Lesson video

Hello, my name's Dr.

George, and I'm going to be helping you with this lesson called, "Nuclear Decay." It's part of the unit, "Nuclear Physics." The outcome of this lesson is, "I can use a nuclear equation to represent alpha and beta decays, and describe the particles involved." Here are the key words, which I'll introduce as we go along.

You can come back to this slide anytime if you want to remind yourself of the meanings.

The lesson has three parts called alpha decay, beta-minus decay, and other types of nuclear decay.

So let's take a look at alpha decay.

An atomic nucleus can have too many or too few neutrons compared to its number of protons, and that makes it unstable.

Over time this nucleus can, at some point, decay, that is change its composition, to become more stable or completely stable.

There are three common methods of nuclear decay.

They're called alpha decay, beta decay and gamma decay.

And these symbols are the Greek letters, alpha, beta and gamma, the first three letters of the Greek alphabet.

Now, is this true or false? This nucleus, which has six protons and no neutrons, is more stable than this one, which has six protons and six neutrons.

And when you've decided, I'd also like you to think about how you know.

So answer the question why.

I'll wait five seconds when I ask a question, but if you need longer, press pause, and press play when you have your answer ready.

And the correct answer is false.

And it's because the nucleus on the left has protons that are closer together, and so the repulsive forces, the electrostatic forces between them, will be larger and that makes that nucleus more unstable.

Alpha decay involves a nucleus ejecting, sending out at high speed, two protons and two neutrons, and they're stuck together to form something that we call an alpha particle.

We can say that the parent nucleus emits an alpha particle and then becomes a new lighter nucleus, called a daughter nucleus.

Here's a way of writing that as a sort of equation.

Now the daughter nucleus has two fewer neutrons than the parent nucleus, but that doesn't affect which element it is.

But having two fewer protons does make it a different element from the parent nucleus.

The alpha particle, which is emitted during alpha decay, happens to be identical to a nucleus of helium-2, an isotope of helium that has two protons and two neutrons.

So that means we can represent alpha particles in two ways when we're writing them in equations, as we will do shortly.

Like this, using the alpha symbol or like this, using the element symbol for helium, He.

And remember what these two numbers mean when we're writing a nucleus.

The upper number is the nucleon number or mass number.

It's the total number of protons and neutrons.

The lower number is the atomic number or proton number It's the number of protons.

Alpha decay is most common for large nuclei.

For example, plutonium-240 is an isotope of plutonium, one of the types of nucleus of plutonium, and it decays by alpha particle emission.

We can write a nuclear decay equation, a kind of equation in symbols, which shows the changes that happen.

We start with plutonium.

It has 94 protons.

And then it decays into a new nucleus, a daughter nucleus, and an alpha particle.

You wouldn't be expected to know that the daughter nucleus has symbol U.

That's actually uranium.

But you would be able to work out its mass number and its atomic number.

The mass number decreases by four because it's lost four nucleons that went into the alpha particle.

And the atomic number decreases by two because the nucleus loses two protons.

Notice also, that another way of checking an equation like this is that the upper numbers on both sides have the same sum.

In this case, they add to 240, and the lower numbers also have the same sum.

They add to 94 on both sides Because the daughter nucleus has two fewer protons, there's a different number of protons from the parent, it's a different element.

And now a question for you.

Uranium-238 decays by alpha decay.

If one of these is the daughter nucleus, which one must it be? And although you don't know that the daughter nucleus is thorium with symbol Th, unless you checked the periodic table, you can know that C is the only possible answer because in A, the atomic number is not right.

It should decrease by two when an alpha particle is released.

This one is not actually an isotope of uranium, although it says it is, because it has an atomic number 90, and we can already see that uranium has atomic number 92.

The two numbers are correct though.

This one is wrong because the mass numbers should decrease by four in alpha emission and it hasn't decreased at all.

So C is right.

The daughter nucleus is of a different element from before.

It has a mass number that's lower by four, and it has atomic number lower by two.

Well done if you realised that.

Now large unstable nuclei, with too many protons, often emit an alpha particle, and that makes them become more stable.

Two protons and two neutrons are very tightly bound together by strong nuclear forces and can form as an alpha particle within a large nucleus.

And then sometimes that particle within the nucleus can be repelled strongly by the rest of the positively charged nucleus.

So strongly that it's emitted.

It travels away from the daughter nucleus at high speed.

Which of the following types of force pushes an alpha particle away from the daughter nucleus? This is a force between charged particles.

It's an electrostatic force.

The alpha particle and daughter nucleus are both positively charged, so they repel each other.

Now after an alpha particle has been emitted, the daughter nucleus often has excess energy, too much energy.

Its protons and neutrons are vibrating energetically.

And we describe the nucleus as being excited.

The excited nucleus can become stable again by emitting gamma radiation that reduces its energy.

And gamma radiation is a type of electromagnetic wave, just as light and radio and microwaves are, but it's a particularly high frequency, electromagnetic wave.

But we can model this gamma radiation as a particle.

That may seem strange, but it turns out that on these subatomic scales there's not such a clear difference between particles and waves.

So we can write the symbol for a gamma particle, or for gamma radiation, like this.

We use the Greek letter gamma and we use two zeros because the gamma radiation has no mass and it has no charge.

You may sometimes see gamma being represented, even in equations, without the two numbers, but it's better to show them to make things clear.

Decay equations that only involve gamma emission aren't usually written because there isn't that much happening.

There's no change in the number or type of particles in the parent nucleus, as you can see in this example.

The only difference between them is the amount of energy they have.

Thorium-234 decays by gamma emission.

Which of these must be the resulting nucleus? And C is the correct answer here because there's no change to the structure of the nucleus.

The number of nucleons, the number of protons, stays the same.

This one's wrong because it has a different mass number, which doesn't happen with gamma emission.

This one says it's uranium, but in fact it's still thorium because it has the same number of protons as before.

And this one has a different atomic number which it shouldn't have.

Gamma decay often follows after another decay.

And in the case of alpha decay followed by gamma decay, we could write a single equation for that.

We start with the parent nucleus, and after these decays, we have a daughter nucleus, an alpha particle and the gamma radiation.

By the way, in this equation, we could have used the symbol He instead of the alpha symbol for the alpha particle.

Now, here's an incorrect alpha decay equation.

Can you see what mistake has been made? The answer is that the symbols for the alpha and gamma particles are the wrong way round.

Gamma is zero zero, showing that it's got no mass or no nucleons and no charge.

And alpha is four two.

Here's a longer question for you.

An unstable large nucleus with too many protons, often decays by emitting an alpha particle, followed by gamma radiation.

Describe the process that occurs during this nuclear decay as shown by the equation and include as much detail as you can in your answer.

Press pause while you do this, and press play when you're ready to check your answer.

Here's an example answer.

The uranium-235 nucleus is unstable because it's large and has too many protons.

Within the nucleus, two protons and two neutrons form an alpha particle.

The positive charge on the rest of the nucleus repels the alpha particle, and it is pushed out of the nucleus at a very high speed.

The remaining nucleus, of thorium 231, is left in an excited state.

It emits gamma radiation to become an unexcited nucleus.

So well done if you included many of the same points.

So we've looked at alpha decay.

Now let's learn about beta-minus decay.

Another way the structure of an unstable nucleus can change is through something called beta-minus decay.

Neutrons are actually unstable if they're not very close to protons.

For example, a neutron that's on its own, will at some point, decay.

And when a neutron decays, it decays into a proton and a fast moving electron, which is known as a beta particle.

So that electron comes from where the neutron was.

It comes from the nucleus.

And beta particles can be represented in two different ways in nuclear decay equations.

Using the Greek letter beta or using an e for electron.

And look at the two numbers.

The mass number is zero.

Now the electron doesn't actually have zero mass, but it's very low, compared with a nucleon, and if you think of it as a nucleon number, well, an electron doesn't have any nucleons.

The lower number is the atomic number.

We sometimes think of it as the number of protons, but we can also think of it as the charge.

And an electron has charge minus one compared with a proton.

During beta-minus decay, a neutron that's not close enough to protons in the nucleus, decays.

It's too unstable.

And so we can represent what happens like this.

The electron's emitted at high speed and escapes the nucleus.

And then after beta-minus decay, there'll be one more proton than there was before and one fewer neutron because we have a proton in place of a neutron now.

And because of the change in the number of protons, the daughter nucleus is a different element from the parent nucleus.

Now a question.

Beta particles are electrons produced during nuclear decay.

Which of these are properties of electrons? Two of these are correct.

They have electric charge minus one and negative, and they're not a nucleon.

They're not a particle that is generally found in the nucleus.

That would be protons and neutrons.

Now during beta-minus decay, just one neutron changes.

Here's an example.

We start with this nucleus.

Let's say this neutron is unstable.

It's a bit too far from the nearest proton.

And then we get this nucleus.

The neutron has changed into a proton and a fast moving electron.

But there's no change in the total number of nucleons.

We had 14 before, there's 14 after, but before, six were protons and eight neutrons.

Now, seven protons and seven neutrons.

We have a proton instead of one of the neutrons.

Beta-minus decay is most likely to occur in a nucleus that has too many neutrons compared with the number of protons.

An example is carbon-14.

It's an isotope of carbon, one of the possible atoms of carbon that exist, but it has too many neutrons to be stable, so it undergoes beta-minus decay.

So here's a representation of a carbon-14 atom.

This is what happens.

It turns out that the daughter nucleus is nitrogen.

That's an atom with one more proton than carbon, and there's this beta particle emitted as well.

The mass number stays the same, as we've seen, and the atomic number, the proton number increases by one.

And again, you can check an equation like this by checking that the upper numbers add up to the same sum on each side, in this case 14.

And the lower numbers add to the same total on each side, in this case six.

Six is seven minus one.

And the reason why the daughter nucleus is no longer carbon, is because it has a different number of protons.

One more proton from before.

Technetium-99 decays by beta-minus decay.

If one of these is the daughter nucleus, which one must it be? And it has to be B.

D is wrong because there's been a change to the mass number, the nucleon number, which doesn't happen in beta-minus decay.

A is wrong because you don't get an isotope of the same element.

There's been a change to the number of protons.

And C is wrong because again, it shouldn't be the same element and the mass number should stay the same while the atomic number should increase by one.

And so B is correct.

You wouldn't know that that's ruthenium, unless you looked it up in the periodic table, but it's the only one of these options that could be correct.

Well done if you picked that one.

After a beta particle's been emitted, the resulting nucleus is often in an excited state, as we saw after alpha emission as well.

And again, the nucleus can lose this excess energy by emitting gamma radiation.

Here's an example.

So, here's the whole process shown in one equation.

The emission of a beta particle and then gamma radiation.

During beta-minus decay, what change to the electrical charge of the nucleus will there be? The charge changes by plus one because there's one more proton than there was before, and a proton has a charge of plus one.

Now, in this task, in each row I'd like you to pick one statement that correctly describes beta-minus decay.

So in the first row, there's no choice for you to make, but in the other rows you need to choose from two or three statements.

Press pause when you do this and press play when you've finished.

I'll show you the correct choices highlighted.

So neutrons are unstable when they're not very close to protons.

Neutrons are likely to be further from protons in a neutron-rich nucleus, a nucleus that has plenty of neutrons, not so many protons.

An unstable neutron is likely to decay.

Nuclear decay is unpredictable.

It doesn't happen immediately.

It will happen at some point if you wait long enough.

So it's likely to decay.

A beta particle is emitted that pushes the nucleus backwards.

You can imagine that if one fast moving part of an object breaks away, the rest of the object moves in the other direction.

It certainly doesn't destroy the nucleus.

And a beta particle is a high speed electron, created in the nucleus.

It doesn't come from outside the nucleus.

It's not one of the usual outer electrons in an atom.

It's created when the decay happened.

And the atom is now of a different element because it now has a different number of protons.

Now let's take a look at other types of nuclear decay.

So alpha and beta-minus decay are the most common forms of nuclear decay, but there are a few others.

And these include neutron emission where a neutron is ejected from the nucleus.

And, as with alpha and beta-minus decay, is often followed by a gamma emission.

Neutrons are represented like this in nuclear decay equations.

N for neutron, nucleon number one, atomic number zero.

It has no charge.

A nucleus with too many neutrons compared to protons may eject a neutron from the nucleus.

For example, this beryllium nucleus, as you can see it has a lot of neutrons compared with its number of protons, but it can emit a neutron and reduce its number of neutrons by one.

You can see that the mass number, the nucleon number's decreased by one, but the proton number hasn't changed.

The neutron emission is followed by gamma emission if the nucleus is left in an excited state and notice that what we get is a different isotope of the same element.

During neutron emission, what change to the electrical charge of the nucleus will there be? And there's just a loss of a neutron, which has no charge.

So there is no change in the charge on the nucleus.

There are still the same number of protons as there were before.

Now, three equations here.

Can you complete these nuclear decay equations by filling in the missing numbers and symbols? And make sure you read the description as well as looking at the equation for each question.

Press pause while you do this and press play when you're ready to check your answers.

And here are the correct equations.

In question one, you needed to fill in the missing alpha particle for the alpha decay and also the missing mass number for the daughter nucleus, which is 237, because it's four less than the mass number of the parent nucleus.

The daughter is called Neptunium, by the way.

In question two, you had to fill in the missing beta particle.

Beta-minus has mass number zero and atomic number minus one.

And you also had to fill in the mass number of the parent nucleus, that's potassium, which is the same as the daughter nucleus because the number of nucleons doesn't change in beta decay.

But in beta decay, one of the neutrons changes into a proton.

So the daughter nucleus has proton number 20, one more than the parent nucleus.

And finally, in the neutron emission, you had to fill in the n, the symbol for the neutron, and you had to fill in the missing daughter nucleus.

There's no change to the number of protons, so the daughter nucleus is of the same element, helium, as the parent nucleus, but the mass number has decreased by one because one neutron has been lost from the nucleons.

Well done if you got most of this right or all of this right.

And now we're at the end of the lesson.

So here's a summary.

Unstable nuclei can decay in different ways, as shown by nuclear decay equations.

In alpha decay an alpha particle forms and is ejected from the nucleus.

And you can see that in this equation.

In beta-minus decay, a neutron converts to a proton and a fast moving electron, a beta particle, and the beta particle is emitted from the nucleus.

In gamma emission, excess energy is emitted from the nucleus as gamma radiation.

And in neutron emission, a single neutron is ejected from the nucleus, as shown here.

Well done for working through this lesson.

I hope you found it interesting, and I hope to see you again in a future lesson.

Bye for now.