Carbon 14 dating 1 | Life on earth and in the universe | Cosmology & Astronomy | Khan Academy

Summary

This educational video delves into the fascinating process of carbon-14 formation in Earth's atmosphere, initiated by cosmic particles colliding with nitrogen-14. It explains how carbon-14 integrates into living organisms through the food chain and remains constant while alive. Post-mortem, the carbon-14 decays into nitrogen-14 at a predictable rate, with a half-life of 5,730 years. This decay rate is the cornerstone of radiocarbon dating, allowing scientists to determine the age of ancient remains by measuring the residual carbon-14.

Takeaways

• 🌏 The Earth's atmosphere is primarily composed of nitrogen, which makes up about 78% of its volume.
• ☀️ Cosmic rays, which are high-energy particles from space, interact with nitrogen in the atmosphere, leading to the formation of neutrons.
• 💥 Neutrons can collide with nitrogen-14 atoms, replacing a proton and creating carbon-14 through a process known as cosmic ray spallation.
• 🔄 Carbon-14 is continuously produced in the atmosphere and integrates into the carbon cycle, becoming part of living organisms through consumption of plants and animals.
• 🌿 Plants absorb carbon dioxide, including carbon-14, from the atmosphere and convert it into organic tissue through photosynthesis.
• 🍃 Once a plant or animal dies, it stops taking in new carbon, including carbon-14, and the existing carbon-14 in its tissues begins to decay.
• 🕰️ Carbon-14 has a half-life of approximately 5,730 years, during which half of its atoms decay into nitrogen-14 through beta decay.
• 🦴 The decay rate of carbon-14 can be used to estimate the age of once-living materials, such as bones or wood, by measuring the remaining carbon-14 in the sample.
• 🔍 By comparing the amount of carbon-14 in a sample to that in living organisms, scientists can determine the time elapsed since the organism's death, providing a dating method for archaeological and geological specimens.

Q & A

What is the most abundant element in Earth's atmosphere?

The most abundant element in Earth's atmosphere is nitrogen, which makes up about 78% of the atmosphere.

What is an isotope and how does it relate to nitrogen-14?

An isotope is a version of an element with a different number of neutrons. Nitrogen-14 is the most typical isotope of nitrogen, having seven protons and seven neutrons, giving it an atomic mass of approximately 14.

How do cosmic rays contribute to the formation of carbon-14?

Cosmic rays, which are high-energy particles from space, interact with nitrogen in the Earth's atmosphere, causing reactions that can lead to the formation of neutrons. These neutrons can then collide with nitrogen-14, replacing a proton and forming carbon-14.

What happens when a neutron collides with nitrogen-14 in the atmosphere?

When a neutron collides with nitrogen-14, it can replace one of the protons in the nitrogen nucleus, turning it into carbon-14. This process leaves the atomic mass number unchanged at 14, but changes the element from nitrogen to carbon.

Why is carbon-14 important for dating ancient materials?

Carbon-14 is important for dating because it is constantly formed in the atmosphere and incorporated into living organisms. Once an organism dies, it stops taking in new carbon, and the carbon-14 it contains begins to decay at a known rate, allowing scientists to estimate the time of death based on the remaining carbon-14.

How does carbon-14 get into living organisms?

Carbon-14 gets into living organisms through the food chain. It combines with oxygen to form carbon dioxide, which is then absorbed by plants. Animals, including humans, consume these plants, incorporating the carbon-14 into their tissues.

What is the half-life of carbon-14, and how is it used in radiocarbon dating?

The half-life of carbon-14 is approximately 5,730 years, which means that half of the carbon-14 atoms in a sample will decay back into nitrogen-14 within that time frame. By measuring the remaining carbon-14 in a sample, scientists can estimate how long ago the organism died.

What happens to the carbon-14 in an organism after it dies?

After an organism dies, it no longer takes in new carbon-14. The carbon-14 that was present at the time of death begins to decay into nitrogen-14 through beta decay, a process that involves the emission of an electron and an electron anti-neutrino.

How can the decay of carbon-14 help determine the age of an archaeological find?

By measuring the amount of carbon-14 remaining in an archaeological find, scientists can estimate its age. For example, if a bone has half the carbon-14 of a living organism, it is roughly 5,730 years old, assuming one half-life has passed. If it has a quarter of the carbon-14, it has likely been through two half-lives, suggesting an age of approximately 11,460 years.

Why is it important to understand the concept of half-life in the context of carbon-14 dating?

Understanding the concept of half-life is crucial for carbon-14 dating because it provides a predictable decay rate for carbon-14. This allows scientists to use the remaining carbon-14 in a sample to calculate the time elapsed since the organism's death, which is the basis for radiocarbon dating.

Outlines

00:00

🌏 Introduction to Carbon-14 Formation

The paragraph introduces the concept of carbon-14 and its formation process. It begins by discussing the Earth's atmosphere, which is composed of 78% nitrogen, symbolized as N with seven protons and seven neutrons. Cosmic rays, which are high-energy particles from the sun, interact with the atmosphere and can transform nitrogen into carbon-14. This occurs when a cosmic ray particle collides with a nitrogen atom, replacing a proton with a neutron, thus creating carbon-14. The paragraph also explains isotopes and how carbon-14 is formed in a non-typical reaction. It emphasizes that carbon-14 is constantly being generated in the atmosphere and can combine with oxygen to form carbon dioxide, which is then incorporated into living organisms through the process of carbon fixation.

05:03

🌿 Carbon-14 in Living Organisms and Radiocarbon Dating

This paragraph delves into the significance of carbon-14 in living organisms and its application in radiocarbon dating. It explains that carbon-14 is taken up by plants and subsequently by animals that consume these plants, with the carbon-14 being integrated into their tissues. The unique aspect of carbon-14 is that it is only absorbed while an organism is alive. Once an organism dies, it stops incorporating new carbon-14, and the existing carbon-14 in its tissues begins to decay back into nitrogen-14 through a process called beta decay. The decay rate of carbon-14 is such that half of it decays every 5,730 years, which is known as its half-life. This decay rate is used to estimate the age of organic materials by measuring the remaining carbon-14. The paragraph illustrates how the amount of carbon-14 in a sample can be used to determine its age, with a sample having half the carbon-14 of a living organism being approximately 5,730 years old, and a sample with a quarter of the carbon-14 being roughly 11,460 years old.

Keywords

💡Carbon-14

Carbon-14, often denoted as ^14C, is a radioactive isotope of carbon that is used to determine the age of an object containing organic material by measuring how much of the carbon has decayed. In the video, it is explained that carbon-14 is formed in the Earth's atmosphere when cosmic rays interact with nitrogen-14, and it is incorporated into living organisms through the food chain. The video emphasizes the importance of carbon-14 in radiocarbon dating, which is a method to determine the age of an object by calculating the amount of carbon-14 remaining in the sample.

💡Cosmic Rays

Cosmic rays are high-energy particles originating from outer space that travel at nearly the speed of light. In the script, cosmic rays are described as particles, which can be single protons or alpha particles, that enter the Earth's atmosphere and interact with nitrogen atoms, leading to the formation of neutrons and subsequently carbon-14. This process illustrates the role of cosmic rays in creating the conditions necessary for the production of carbon-14.

💡Isotope

An isotope is a variant of a chemical element that has a different number of neutrons in its nucleus, resulting in a different atomic mass. The video explains isotopes in the context of nitrogen and carbon, where nitrogen-14 and carbon-14 are isotopes of nitrogen and carbon, respectively. The video script uses the term to contrast the stable nitrogen-14, which is abundant in the atmosphere, with the radioactive carbon-14, which is formed through cosmic ray interactions.

💡Nitrogen-14

Nitrogen-14 is the most common isotope of nitrogen, making up about 99.6% of the nitrogen on Earth. It has seven protons and seven neutrons, giving it an atomic mass of approximately 14. In the video, nitrogen-14 is highlighted as the starting point for the formation of carbon-14 when it interacts with a neutron resulting from a cosmic ray collision, leading to one of its protons being replaced by a neutron.

💡Beta Decay

Beta decay is a type of radioactive decay in which a proton is transformed into a neutron, or vice versa, within an atomic nucleus, resulting in the emission of an electron or positron. The video script describes how carbon-14 undergoes beta decay, where one of its neutrons turns into a proton, emitting an electron and an electron anti-neutrino in the process, thus transforming back into nitrogen-14. This decay process is crucial for the radiocarbon dating method discussed in the video.

💡Half-Life

The half-life of a radioactive isotope is the time required for half of the atoms of the isotope to decay. For carbon-14, this half-life is approximately 5,730 years. The video script explains the concept of half-life in the context of carbon-14 decay, which is fundamental to radiocarbon dating. The video uses the half-life to illustrate how the remaining amount of carbon-14 in a sample can be used to estimate its age.

💡Radiocarbon Dating

Radiocarbon dating is a method used to determine the age of an object containing organic material by measuring the amount of carbon-14 it contains and calculating how much has decayed. The video script introduces radiocarbon dating as a technique that relies on the constant formation of carbon-14 in the atmosphere and its uptake by living organisms. Once an organism dies, it stops incorporating carbon-14, and the remaining carbon-14 decays, allowing scientists to estimate the time of death based on the remaining carbon-14.

💡Atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet, which includes nitrogen, oxygen, and trace amounts of other gases. In the video, the atmosphere is described as the site where cosmic rays interact with nitrogen to form carbon-14. The script emphasizes the atmosphere's role in the carbon cycle and the incorporation of carbon-14 into living organisms.

💡Neutron

A neutron is a subatomic particle found in the nucleus of an atom, with no net electric charge and a mass similar to that of a proton. The video script explains that neutrons are formed when cosmic rays collide with atoms in the Earth's atmosphere. Neutrons are crucial for the formation of carbon-14, as they can replace a proton in nitrogen-14, leading to the creation of carbon-14.

💡Carbon Dioxide

Carbon dioxide, with the chemical formula CO2, is a molecule composed of one carbon atom double-bonded to two oxygen atoms. In the video, carbon dioxide is mentioned as a molecule that can contain carbon-14. Once carbon-14 combines with oxygen to form carbon dioxide, it becomes part of the carbon cycle, being absorbed by plants and eventually incorporated into the tissues of animals that consume those plants.

💡Carbon Fixation

Carbon fixation is the process by which living organisms convert carbon dioxide from the atmosphere into organic compounds, such as glucose, through photosynthesis or chemosynthesis. The video script describes carbon fixation as a process where plants use light energy from the sun to convert gaseous carbon, including carbon-14, into organic tissue, which is a fundamental part of the carbon cycle and the basis for life on Earth.

Highlights

Introduction to the concept of carbon-14 and its role in dating objects.

Carbon-14 is formed in the Earth's atmosphere through interactions with cosmic rays.

Nitrogen-14 is the most abundant isotope in the atmosphere, making up 78%.

Cosmic particles can transform nitrogen-14 into carbon-14 by replacing a proton with a neutron.

Carbon-14 is constantly formed in the atmosphere, but not in large quantities.

Carbon-14 combines with oxygen to form carbon dioxide, which is then incorporated into living organisms.

Plants fix carbon dioxide from the atmosphere, including carbon-14, into their tissues.

Carbon-14 is taken up by animals that consume plants, including humans.

Carbon-14 is only incorporated into an organism's tissues while it is alive.

Upon death, the carbon-14 in an organism's tissues begins to decay back into nitrogen-14.

The decay of carbon-14 occurs via beta decay, emitting an electron and an electron anti-neutrino.

The half-life of carbon-14 is approximately 5,730 years, during which half of the carbon-14 decays.

The decay rate of carbon-14 can be used to estimate the age of ancient objects.

By measuring the remaining carbon-14 in a sample, one can estimate how long ago the organism died.

The method of carbon-14 dating is based on the principle that living organisms maintain a constant ratio of carbon-14 to carbon-12.

Carbon-14 dating is a probabilistic method, with each atom having a 50% chance of decaying within its half-life.

The video discusses the practical application of carbon-14 dating in archaeology and other fields.

Transcripts

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.