Lesson video

Hello, my name's Dr.

George, and this lesson is called "Unstable Nuclei." It's part of the unit Nuclear Physics.

The outcome of the lesson is I can explain why some nuclei are stable and some are unstable.

Here are the keywords for the lesson.

I'll explain them as we go along, but you can come back to this slide anytime if you need to remind yourself of any of the meanings.

The lesson has three parts.

They're called atomic structure, elements and isotopes of elements.

You may already know that an atom can contain three types of particle.

Electrons are found around the nucleus, and they're negatively charged.

They have a very small mass compared to the other particles.

Protons and neutrons are found in the nucleus, and they have similar masses to each other.

Protons have positive charge, and neutrons are uncharged.

And this diagram is not to scale.

In fact, the nucleus is much, much smaller compared with the size of the atom.

Here's a table summarising the properties of these three subatomic particles that atoms are made of.

It shows the locations of each of them and the relative mass.

So if we count a proton as having mass one, then neutron has almost exactly the same mass, whereas an electron has only 1/2,000 of the mass of either of those.

And the charges are shown on the right.

Protons and neutrons can both be described as nucleons.

Nucleon means particle that's found in the nucleus.

Which of these are properties of protons? And when asked a question, I'll wait five seconds, but if you need longer, press pause, and press play when you have your answer ready.

And here are the correct answers.

Protons each have a charge of plus one, and they are a type of nucleon because they are particles found in the nucleus.

Well done if you picked all three of those options.

All protons have the same positive electric charge and objects that have the same type of charge repel each other.

The forces that act between charged objects are called electrostatic forces.

And the repulsive force becomes larger when the particles are closer together.

In a nucleus, protons are packed closely together, and so they repel each other strongly in all directions.

If this were the only force acting, the nucleus would break apart, but there's another force that acts between all protons and neutrons.

It's an attractive force and it's known as the strong nuclear force.

It attracts all of the nucleons together.

So now a question, which of the following forces causes protons to repel each other in the nucleus? And the correct answer is electrostatic force.

Protons all have a positive charge, so they all repel each other because of electrostatic forces.

The strong nuclear force is the one that holds nucleons together.

Now here's a gap-fill exercise for you.

These sentences describe the forces inside the nucleus of an atom of carbon.

Can you fill each gap using either electrostatic or strong nuclear? Press pause when you do this, and press play when you're ready to check your answers.

So here are the answers.

First of all, protons repel each other with the electrostatic force.

Protons attract each other with the strong nuclear force.

There wouldn't be an electrostatic force of attraction between two like charges.

Protons attract neutrons with the strong nuclear force.

And neutrons attract each other with the strong nuclear force.

So all particles in the nucleus experience the strong nuclear force and it attracts them together.

In the carbon nucleus, the strong nuclear force holding the protons together is stronger than the electrostatic force pushing them apart.

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

And now let's move on to the second part of the lesson, elements.

There are more than 100 different elements.

Some of these are common, for example, silicon, which makes up about 28% of the Earth's crust, although it's rare to see it in pure form like this.

And some of the elements are very rare on Earth.

For example, gold makes up only.

004% of the Earth's crust.

The structure of a particular atom of an element can be described using two numbers and one or two letters.

So the element symbol is written here.

This one is C for carbon.

And then there's the mass number.

It's the upper number here.

It's the total number of nucleons, protons plus neutrons, in the nucleus of an atom.

It's also known as the nucleon number for that reason.

And the other number is the atomic number, also called the proton number.

And it's simply the number of protons in the nucleus of that atom.

The more accurate terms for these two numbers are nucleon number and atomic number, but mass number and atomic number are often used instead.

Every atom is electrically neutral.

It's not positively charged or negatively charged overall.

That's because it has the same number of negative charges as positive charges, and that means it has the same number of electrons as protons.

So the atomic number is equal to the number of protons, as we've seen, but also equal to the number of electrons around the nucleus, because those two are equal to each other.

In the aluminium atom shown here, there are 13 protons, and that means there are also 13 electrons.

Now, how many electrons does the atom shown here have? The correct answer is 16, because the number of electrons is the same as the number of protons, which equals the atomic number, the lower number, 16.

The number of neutrons is not shown on an atomic symbol, but it can be found by subtracting the atomic number, the number of protons, from the mass number, the total number of nucleons.

This is the symbol for an iron atom, Fe is the chemical symbol for iron.

And we can find the number of neutrons in the nucleus of this atom by subtracting 26 from 56.

Number of nucleons, 56, minus the number of protons, 26, gives 30 as the number of neutrons.

How many neutrons are in the nucleus of the oxygen atom shown here? And the correct answer is nine, because the mass number, 17, is the number of nucleons, take away the atomic number, eight, which is the number of protons, gives us nine as the number of neutrons.

Another question, how many protons, neutrons and electrons are there in the atom of chlorine shown here? The answer is b.

There are 17 protons, as shown by the atomic number, and there must be an equal number of electrons because an atom is neutral, so 17.

And the number of neutrons is the nucleon number, 35, take away the number of protons, 17.

So that's 18 neutrons.

Well done if you're getting these right.

And now some more questions for you.

For each of the atoms shown, can you say how many protons, neutrons and electrons there are? Press pause when you do this, and press play when you're ready to check your answers.

And here are the answers.

In each case, the number of protons and electrons equals the lower number, the atomic number, shown next to the symbol.

And the number of neutrons is the mass number, the nucleon number, take away the atomic number.

So that's the upper number take away the lower number.

You might like to press pause while you check your answers against these, and press play when you're ready to continue.

And now, let's look at isotopes of elements.

The number of protons in an atom is what decides which element that atom belongs to.

And each element has a different atomic number.

For example, every single atom of carbon contains six protons.

That's what makes it a carbon atom.

Every atom of oxygen contains eight protons.

And every atom of gold contains 79 protons.

And the chemical behaviour of every single atom of a particular element is the same, because they all have the same number of electrons in the same arrangement.

But although every atom of a particular element has the same number of protons, the number of neutrons in the nucleus can vary.

Most atoms of carbon contain six neutrons, but some contain seven.

As you can see here, the nucleon number minus the proton number is seven.

And a few carbon atoms rarely contain eight neutrons.

These three types of carbon are known as isotopes of carbon.

So isotopes are different atoms of the same element, but what makes them different is having different numbers of neutrons.

We can call these carbon 12, carbon 13 and carbon 14 for their numbers of nucleons.

All elements have different isotopes.

For example, most atoms of hydrogen have one proton and no neutrons, but some hydrogen atoms have a proton and a neutron.

Check that you can see why the numbers are two and one.

And rarely, hydrogen atoms can have one proton and two neutrons.

Again, check that you understand why the numbers are three and one.

All three isotopes of hydrogen have a single proton.

That's what makes them hydrogen.

If the nucleus had a different number of protons, then it would be an atom of a different element.

Now, oxygen has an atomic number of eight, which of the following are isotopes of oxygen? You might need to press pause when you think about this.

Three of these are actually isotopes of oxygen, and what makes them oxygen is that they all have eight protons.

The number of neutrons can vary, which makes them different isotopes of oxygen.

Here are the symbols for these three isotopes, all with atomic number eight, but different nucleon numbers, different mass numbers.

Isotopes of the same element have the same number of protons, as we've seen, and so their atoms also have the same number of electrons.

It's the number of electrons and their arrangement around an atom that determines how an atom will behave chemically, what kind of chemical reactions it will undergo.

And that's because the electrons are on the outside of an atom, and they're the part of the atom that can interact with other atoms. So chemistry is all about electrons.

For example, all isotopes of carbon will react with oxygen in the same way as each other.

They'll all combust in oxygen to form carbon dioxide.

So you generally can't distinguish between different isotopes of the same element just by looking at the chemical reactions they undergo.

But since we're looking at nuclear physics, let's focus back on nuclei.

Some isotopes of an element are unstable, and that means that, at some point, the nucleus of these isotopes will change or break apart.

This process is called nuclear decay or radioactive decay.

And there are several different types of nuclear decay with different causes.

Let's take a look at some of those.

A nucleus can have too many protons compared with the number of neutrons it has, and that's when the repulsive electrostatic forces between the positively charged protons is larger than the strong nuclear force holding the nucleus together, and then the nucleus can decay.

For small nuclei, an equal number of protons and neutrons is the most stable.

For example, carbon 12 has six protons and six neutrons, and it's stable.

And oxygen 16 has eight protons and eight neutrons, and that's stable.

But for elements with larger atoms, more neutrons than protons are needed to make the nucleus stable.

Here are two examples.

An isotope of copper on the left and gold on the right.

And if you calculate the number of neutrons in each, you'll find that it's larger than the number of protons.

So thinking about that, which of the following isotopes of nitrogen is most likely to be stable? And you've already seen that carbon 12 and oxygen 16 are relatively small atoms, and they're stable when they have equal numbers of protons and neutrons.

And so for nitrogen 14, it's going to be most stable when it has equal numbers of protons and neutrons.

That would be seven protons for nitrogen, and so seven neutrons.

Now, isolated neutrons, that's neutrons that are on their own, are actually unstable.

A neutron on its own can decay into a proton and an electron.

So that means that this particle actually changes into two different particles.

But neutrons are more stable when they're close to protons inside a nucleus.

So we've seen that if a nucleus has too many protons compared to neutrons, it will be unstable, but it can also be unstable if it has too many neutrons compared to protons.

One of the neutrons might end up being too far away from a proton, and then it could decay.

Carbon 16, for example, has too many neutrons, and so it's unstable.

In this model here, this neutron seems to be particularly far from any protons, and it could decay.

Now, take a look at these isotopes of neon.

Which of these is most likely to be stable? And the answer, of course, is the one that doesn't have too few neutrons and doesn't have too many neutrons, neon 21.

An element can have several different stable isotopes and many unstable ones.

For example, there are three stable isotopes of the element neon, shown here.

All of them have similar numbers of protons and neutrons.

But there are many more unstable isotopes of neon.

Here are some of them, and here are some more.

Now, which two of these are stable isotopes of carbon? You couldn't know for sure which of them are stable, but if I tell you that it's two of them out of the six, you should be able to use what you know to work out which two it should be.

And the answer is c and d.

Carbon is a relatively small atom, and the most stable isotopes are the ones with the closest to having equal numbers of protons and neutrons.

So these isotopes have too few neutrons compared to protons, while these have too many neutrons compared to protons.

Well done if you picked out c and d.

And now a few questions for you about carbon 12 and carbon 16, two of the isotopes of carbon.

Press pause when you write your answers, and press play when you're ready to check them.

So here are the answers.

Carbon 12 is a stable isotope because it's a light element that has an equal number of protons and neutrons.

So the attractive forces, strong nuclear, are greater than the repulsive forces, electrostatic, between protons.

Carbon 16 is unstable because it has too many neutrons.

One of the neutrons may move too far from a proton to be stable, and it may decay.

Carbon 16 atoms are unstable and will decay over time.

Any carbon 16 atoms will probably have decayed by now.

So there are none left.

Well done if you've got some or all of those right.

And now we're at the end of the lesson, and here's a summary.

Particular atoms of an element can be described with just two numbers, the mass number, written above, and the atomic number, written below, next to the element symbol.

The mass number, or nucleon number, is the number of protons plus the number of neutrons.

While the atomic number, or proton number, is the number of protons, which is the same as the number of electrons.

Isotopes of an element have the same chemical properties as each other, because they have the same number of electrons, they have different numbers of neutrons in their nuclei.

Unstable nuclei have too many neutrons, such as oxygen 22, neutrons that are too far away from protons become unstable.

Or they may have too few neutrons, such as oxygen 13, that help hold the positive protons in the nucleus with the strong nuclear force.

Well done for working through this lesson, and I hope you now feel that you understand what isotopes are and what makes some of them stable and others unstable.

I hope to see you again in a future lesson.

Bye for now.