Carbon 14 Dating 1

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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
give or take.