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What I want to do with this video is to kind of introduce you to the idea of

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how carbon 14 comes about

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and how it gets into all living things

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and then either later this video or in future videos we will talk about how

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it's actually used to date things.

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How we use it to actually to figure out that that bone is 12000 years old

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or that that person died 18000 years ago whatever it might be.

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So let me draw the earth

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the surface of the earth is like that

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it is just a little section of the surface of the earth

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earth and then we have the atmosphere of the earth

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I'll draw that in yellow.

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So you have the earth's atmosphere right over here.

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78% - the most abundant

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element in out atmosphere is nitrogen

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I'll write nitrogen, its symbol is just N

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and it has 7 protons and it also has 7 neutrons

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so it has an atomic mass of roughly 14

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this is the most typical isotope of nitrogen

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we talk about the word isotope in the chemistry playlist.

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An Isotope.

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The protons define what element it is.

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But this number up here can change

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depending on the number of neutrons you have

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so the different versions of a given element

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- those are each called isotopes -

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I just view them in my head as versions of an element.

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Anyway, we have our atmosphere

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then coming from our sun

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we have what's commonly called cosmic rays

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but they are actually not rays

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they are cosmic particles

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they are mainly...you could view them just as a single proton

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which is the same thing as a hydrogen nucleus

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they can also be

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alpha particles which is the same thing as a helium nucleus

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and there are few electrons.

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And they are going to come in

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and they are going to bumb into things in our atmosphere

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they are actually going to form neutrons

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and we'll show a

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neutron with a lowercase n

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and 1 for its mass number

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and then we don't write anything as it has no protons down here

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like we had for nitrogen with 7 protons

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so it's not an element

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it's a subatomic particle.

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But you have these neutrons formed

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and every now and then

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and i'm not just.. let's be clear.. this isn't a typical reaction

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but every now and then

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one of those neutrons

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will bump into one of the nitrogen 14's in just the right way

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so that it bumps off one of the protons in the nitrogen

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and essentially replaces that proton with itself

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Let me make it clear.

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So it bumps off one of the protons,

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so instead of 7 protons

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we now have 6 protons

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but this number 14 does not go down to 13

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because it replaces it with itself

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so this still stays at 14

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and now since it only has 6 protons

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this is no longer nitrogen by definition

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this is now carbon

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and that proton that was bumped off

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just kind of gets emitted

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let me do this in another color

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plus a proton that just flying around

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you could call that hydrogen,

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hydrogen 1,

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and it can gain an electron someways

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if it doesn't gain an electron

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it is just a hydrogen ion

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a positive ion

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either way or a hydrogen nucleus

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but this process - once again it's not a typical process

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but it happens every now and then - this is how carbon 14 forms.

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So this right here is carbon 14.

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You can essentially view it

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as a nitrogen 14 where

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one of the protons is replaced with a neutron.

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And what's interesting about this is that

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this is constantly being formed in our atmosphere

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not in huge quantities but in reasonable quantity.

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Let me write this down.

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...constantly being formed...

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Constant Formation

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And what happens is...

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and let me be very clear

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Let's look at the periodic table over here.

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Typical carbon:

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Carbon by definition has 6 protons

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but the typical isotope,

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the most common isotope of carbon

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is carbon 12.

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So...carbon 12.

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Carbon 12 is the most common.

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So most of the carbon in your body is carbon 12

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but what's interesting is,

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is that a small fraction of carbon 14 forms

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and this carbon 14 can then also combine with oxygen

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to form carbon dioxide.

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And then that carbon dioxide

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gets absorbed into the rest of the atmosphere into, our oceans

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it can be fixed by plants, so when people talk about

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carbon fixation they are really talking about

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using mainly light energy from the sun to take gaseous carbon

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and turn it into actual kind of organic tissue.

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And so this carbon 14 makes it's way

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it's constantly being formed

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it makes its way into

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oceans...it's already in the air but it

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completely mixes through the whole atmosphere

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...oceans and the air...

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and then it makes its way into

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plants. And plants are really just

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made out of that fixed carbon

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that carbon that was taken in in gasous form

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and put into, I guess you could say,

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into a kind of solid form,

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put into a living form.

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That's what wood pretty much is.

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It gets put into plants

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and it gets put into the things that eat

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the plants. So that could be us.

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Now why is this even interesting?

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I've just explained a mechanism where

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some of our body, even though carbon 12

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is the most common isotope,

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some of our body, while we're living

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gets made up of this carbon 14 thing.

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Well, the interesting thing is:

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the only time you can take in this carbon 14

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is while you're alive,

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while you're eating new things.

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Because as soon as you die

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and you get buried under the ground

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there is no way for the carbon 14

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to become part of your tissue any more

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because you're not eating anything

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with new carbon 14.

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And what's interesting here

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is: once you die, you're not going to

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get any new carbon 14, and that carbon 14

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that you did have at your death

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is going to...

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So the carbon 14 that you did have

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is going to decay

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via beta decay.

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And we learned about this.

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back into nitrogen 14.

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So this process reverses.

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So it'll decay back into nitrogen 14.

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In beta decay you emit an electron

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and an electron anti-neutrino.

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I'm not going into the details of that.

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But essentially what you have happening here is:

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You have one of the neutrons

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is turning into a proton

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and emitting this stuff

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in the process.

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Now why is this interesting?

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So I just said while your're living

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you have kind of straight up carbon 14.

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As soon as...and carbon 14 is constantly

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doing this decay thing.

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But what's interesting is as soon as you die

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and you're not ingesting any more plants

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or breathing from the atmosphere if you

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are a plant or fixing from the atmosphere

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and this even applies to plants:

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once the plant dies, it's no longer

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taking in carbon dioxide from the atmosphere

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and turning it into new tissue

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the carbon 14 in that tissue gets "frozen"

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and this carbon 14 does this decay

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at a specific rate. And then you can use that rate

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to actually determine how long ago that thing

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must have died. So the rate

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at which this happens, the rate of carbon 14

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decay is essentially: half disappears, half gone

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in roughly 5730 years.

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And this is acutally called a half-life.

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And we talk about it in other videos.

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This is called...a half life.

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And I want to be clear here:

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You don't know which half

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of it is gone. It's a probabilistic thing.

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You can't just say: "Oh, all of the carbon 14's

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on the left are going to decay

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and all the carbon 14's on the right aren't going to

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decay in 5730 years."

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What it's essentially saying is: Any given

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carbon 14 atom has a 50% chance of

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decaying into nitrogen 14

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in 5730 years. So over the course of 5730 years

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roughly half of them will have decayed.

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Now why is that interesting?

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Well, if you know that all living things

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have a certain proportion of carbon 14

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in their tissue as kind of part

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of what makes them up

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and then if you where to find some bones...

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Let's just say you find some bone.

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Right here.

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You dig it up on some type of

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archeology dig. And you say: "Hey, that bone

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has one half the carbon 14 of all the

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living things that you see right now."

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So you could...it would be a pretty reasonable

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estimate to say: "Well, that thing must

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be 5730 years old." Even better maybe you

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dig a little deeper and you find another bone

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Maybe you find another bone.

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Maybe a couple of meters even deeper.

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And you say: "Wow, this thing right over here has

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one fourth the carbon 14 that I would expect

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to find in something living."

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So how old is this?

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If it only has one fourth the carbon 14,

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it must have gone through two half-lives

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after one half-life, it would have had

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it would have one

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half the carbon and then after another half-life

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half of that also turns into nitrogen 14.

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And so this would involve two half-lives.

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Which is the same thing as two times 5730 years.

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Or you would say that this thing is what?

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...this thing is ten thousand...11460 years old

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give or take.