If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:03
EVO‑1 (EU)
EVO‑1.N (LO)
EVO‑1.N.1 (EK)

Video transcript

what I want to do in this video is kind of introduce you to the idea of one how carbon-14 comes about and how it gets into all living things and then in either later this video or in future videos we'll talk about how it's actually used to date things how we use it to actually figure out that that bone is 12,000 years old that that person died 18,000 years ago whatever it might be so let me draw the earth so let me just draw the surface of the earth like that it's just a little section of the surface of the earth and then we have the atmosphere of the earth I'll draw that in yellow so then you have the Earth's atmosphere right over here let me write that down atmosphere atmosphere and 78% the most abundant the most abundant element in our atmosphere is nitrogen it is 78% nitrogen and I'll write nitrogen that's its symbol is just N and it has 7 protons and it also has 7 neutrons so it has an atomic mass of roughly 14 then this is the most typical isotope of nitrogen and we talk about the word isotope in the chemistry playlist an isotope that the protons define what element it is but this number up here can change depending on the number of neutrons you have so the different versions of a given element those are each called isotopes I just view them in my head as versions of an element so anyway we have our atmosphere and then coming from our Sun we have what's commonly called cosmic rays but they're actually not raised or cosmic particles they're mainly you could view them as just single protons which is the same thing as a hydrogen nucleus they can also be alpha particles which is the same thing as a helium nucleus and there's even a few electrons and they're going to come in and they're going to bump into things in our atmosphere and they're actually going to form neutrons so they're actually going to form neutrons and we'll show a neutron with a lowercase n and a 1 for its mass number with N and we don't write anything because it has no protons down here like we had for nitrogen we had seven protons so it's not really an element it is a subatomic particle but you have these neutrons form and every now and then and I'm not you know let's just be clear this isn't like a typical reaction but every now and then one of those neutrons will bump into one of the nitrogen 14s in just the right way into one of the nitrogen 14s in just the right way so that it it bumps off one of the protons in the nitrogen so it bumps off one of the protons in the nitrogen and essentially replaces that proton with itself so let me make it clear so it bumps off one of the protons so instead of seven protons we now have six protons but it's this number fourteen doesn't go down to thirteen because it replaces it with itself so this still stays at fourteen and now since it only has since it not only has six protons this is no longer nitrogen by definition this is now carbon and that proton that was bumped off just kind of gets emitted so then let me just do that another color so plus and a proton that's just flying around you could call that hydrogen hydrogen one and it can gain an electron some ways if it doesn't gain an electron it's just ah it's just a hydrogen ion a positive ion either way or a hydrogen nucleus but this process and once again it's not a typical process but it happens every now and then this is how carbon-14 forms so this right here is carbon-14 you can essentially view it as a nitrogen 14 where one of the protons is replaced with a neutron and what's interesting about this is this is constantly being formed in our atmospheres not in huge quantities but in in reasonable closet so let me write this down constantly being formed constant constant formation and what happens is and let me be very clear let's look at the periodic table over here typical carbon so carbon by definition if you have six protons carbon by definition it has six protons but the typical isotope the most common isotope of carbon is carbon-12 so carbon 12 carbon 12 is the most common is the most common so most of the carbon in your body is carbon-12 but what's interesting is is that a small fraction of carbon-14 forms and then this carbon-14 can then also combined with oxygen to form carbon dioxide and then that carbon dioxide gets absorbed into the rest of the atmosphere into our oceans it can be fixed by plants when people talk about carbon fixation they're really talking about using mainly light energy from the Sun to take gaseous carbon and turn into actual kind of organic tissue and so this carbon-14 this carbon-14 makes its way makes its way it's constantly being formed it makes its way into oceans oceans it's already in the air but it completely mixes through the whole atmosphere oceans and the air and then it makes its way into plants and then it makes it way and plants are really just they're really just made out of that fixed carbon that carbon that was taken in gaseous form and put in to put into I guess you could say into kind of solid form put it into an living form that's what would pretty much is it puts it it gets put into plants and then it gets put into the things that eat the plants so that could be that could be us now why is this why is this even interesting I've just explained a mechanism where some of our body even though carbon-12 is the most common isotope some of our body while we're living gets made up of this carbon-14 thing well the interesting thing is the only time you can take in this carbon-14 is while you're alive while you're eating new things because as soon as you die and you get buried under the ground there's no way for the carbon-14 to become part of your tissue anymore because it's not being you're not eating anything with new carbon-14 and what's interesting here is once you die you're not going to get any new carbon-14 and that carbon-14 that you did have at your death is going to so the carbon-14 that you did have is going to decay is going to decay via beta decay and we learn about this back into back into nitrogen-14 so the kind of this process reverses so it'll decay back into nitrogen-14 and beta decay you emit an electron and an electron antineutrino and we'll go into the details of that but essentially what you have happening here is you have one of the neutrons is turning into a proton and emitting and emitting this stuff in the process now why is this interesting so I just said while you're living you have kind of straight-up carbon-14 as soon as and carbon-14 is constantly doing this decay thing but what's interesting is as soon as you die and and and you're not ingesting any more plants or any or breathing from the atmosphere if you are a plant or fixing from the atmosphere and this even applies to plants once the plant dies it's no longer taking in carbon dioxide from the atmosphere and turning it into new tissue the carbon-14 in that tissue gets frozen and this carbon-14 does this decay over at a specific rate at a specific rate and then you can use that rate to actually determine how long ago that thing must have died so the rate the rate at which this happens so the rate of carbon-14 decay is essentially half disappears half gone half gone in roughly five thousand seven hundred and thirty five thousand and seven hundred and thirty years and this is actually called a half-life and we talked about it in other videos this is called half-life and I want to be clear here you don't know which half of its gone it's a probabilistic thing you can't just say oh all of the carbon-14 s on the left are going to decay in all the carbon-14 s on the right aren't going to decay in that 5730 years what's essentially saying is any given carbon-14 atom has a 50% chance of decaying into nitrogen-14 in 5730 years so over the course of 5,730 years roughly half of them will have decayed now why is that why is that interesting well if you know that all living things have a certain proportion of carbon-14 in their in their in their tissue it's kind of part of what makes them up and then if you were to find some bone let's just some say find some bone right here that you know you dig it up on some type of archaeology dig and you say hey that bone has 1/2 the carbon-14 1/2 the carbon-14 of all the living things that you see right now so you could be a pretty reasonable estimate to say well that thing must be 5730 years old even better maybe you you dig a little deeper and you find another bone maybe you find another bone maybe a couple of feet even deeper and you say wow you know this thing right over here has 1/4 the carbon-14 this has 1/4 the carbon-14 that I would expect to find in something living so how old is this well if only has one 4th the carbon-14 it must have gone through two half-lives after one half-life it would have had it would have 1/2 the carbon and then after another half-life half of that also turns into a nitrogen 14 and so this would involve two half-lives two half-lives which is the same thing as two times five thousand and seven hundred thirty years or you would say that this thing is this thing is what this thing is ten thousand you would say this thing is eleven thousand four hundred and sixty years old give or take
Biology is brought to you with support from the Amgen Foundation