BYU-Idaho Online Learning
Video Transcript
[A slideshow is shown. The slideshow is called “Radioactive Decay.”]
Instructor: Hi, I just want to take a minute and talk about radioactive decay. I had a student email me and say that this was a concept that they found very confusing. And so I thought making a little video for you might help. So let's just talk real quickly about what radioactive decay is and how it works.
We use radiometric dating a lot in science. It helps us to determine the age, especially of the Earth and things of that nature. Radio as—oh sorry—radioactive isotopes decay over time into different elements. So a normal carbon, stable carbon, would be C12. So if you see carbon C12, it's normal, it's stable. There's six protons and six neutrons. Now if this carbon ends up with eight neutrons somehow, it becomes radioactive. And we call this a radioactive isotope of carbon, and it's C14. C14, once it's radioactive, it can then decay. So once they're—if they're stable they don't decay, but if they're radioactive then they will and they decay over time. So carbon-14, it decays over time and turns into nitrogen-14, which is stable for nitrogen, so then it stops decaying once it's stable. That takes about 5,730 years. So when you hear someone say carbon dating, that's a form of radiometric dating where we are dating carbon. If you have a sample of a rock and it has carbon in it. And you see that some of that is carbon-14, and you see that some of it is nitrogen, you'll know that time has elapsed such that the carbon decayed into nitrogen. And so we know how much time—the half-life of it would have been 5,730 years. So we'll know it's at least that old. Now some things like the earth are going to be dated much, much, much, much older than what you would want to use carbon for. So in those cases, we can use other things. One example is uranium. Uranium-238, it decays and turns into lead, and that has a half-life. It takes about 4.46 billion years to decay to lead. If you find a sample that has uranium-238 in it and it has lead in it, you'll know that sample's about 4.46 billion years old. Because that's how long it takes for it to decay.
Let's talk about what a half-life is. A half-life—oh, this is what's in our reading—it says it's the period of time required for half of the nuclei in a large sample of radioactive isotope to decay. Said another way, the half-life is the time required to produce equal proportions of parent and daughter atoms from an initial state composed entirely of parent atoms. So basically a half-life is the amount of time required for a quantity to fall to half of its value. If you look at this little chart on the right, it shows you how many half-lives have let—have elapsed.
[A table is shown. The table has two columns. The first column has is called “Number of half-lives elapsed” and the second column is called “Fraction remaining.” The first row has “0” and “1/1” respectively. The second row has “1” and “½”. The table continues until seven. The table then has a break signified by “...” in a row. The final row in the table has “n” in the first column and “1 divided by 2 to the power of n.”]
If none have elapsed, if it's a new sample, you should still have one out of one, a whole sample remaining. After one half-life has elapsed, you should have half of your sample that's still remaining of that parent atom. After two, you should have a quarter. I'm going to show you how this works in just a second in a little more detail. So, carbon dating, radioactive decay occurs. Just recognize that carbon-14 will then decay into nitrogen-14, okay. So that's how this is working.
[A new slide is shown. The text “0 half-lives have elapsed” is shown. A blank table with 6 columns and 6 rows is shown.]
Let's just say we find a rock, a sample, something out there. And let's just say that we can use carbon even for this example, let's say that this is an entire piece of carbon. The whole thing is all made out of carbon. So we know when we see this that no half-lives have elapsed. This is an entirely brand new specimen. So how old is this? It's brand new.
[Half the table gets filled in with a color. The text changes to say “1 half-life has elapsed.”]
But then we look and we say, oh, now we have this dark part, and we're going to call this dark part nitrogen. So half of the sample is now the stable nitrogen. But half of this sample is that carbon-14 that we started with. We know because it's half and half that one half-life has elapsed. If this is going to continue to decay, how much of this sample, well then continue to decay? Now remember, that dark part, that nitrogen that's stable, that will not decay. The daughters do not decay. It's the parents that will continue to decay. That white part, that carbon-14, that radioactive unstable part that decays. So only half—for another half-life to elapse, only half of the white part will continue to decay. Then it would look like this and that tells us that two half-lives have elapsed.
[Half of the remaining white part of the table changes color. The text changes to say “2 half-lives have elapsed.”]
Okay? If we're going to continue on we say, okay, now we have all these dark boxes of nitrogen, but we still have this radioactive carbon, this white part that can still decay. And when another half-life elapses, half of that will continue to decay, and so on and so forth.
[Half of the remaining white part of the table changes color. Only 4 and a half boxes remain white. The text changes to say “3 half-lives have elapsed.”]
So this—in this example here, four half-lives for the lapsed.
[Half of the remaining white part of the table changes color. Only 2 and a quarter boxes remain white. The text changes to say “4 half-lives have elapsed.”]
That's how we determine how many half-lives have elapsed. Remember that parent—the parents will continue to decay, but the daughters, they're stable, they do not continue to decay.
[The slide changes. Two images of crystal-like rocks are shown. The second image has two types of material in it.]
One example that you'll read about is zircons. This helps us determine the age of the Earth. case, because it's so—age of Earth is—we need something really large to help data. So we use the uranium-lead dating. Uranium-lead dating is where uranium decays to form lead. Zircons include uranium, but they don't have any lead in their formation. If we can find a zircon, it will help us to determine how old the Earth is, because it has uranium in it. If we find a zircon with lead, we'll know that a certain amount of time has elapsed. In our reading, we read that radiometric ages of igneous crystals indicate when the crystal solidified from magma. What that means is when magma forms, that's like lava or magma, either one, and it hardens, it creates a rock, right? When that initially happens, that's an igneous rock. Those crystals form this hard rock. In the case of a zircon, when it's formed from magma, it forms a pure solid zircon with uranium. Then it says, igneous crystals contain only parent atoms and no daughter atoms when it forms. So only the uranium, only that parent.
Now uranium has—uranium-238 has a half-life of about 4.5 billion years. If you find a zircon that has lead in it, then we know that at least 4.5 billion years have elapsed. The oldest sample that we have found of zircon has some lead in it, as you can see that picture on the bottom right. So that tells us that this sample is about 4.5 billion years old. If it's made up of about half of its lead—half of the uranium I should say, in the sample. That's how we date the Earth. We say the oldest sample of zircon that we have found has—looks to have had one half-life elapsed. About half of the uranium has decayed into lead. So we know that the Earth is older than the oldest zircon that has been found on the Earth. We would say, you know, this formed from magma when the Earth formed, so it has to be at least this old. It could've been after. So the Earth is at least 4.5 billion years old. So that's where that estimate comes from.
If you have any other questions, please make sure you ask. I'm happy to make more videos, if it helps, explain things to you. And if this didn't make sense, please let me know. I'd be happy to find another way to discuss it and help you with it.
[End of video.]