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 either later in this
video or in future videos we'll talk about
how it's actually used to date things, how we
use it actually figure out that that bone is
12,000 years old, or 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. And 78%, the most abundant
element in our atmosphere is nitrogen. It is 78% nitrogen. And I'll write nitrogen. Its symbol is just N.
And it has seven protons, and it also has seven 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, 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 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 rays. They're cosmic particles. You can 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. 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 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-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. 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 this number 14
doesn't go down to 13 because it replaces
it with itself. So this still stays at 14. And now since it
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 in another color. So plus. And a proton that's
just flying around, you could call that hydrogen 1. And it can gain an
electron some ways. If it doesn't gain
an electron, 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 atmosphere, not in huge quantities, but
in reasonable quantities. So let me write this down. Constant formation. And let me be very clear. Let's look at the
periodic table over here. So carbon by definition
has six protons, but the typical isotope, the
most common isotope of carbon is carbon-12. So carbon-12 is the most common. So most of the carbon in
your body is carbon-12. But what's interesting
is that a small fraction of carbon-14 forms,
and then 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. When people talk
about carbon fixation, they're 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, 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-- 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 gaseous
form and put into, I guess you could say,
into kind of a solid form, put it into a living form. That's what wood pretty much is. It gets put into
plants, and then 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's no way for the carbon-14 to
become part of your tissue anymore 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 you're death is going to decay
via beta decay-- and we learned about this--
back into nitrogen-14. So kind of this
process reverses. So it'll decay back into
nitrogen-14, and in beta decay you emit an electron and
an electron anti-neutrino. I won't 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 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. And carbon-14 is constantly
doing this decay thing. But what's
interesting is as soon as you die and you're
not ingesting anymore plants, or breathing from the
atmosphere if you are a plant, or fixing from the atmosphere. And this even applies to plants. Once a 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've died. So the rate at
which this happens, so the rate of carbon-14 decay,
is essentially half disappears, half gone, in
roughly 5,730 years. And this is actually
called a half life. And we talk about
in other videos. This is called a half life. And I want to be clear here. You don't know which
half of it's gone. It's a probabilistic thing. You can't just say all 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 that 5,730 years. What it's essentially saying
is any given carbon-14 atom has a 50% chance of
decaying into nitrogen-14 in 5,730 years. So over the course of 5,730
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 were to
find some bone-- let's just say find some
bone right here that you dig it up on some
type of archaeology dig. And you say, hey, that bone has
one half the carbon-14 of all the living things that
you see right now. It would be a pretty
reasonable estimate to say, well, that thing
must be 5,730 years old. Even better, maybe you
dig a little deeper, and 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
that I would expect to find in
something living. So how old is this? Well, if it only has
1/4 the carbon-14 it must have gone
through two half lives. After one half life, it would
have had 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, which is the same thing
as 2 times 5,730 years. Or you would say that
this thing is what? You'd say this thing is 11,460
years old, give or take.