Alpha Decay: Study with Video Lessons, Practice Problems & Examples

Alpha decay occurs when an unstable nucleus emits an alpha particle, consisting of 2 protons and 2 neutrons, resulting in a decrease in atomic mass by 4 and atomic number by 2. For example, polonium-210 undergoes alpha decay to form lead-206. The alpha particle, equivalent to helium, is the largest and most damaging radioactive particle due to its high ionizing power, yet it has low penetrating ability, making it difficult to enter the body. Protection from alpha particles is provided by clothing and air.

Alpha Decay Concept 1

Hey, guys. In this new video, we're going to take a look at alpha decay. So remember, Rutherford talked about the 3 major types of decays. There's alpha decay, beta decay, and gamma emission. Here, we have alpha decay. We're going to say alpha decay occurs when an unstable nucleus emits a particle composed of 2 protons and 2 neutrons. Now, just think about it. We say that our atomic mass equals the number of protons plus the number of neutrons. So, here our atomic mass is, we lose 2 protons and 2 neutrons, so 2 + 2 gives me 4. Your atomic mass is protons, and we're going to say the alpha particle is represented by 42 for your atomic mass over your atomic number. And here we have our alpha symbol. Now, we can also say that on our periodic table, we have an element that also has an atomic mass of 4 and an atomic number of 2. That element is Helium. So, we can say that the alpha particle can also be represented by the element Helium because Helium has the same atomic mass as an alpha particle and it has the same atomic number as an alpha particle. And remember, we're using the term decay. So, decay means that this alpha particle will be a product. So, if you wanted to look at an example of this, we could think of, for example in your periodic table, you could have, polonium, when shown in your periodic table as PO. Polonium, we're going to say, let's talk about isotope-210. Now remember, with these nuclear reactions, they can happen with different isotopes of an element. So, on your periodic table, we'll be doing different types of decays with different types of isotopes. So don't worry if your atomic mass on your periodic table doesn't match my atomic mass. That's because I'm dealing with a certain isotope of that element. Remember, isotopes have the same atomic number, so that's the same element, but they have a different number of neutrons. So we'll have different atomic masses. So here, polonium-210 means the atomic mass is 210. If you look on your periodic table, polonium has an atomic number, the number of protons of 84. Now, we're going to undergo alpha decay. Alpha decay means we're going to spin out or emit an alpha particle. You can represent it like this or like this. Here, I'll just choose to show it as Helium. So, we're going to emit 42 Helium. Now, nuclear reactions are different from regular reactions, but there are some similarities. Just like you have to have a regular chemical reaction balanced, you have to have a nuclear reaction also balanced. So here, our total atomic mass is 210. Here, we have already an atomic mass of 4. So, we need to create an element that when I add it to the 4, gives me back this mass of 210. So, the new element has to be 206, because 206 + 4 gives me 210. Also, your atomic numbers need to match on both sides. This atomic number is 2, we need it to add up to 84. So we'd say that the new element would have to have 82 because 82 plus 2 gives me 84. And what element would that be? Well, that would be lead. So, what we'd say here is that we'd say the alpha decay of polonium-210 creates a brand new element, lead-206. The Helium or the Alpha Particle is just something that we emit, that's just waste. The new element that we're concerned with is lead-206. So, this represents an alpha decay. And it's as simple as that. Make sure that your atomic mass adds up on both sides. Make sure your atomic numbers add up on both sides.

Alpha Decay Concept 2

Now, if we want to talk a little bit more about this alpha particle, we're going to say in terms of size of radioactive particles, we're going to say that the alpha particle is the largest. So, it's bigger than your beta particle, it's bigger than your gamma particle. So, your alpha particle is the largest of them. Now, it is the most damaging to biological cells because it has the highest ionizing power, which means that somehow if you got it into your body, it would just shred your insides. It would irradiate all of your biological cells in your body. A person who is exposed to an alpha particle internally has a very low chance of survival. The good thing is, because it has the highest ionizing power and because it's so large, it's extremely difficult for it to penetrate us, penetrate our skins and get into our cells. So, we're going to say, it has the lowest penetrating power. We're going to say that our clothes, even the air around us provides protection against alpha particles getting into our bodies. Now, how could you get an alpha particle inside of you? Maybe you work in a nuclear facility, where you have contaminated water or contaminated food or there was some chemical leak and it got exposed in our environment in some way and then you ingested it. But, it's extremely hard for things like this to occur. So, alpha particles are extremely damaging to our insides but the good thing is, they're extremely hard to get into our bodies.

Alpha Decay Example 1

Now, here, we have to write the balanced nuclear equations for each of the following alpha emissions. So, for an alpha emission again, that's the same thing as decay. That means that the alpha particle will be a product. So here we have Curium-248. Remember, these numbers here are atomic masses. You have to actually look on your periodic table to find out the atomic number of curium and bismuth. So, curium on our periodic table is 96. We know that we're going to basically emit an alpha particle. So, it's going to have to be a product. Again, we have 4 already, so we have to make sure that the new element next to it adds up to 248 on the other side. So it's going to be 244 +4 gives me 248. And down here, this has to be 94. And what does that give me? That gives me plutonium (Pu).

^{248} Cm → ^{244} Pu + ^4 He

And then, bismuth 207, that's the atomic mass. The atomic number on the bottom would be 83. So, we have Bi, we emit a Helium or alpha particle. So, we have 4 here but we need 207, so we have 203 over 81 and that gives me Thallium (Tl). It's that simple.

^{207} Bi → ^{203} Tl + ^4 He

Now, if they were to ask for an alpha absorption or alpha capture, then that means the alpha particles wouldn't be products, they'd be reactants. For example, you have ^{40}Ca which is calcium and then you do alpha capture. So, it will be on the same side. And then all you have to do is just add these numbers up. So, it would be 40 + 4 is 44, over 22, to give you your new element. So, you just look on your periodic table and see what element has an atomic number of 22, and that will represent an alpha capture.

Here’s what students ask on this topic:

What is alpha decay and how does it affect the atomic mass and atomic number of an element?

Alpha decay occurs when an unstable nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons. This emission results in a decrease in the atomic mass by 4 units and a decrease in the atomic number by 2 units. For example, if polonium-210 undergoes alpha decay, it emits an alpha particle (helium nucleus) and transforms into lead-206. The atomic mass decreases from 210 to 206, and the atomic number decreases from 84 to 82.

Why is an alpha particle considered the most damaging to biological cells?

An alpha particle is considered the most damaging to biological cells because it has the highest ionizing power among radioactive particles. This means it can cause significant damage to the cells it interacts with, potentially leading to severe biological effects. However, due to its large size, it has low penetrating power and is unlikely to penetrate the skin. The primary risk comes from internal exposure, such as ingestion or inhalation of alpha-emitting substances.

How does alpha decay differ from beta decay and gamma emission?

Alpha decay involves the emission of an alpha particle (2 protons and 2 neutrons), resulting in a decrease in atomic mass by 4 and atomic number by 2. Beta decay involves the emission of a beta particle (an electron or positron), which changes a neutron to a proton or vice versa, altering the atomic number by 1 but not the atomic mass. Gamma emission involves the release of gamma rays (high-energy photons) without changing the atomic mass or atomic number, as it only releases excess energy from the nucleus.

What are the protective measures against alpha particles?

Protective measures against alpha particles include wearing clothing and ensuring that the air around you is clean, as alpha particles have low penetrating power and can be stopped by these barriers. The primary concern is preventing ingestion or inhalation of alpha-emitting substances, which can be achieved by following safety protocols in environments where radioactive materials are present, such as nuclear facilities.

What is an example of an element undergoing alpha decay?

An example of an element undergoing alpha decay is polonium-210. When polonium-210 undergoes alpha decay, it emits an alpha particle (helium nucleus) and transforms into lead-206. The atomic mass decreases from 210 to 206, and the atomic number decreases from 84 to 82. The reaction can be represented as:

${\mathrm{Po}}^{210}\to {\mathrm{Pb}}^{206}+{\mathrm{He}}^4$