strong nuclear force has two advantages over the electric force. The first one is, 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 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. 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. And so, it's being repelled
by all the other protons. But, the nuclear force is short ranged. This means even though it is
super strong in everything, only the few protons and
neutrons in its vicinity, they are the ones, in its neighborhood, they're the only ones that can attract it. The ones that are far away,
well, they can't reach it. Their 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 neurons 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. Which means if you have too many particles and the nucleus is too big, eventually electric force will overpower our strong nuclear force, and that's what will make it unstable. And so now, hopefully you can see how or why as the nucleus becomes
more and more heavier, it becomes bigger and bigger. And because of the short range
nature of the nuclear force, eventually electric force can win out. That's what makes these things unstable. So, if you now imagine
elements with 1000s of protons, there's no chance that they will stay put. The electric force would
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 kind of hold onto 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 this particle was, what particle this was, and we just call it the alpha particle. But, later on we realized that
it's just a helium nucleus with two protons on 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 less 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 onto 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 decays, 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 because helium nucleus is in
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. 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, two less protons, which means that 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. That means it should now
have 234 particles in it. That should be its new mass number. So my new, my daughter
nucleus should look like this. But what is it? I don't remember which,
what is the element? 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 90 to be over 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 be, you know it's atomic
number and the mass number? Can you pause and try? All right, 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
plus two, 95 protons in it. And similarly, four particles came out. 237 is in the daughter nucleus. That means the parent must
have had 237 plus four, 241 total number of particles in it. And therefore, my parent
nucleus would be having an atomic number 95,
which is over here, Am. 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 period 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 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 a total number of
protons and neutrons here. And if you keep track of that, we'll be able to predict what our daughter nucleus would be or what our per 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. And you have the americium
source at the bottom. Now the americium is gonna 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. And 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. And that's why we 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 current, 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. And therefore, although they
have a high ionization power, they have very low penetration power. And 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.