Carbon 14 Dating 1 Carbon 14 Dating 1
Carbon 14 Dating 1
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- What I want to do with this video is to kind of introduce you to the idea of
- how carbon 14 comes about
- and how it gets into all living things
- and then either later this video or in future videos we will talk about how
- it's actually used to date things.
- How we use it to actually to figure out that that bone is 12000 years old
- or that that person died 18000 years ago whatever it might be.
- So let me draw the earth
- the surface of the earth is like that
- it is just a little section of the surface of the earth
- earth and then we have the atmosphere of the earth
- I'll draw that in yellow.
- So you have the earth's atmosphere right over here.
- 78% - the most abundant
- element in out atmosphere is nitrogen
- I'll write nitrogen, 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
- this is the most typical isotope of nitrogen
- we talk about the word isotope in the chemistry playlist.
- An Isotope.
- 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.
- Anyway, we have our atmosphere
- then coming from our sun
- we have what's commonly called cosmic rays
- but they are actually not rays
- they are cosmic particles
- they are mainly...you could view them just as a single proton
- 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 are few electrons.
- And they are going to come in
- and they are going to bumb into things in our atmosphere
- they are actually going to form neutrons
- and we'll show a
- neutron with a lowercase n
- and 1 for its mass number
- and then we don't write anything as it has no protons down here
- like we had for nitrogen with 7 protons
- so it's not an element
- it's a subatomic particle.
- But you have these neutrons formed
- and every now and then
- and i'm not just.. let's be clear.. this isn't a typical reaction
- but every now and then
- one of those neutrons
- will bump into one of the nitrogen 14's in just the right way
- so that it bumps off one of the protons in the nitrogen
- and essentially replaces that proton with itself
- Let me make it clear.
- So it bumps off one of the protons,
- so instead of 7 protons
- we now have 6 protons
- but this number 14 does not go down to 13
- because it replaces it with itself
- so this still stays at 14
- and now since it only has 6 protons
- this is no longer nitrogen by definition
- this is now carbon
- and that proton that was bumped off
- just kind of gets emitted
- let me do this in another color
- plus a proton that just flying around
- you could call that hydrogen,
- hydrogen 1,
- and it can gain an electron someways
- if it doesn't gain an electron
- it is just a hydrogen ion
- a positive ion
- either way or a hydrogen nucleus
- but this process - 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 that
- this is constantly being formed in our atmosphere
- not in huge quantities but in reasonable quantity.
- Let me write this down.
- ...constantly being formed...
- Constant Formation
- And what happens is...
- and let me be very clear
- Let's look at the periodic table over here.
- Typical carbon:
- Carbon by definition has 6 protons
- but the typical isotope,
- the most common isotope of carbon
- is carbon 12.
- So...carbon 12.
- Carbon 12 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 this carbon 14 can then also combine 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, so when people talk about
- carbon fixation they are really talking about
- using mainly light energy from the sun to take gaseous carbon
- and turn it into actual kind of organic tissue.
- And so this carbon 14 makes it's way
- it's constantly being formed
- it makes its way into
- 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 plants are really just
- made out of that fixed carbon
- that carbon that was taken in in gasous form
- and put into, I guess you could say,
- into a kind of solid form,
- put into a living form.
- That's what wood pretty much is.
- It gets put into plants
- and it gets put into the things that eat
- the plants. So that could be us.
- Now 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 is no way for the carbon 14
- to become part of your tissue any more
- because 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
- via beta decay.
- And we learned about this.
- back into nitrogen 14.
- So this process reverses.
- So it'll decay back into nitrogen 14.
- In beta decay you emit an electron
- and an electron anti-neutrino.
- I'm not going 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 this stuff
- in the process.
- Now why is this interesting?
- So I just said while your'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 you're not ingesting any more plants
- 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
- 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
- at which this happens, the rate of carbon 14
- decay is essentially: half disappears, half gone
- in roughly 5730 years.
- And this is acutally called a half-life.
- And we talk about it in other videos.
- This is called...a half life.
- And I want to be clear here:
- You don't know which half
- of it is 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
- and all the carbon 14's on the right aren't going to
- decay in 5730 years."
- What it'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 5730 years
- roughly half of them will have decayed.
- Now why is that interesting?
- Well, if you know that all living things
- have a certain proportion of carbon 14
- in their tissue as kind of part
- of what makes them up
- and then if you where to find some bones...
- Let's just say you find some bone.
- Right here.
- You dig it up on some type of
- archeology dig. And you say: "Hey, that bone
- has one half the carbon 14 of all the
- living things that you see right now."
- So you could...it would be a pretty reasonable
- estimate to say: "Well, that thing must
- be 5730 years old." Even better maybe you
- dig a little deeper and you find another bone
- Maybe you find another bone.
- Maybe a couple of meters even deeper.
- And you say: "Wow, this thing right over here has
- one fourth the carbon 14 that I would expect
- to find in something living."
- So how old is this?
- If it only has one fourth the carbon 14,
- it must have gone through two half-lives
- after one half-life, it would have had
- it would have one
- half the carbon and then after another half-life
- half of that also turns into nitrogen 14.
- And so this would involve two half-lives.
- Which is the same thing as two times 5730 years.
- Or you would say that this thing is what?
- ...this thing is ten thousand...11460 years old
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