Hello, my name's Dr.

George, and this lesson is called Evidence of Atomic Structure.

It's all about what's inside an atom and how we know.

It's part of the unit nuclear physics.

The outcome for this lesson is I can describe how alpha particles were used to show the size and nature of atomic nuclei.

Here are the key words for the lesson.

I'm not going to read through them now, because I'll introduce them as we go along, but you can come back to this slide anytime to remind yourself of the meanings.

The lesson has three parts.

They're called background radiation, alpha particle scattering experiment, and atomic structure.

In 1896, the French scientist Henri Becquerel investigated the properties of some samples of minerals, including uranium ore.

During a pause in his experiment, he stored a wrapped sample of ore in a drawer along with some photographic film.

The ore was wrapped in thick paper and placed on top of the film.

He later developed the film and found it contained an exposed image matching the sample.

An exposed image is what you would expect if light had shone on the photographic paper, but it hadn't.

Becquerel concluded that the uranium in the ore must be giving off some sort of invisible rays that could pass through the wrapping paper and expose the film in a similar way to the way light can.

His doctoral student was Marie Curie, and she continued the investigation of these rays, naming the effect radiation.

She earned two Nobel Prizes for her work on radiation.

So radiation in this context consists of tiny particles or high energy electromagnetic waves emitted, shot out of, unstable atoms. Although bear in mind that the word radiation is used for any spreading out of energy by waves or particles.

So even light can be called radiation.

But in this lesson we're talking about particularly high energy radiation, and many substances in our surroundings emit this invisible radiation, and such substances are called radioactive substances.

Most sources of radiation are actually natural, although some are artificial.

Now which of the following are properties of the radiation discovered by Becquerel and explored by Curie? And each time I ask a question, I'll wait five seconds, but you may need longer, in which case, press pause and press play when you have your answer ready.

There's more than one correct answer here.

This radiation is invisible to the naked eye.

It can expose photographic film, as Becquerel discovered, and it can pass through some solid materials such as the thick paper that was wrapped around the ore.

Natural sources of this radiation include rocks.

Igneous rocks in particular, often contain radioactive atoms, usually more than other rock types.

Igneous rocks are rocks formed from volcanic activity.

Also cosmic rays.

The sun and other stars emit high energy radiation due to nuclear reactions taking place inside them, and some of that radiation reaches the earth's atmosphere.

And also, food.

Plants take in minerals from soil and rocks, and some of those may be radioactive.

So food is a little bit radioactive, as are you.

Which of the following crops are likely to be the most radioactive? Press pause while you read the options, and press play when you've chosen your answer.

The answer is the one that grows in soil containing igneous rock.

As we saw earlier, igneous rock tends to be more radioactive than the other two rock types, metamorphic and sedimentary.

Now, artificial sources of radiation include medical procedures.

These include X-rays and radiotherapy, which uses radioactive sources.

Also, nuclear weapons.

When these are tested, some radioactive materials may be released.

And nuclear power.

Some waste products are radioactive.

Radiation can be detected in different ways.

As Becquerel saw, it darkens photographic film that's exposed to it.

But there's also a device called a Geiger counter, which detects individual bits of radiation, making a click for each one and recording the total number.

There is some background radiation just about everywhere from a range of natural and artificial sources.

So if you take a Geiger counter and switch it on, you will find that it clicks occasionally, even when there's no obvious radioactive sources around.

The level of background radiation can be very different in different locations.

In areas where there's a lot of igneous rock, there's usually higher levels of radiation, because of more radioactive atoms in the rock.

And the tops of mountains usually have higher levels of radiation, because being higher up in the atmosphere, they're exposed to more cosmic rays.

So bearing that in mind, in which of the following occupations are you likely to be exposed to the highest level of background radiation? The correct answer is airline pilot.

Because an airline pilot spends time relatively high up in the atmosphere, and so is exposed to higher levels of cosmic rays.

An astronaut would be exposed to even more cosmic rays.

And now here are some written questions for you.

So while you are writing your answers, press pause, and when you're ready, press play, and I'll show you some example answers.

So here's some possible answers.

Natural sources include foods, rocks, cosmic rays, and you may have heard of radon gas, which can come up from certain types of rocks in the ground.

And the radioactive substances within food and rocks are called radioactive isotopes.

Artificial sources include nuclear weapons testing, nuclear power and sources used in medicine and research.

In question two, the explanation for why our bodies contain some radioactive atoms is because of what we eat.

Plants absorb minerals through their roots, and these minerals can include some radioactive atoms and therefore, the food we eat contains some radioactive particles.

We can also breathe in some radioactive particles.

You may have heard of carbon-14, a naturally occurring radioactive isotope of carbon, which is found in the air.

Finally, you were asked how to test which of two rocks is more radioactive.

The rocks could both be placed near photographic film so that the film is exposed to the radiation.

You'd need to keep the two well apart so that each piece of film is only affected by one of the samples.

The more radiation from a rock in a fixed time, the more radiation is emitted.

A Geiger counter will produce more clicks in a fixed time near to the rock that emits more radiation.

So well done if you answered some of these correctly.

Now let's move on to think about the alpha particle scattering experiment.

Matter is composed of atoms that are too tiny to be seen individually, even with the most powerful light microscopes.

Early chemists assumed that atoms were like small balls, spheres, which could not be divided into parts, and had a radius of around 10 to the power -10 metres.

They assumed that each element was made from a different type of atom.

But then in 1897, a physicist called JJ Thompson discovered that when metals are heated, they can release particles that are much smaller than atoms. And he experimented to try to find the properties of these particles, and he found that they could be deflected by an electric field in a particular direction, which meant that they had negative charge.

At the time, he called them cathode rays, but they're now called electrons.

Their existence showed that atoms are made of smaller particles.

If matter is made of atoms, but something smaller than an atom can come out of matter, then there must be small parts inside the atoms. Here's a question for you.

Why is an electron beam deflected when it's between a metal plate with a positive charge and a metal plate with a negative charge, as shown in this diagram? And it's because a force acts on it, and that force is called an electrostatic force, because it's a force caused by charged objects.

After the discovery of the electron, JJ Thompson realised that a new model of the atom was needed and he proposed what he called the plum pudding model of the atom.

Since negatively charged particles, electrons, can come out of atoms, but overall, matter is usually neutral, so atoms are neutral.

He assumed there must be something else in atoms that has positive charge so that the positive and negative charge is cancelled out.

And he imagined a positively charged sphere with negatively charged electrons dotted around inside it.

And he thought this was similar to a type of pudding that you don't often see these days, plum pudding.

The sphere is like the sponge of the pudding, and the electrons are like plums within it.

He assumed that since electrons can leave the atom, they must be separate parts, and because they have negative charge, they'll repel each other.

So they must be spread out throughout the atom.

Now the colours are changed in these images, but pay attention to the labels.

Which of the following shows JJ Thompson's plum pudding model of the atom? The correct answer is C.

The sphere is a positive part, and the small dots are the negative electrons.

A few years later in 1909, a physicist called Ernest Rutherford decided to explore Thompson's model of the atom using a beam of alpha particles.

Alpha particles are positively charged and very fast moving, and they're emitted by some unstable atoms. So they had been discovered by this stage, and Rutherford thought if he fired these at atoms, he could see how the atoms affected them, and perhaps learn more about what's inside the atom.

The alpha particles were fired at very thin gold foil.

He used gold because it can be made extremely thin, just a few 10ths of atoms thick.

And this was set up in a vacuum chamber, a container in which the air had been removed so the air particles wouldn't get in the way of the alpha particles.

On the outside of the chamber, there was a screen made of fluorescent material.

Each time an alpha particle hit it, it emitted a tiny flash of light that could be seen by the naked eye in a dark room.

Judging by Thompson's plum pudding model, it was thought that alpha particles fired through atoms in the gold foil would go straight through, possibly deflected, moved off their straight path a little by the electrons, but not much.

Because at the time the approximate size and mass of the atom were known.

And so it was realised that the atom was not a very dense, solid thing, but actually quite an insubstantial thing, almost like a cloud.

So it should be easy for these small fast moving alpha particles to punch straight through.

And this diagram shows the sort of thing that Rutherford expected to see if Thompson's model was correct.

Before we go on, a quick question for you.

Why would an alpha particle's path be changed when it passed near an electron? And it's because there would be an electrostatic attraction.

We saw that the alpha particles are positive, electrons are negative, and so there would be an attraction and the type is electrostatic.

That's the type of force that acts between charges.

The results of this alpha particle experiment were very surprising, and they showed that the plum pudding model of the atom was incorrect.

Most alpha particles did pass through with no deflection.

That wasn't surprising.

And a few were deflected a little, which also wasn't surprising.

But the strange thing was that a very small fraction of the alpha particles were deflected a lot by the atoms in the gold foil, like the two shown here.

Some of the alpha particles even came straight back towards where they'd come from.

So this led Rutherford to come up with a new model of the atom in which the electrons are around the outside of a tiny positively charged nucleus.

Let's take a look at the individual observations from the experiment and the conclusions that they led to.

So most alpha particles passed straight through atoms with no deflection.

And Rutherford's conclusion was that these alpha particles didn't pass close to any charged particles in the atoms. So they didn't experience any significant force.

And he thought that meant that most of the atom is empty space.

To explain why some alpha particles were deflected slightly, he concluded that there are charged particles in the atoms which do interact with some of the alpha particles if they come close enough.

But then he needed to explain why some alpha particles were deflected a lot, and he concluded that these alpha particles happened to interact, come close to something, with a large mass and a positive charge.

So the positive alpha particles were repelled by something positive, and they weren't able to simply knock that positive thing out of the way because it had a large mass.

But note also that only a few, very few, alpha particles were deflected this way, and that led Rutherford to conclude that most of the mass and all of the positive charge in an atom is found in a very small part of it, the nucleus.

So most of the alpha particles didn't happen to go anywhere near a nucleus, and so were hardly deflected or not deflected at all.

Why are some alpha particles deflected backwards by atoms in gold foil? There are two correct answers here.

The nucleus of an atom is positively charged, so it repels alpha particles, and the nucleus of an atom contains most of the atom's mass.

So the nucleus is heavy compared with an alpha particle, and the alpha particle can't simply knock it out of the way.

It is true that most of an atom is empty space, but that doesn't explain why this deflection can happen.

It's also true that electrons in atoms are negatively charged, but they wouldn't repel an alpha particle, making it deflect backwards.

So out of all this, Rutherford developed the nuclear model of an atom.

Here's a picture representing it.

The radius of the atom is about 10,000 times bigger than the radius of the nucleus.

He worked this out from the results of his experiment.

So you can see that this diagram isn't to scale, because this nucleus is far too big compared with the size of the atom.

And the nucleus is positive and contains most of the mass of the atom.

And there are negative electrons orbiting the nucleus, going around it like planets around the sun, and most of the atom is empty space.

This was very surprising at the time, the idea that what seems like very solid matter, including ourselves, is nearly all actually empty space.

In many textbooks, atoms are drawn like the one shown here.

Which of the following statements describe what a real atom is like? Option A isn't true.

The nucleus doesn't take up most of the space in an atom.

In fact, it's very small.

Option B is true.

Atoms have electrons around a nucleus.

C is not true.

The nucleus of an atom doesn't control the atom.

If you thought that was true, you may have been thinking about the nucleus of a living cell, which contains the instructions for how the cells should behave.

The nucleus of a cell is a very different thing from the nucleus of an atom.

It's much, much bigger, but they're both called nucleus because they're the central, most important part of the object.

The nucleus of an atom does have a positive electric charge, and the diameter of an atom is about 10,000 times bigger than the diameter of its nucleus.

So well done if you picked out those correct statements.

Now, some pupils are discussing Rutherford's alpha scattering experiment and what the results tell us about atoms. Lucas says, "Pictures of atoms in textbooks are not drawn to scale." Laura says, "The diameter of a nucleus is a lot smaller than the diameter of an atom." Jacob thinks that an atom is not electrically charged, so the particles in it don't have a charge.

Alex thinks most of an atom is empty space, and June thinks most of the mass of an atom is in its nucleus.

So questions for you, which pupils are correct, and explain your answer, and which pupils are incorrect, and what would you say to them to help them understand atoms better? So press pause while you discuss or write down your answers, and press play when you're ready to check them.

So here are the answers.

All of the statements are correct except Jacob's.

Alex said most of an atom is empty space.

And we know from Rutherford's experiment that it must be because most of the alpha particles went straight through, so they passed without hitting or coming close to anything.

John said, "Most of the mass of the atom is in its nucleus," and Rutherford found that the alpha particles that hit the nucleus bounce backwards instead of being able to just push the nucleus out of way, which shows that the nucleus has much more mass than an alpha particle.

Lucas said, "Pictures of atoms in textbooks are not drawn to scale," and he is right.

If they were, the electrons wouldn't fit on a page.

In fact, if the nucleus were drawn with a diameter of 0.

5 centimetres, half a centimetre, the atom drawn to scale would be about 50 metres across.

And that's why atoms usually aren't drawn to scale.

If we squash the atom to a size that would fit on a page, the nucleus would be too small to see.

Laura said, "That the diameter of a nucleus is a lot smaller than the diameter of an atom," and we've already seen that most of the atom is empty space from Rutherford's experiment.

So the nucleus must be very small in comparison to the whole atom.

And finally, Jacob said, "An atom is not electrically charged, so the particles in it don't have a charge," but this statement's not correct.

The alpha particles were repelled if they pass close to the nucleus, showing that the nucleus has the same type of charge as the alpha particles, positive.

The reason why atoms are neutral overall is because they contain an equal amount of positive and negative charges which balance each other out.

So well done if you realised which statements were correct and which one wasn't, and gave some of the right reasons.

Now let's take a look at atomic structure in more detail.

The model of the atom that we use today is very similar to the nuclear model of the atom, but it's called the Bohr nuclear model, because there's been some changes since Rutherford's time.

A physicist called Niels Bohr used quantum physics to work out that electrons don't actually orbit the nucleus the way planets go around the sun.

He developed a slightly different model called the electron shell model.

We're not going to go into that here, but here's a diagram representing the shell model in which the electrons are at particular distances only from a nucleus.

Other observations showed that the nucleus of atoms could only have specific values of electric charge.

Shortly after this, evidence was found that the nucleus of an atom contains positively charged particles, which we now call protons, and that's why the nucleus can only have specific values of positive charge, 'cause it can only have a whole number of protons.

Then in 1932, a physicist called James Chadwick demonstrated that the nucleus of an atom also contains particles called neutrons, which have a similar mass to protons, but no electric charge.

They're neutral.

So this model of the nucleus is different from Rutherford's.

Rutherford only knew that the nucleus was positive, small and dense, but he didn't know about these separate particles within it.

Now, what is the correct order in which the following were added to the atomic model? Starting with the earliest.

The correct answer is electron shells, then protons, then neutrons.

And the model of the atom is a really good example of how a scientific model can change over time based on new evidence from new observations.

How can you sort the images of the models of an atom into chronological order? So starting with the earliest in time and finishing with the latest one.

And then give a brief description of each model.

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

So here's the right order, and alongside them, descriptions of each.

First of all, D.

The atom is a tiny sphere that cannot be split.

So this was before JJ Thompson's discovery of the electron.

Then B, the plum pudding model in which negative electrons are spread throughout a positively charged sphere.

And this is still before Rutherford's scattering experiment.

Then in C, we have Rutherford's nuclear model in which a tiny, positively charged nucleus contained most of the atom's mass, and they're surrounded by orbiting electrons.

And finally, A, Bohr's nuclear model in which electrons move in shells around a positively charged nucleus.

This nucleus contains both positively charged protons and neutrons, which have no charge.

So well done if you included a lot of those points in your answer.

Now we've reached the end of the lesson.

So here's a summary.

Unstable atoms can emit invisible radiation, which causes background radiation almost everywhere.

The discovery of the electron led to the plum pudding model of the atom with electrons embedded in a positive sphere.

The alpha particle scattering experiment by Rutherford's research team showed that an atom is mostly empty space.

It has a small, positively charged nucleus that contains most of the mass, surrounded by negatively charged electrons.

Bohr showed that electrons are arranged in shells, and Chadwick found that there are neutrons along with the protons in the nucleus.

So 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.