Lesson video

Welcome to today's lesson on developing a model for atoms. It's part of the unit, atomic structure and the periodic table.

My name is Mrs. Mytum-Smithson.

Don't worry if you don't know anything about the history of the atom.

This lesson will tell you all about it, and by the end of this lesson, you should be able to describe the structure of atoms, the sub-atomic particles they contain, and developments leading to this model.

Today's keywords are plum pudding model, Rutherford's experiment, nuclear model.

On the next slide, there's some sentences describing these.

So if you want to read those, pause the video, read them, and then press play when you're ready to start the lesson.

Today's lesson consists of three learning cycles.

First, we're going to learn about the early atomic models and the plum pudding model.

Then we're going to look at the gold foil experiment and nuclear model.

And finally, we're going to look at the current model.

So let's start with our first learning cycle, early atomic models and the plum pudding model.

Atoms were first proposed as the smallest units of matter by the ancient Greek philosopher, Democritus.

Now this was a long time ago, 460-370 BC.

So he proposed that if you kept on cutting a substance, and you cut it and cut it, and divided it and divided it, eventually you'd come across a small unit.

And this was called the atom.

So the word atom comes from the Greek word atomos, meaning uncuttable or indivisible.

Many years after the atom was first suggested by Democritus, John Dalton built on Democritus' idea and suggested in 1803 that atoms were tiny, hard, solid spheres that could not be divided.

So here's one of those hard, solid tiny spheres that could not be divided.

He said that each element was made of identical atoms, so all carbon atoms were the same.

However, each elements atoms were different from another elements atoms. So for example, a carbon atom would be different to a magnesium atom, which would be different to a iron atom.

So here's three different atoms. So these would represent three different elements.

Let's do a quick check for understanding now.

What I want you to do is select two correct statements about Dalton's model of atoms. Did he suggest that all atoms of an element were the same, all atoms of an element were different, all atoms of all elements were identical, or each element had different atoms? I want you to select two correct statements.

So pause the video for some thinking time and come back, and we'll look through the answers.

So well done if you said that all atoms of an element were the same and each element had different atoms. So all of the carbon atoms were the same, all of the magnesium atoms were the same, but a magnesium atom would be different to a carbon atom.

Well done if you got that correct.

Which one of these is the best representation of Dalton's model? So is it an empty flat circle, a solid sphere, or a solid cylinder? Pause the video, choose your image, and then press play when you've got your answer.

Well done if you said a solid sphere, that's B.

Dalton did say that they were tiny, solid spheres.

Well done if you got that correct.

In 1897, JJ Thomson carried out some experiments, and he showed that atoms were not solid spheres like John Dalton had suggested.

They were made up of smaller, subatomic, negatively charged particles.

Remember, subatomic just means something that's smaller than an atom.

And these subatomic particles were called electrons.

JJ Thomson's new theory was called the plum pudding model of the atom.

And this suggested that the atom was made up of a ball of positive charge and it had electrons embedded in it.

So these electrons were negative and they were embedded in it, so they were stuck in position, they did not move around.

So here, we've got a plum pudding, and we've got the cake part, and that represents the ball of positive charge.

Back in JJ Thomson's days, all dried fruit were called plums. And so we've got the raisins here, they're the dried fruit, and they're representing the negative electrons.

So they're the plums in the plum pudding.

True or false? JJ Thomson said that there must be something smaller than an atom.

Is that true or is that false? Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said that was true.

He did say that there was something smaller than an atom.

Now I want you to justify your answer.

Is it the smaller particles were negative and called electrons or is it the smaller particles were positive and called electrons? Pause the video now for some thinking time.

Press play when you've got your answer.

Well done if you said that the smaller particles were negative and called electrons.

Here, I've got three images for you.

Which image shows JJ Thomson's model? So we've got A, which is a ball of positive charge with negative electrons in it.

We've got B, which is a ball of negative charge with positive electrons in it.

Or we've got a solid sphere which is C.

Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said that it was A.

It is indeed a ball of positive charge with those negative electrons embedded in it.

True or false? JJ Thomson's model was called the currant bun model.

Pause the video for some thinking time and then press play when you've decided if that statement is true or false.

Well done if you said that that was false, it was not called the currant bun model.

So let's see some justifications.

So the plum pudding model showed that the raisins represented the negative electrons embedded in the positive pudding, or was it B, the plum pudding model shows that the raisins represented the positive electrons floating around the negative pudding.

Pause the video for some thinking time, select your answer, and then press play.

And I'll tell you if you're right or not.

So well done if you said A, the plum pudding model shows that the raisins represented the negative electrons embedded in the positive pudding.

So that means that they cannot move around.

Well done for working hard so far this lesson, now we're going to see how much you know.

So here is part one of task A, draw and label diagrams to show, A, John Dalton's model, and B, JJ Thomson's model.

Pause the video while you draw and label those diagrams, then press play and we'll see how well you've done.

Well done for completing that task.

Let's see if your models look like these models.

So John Dalton's model was a solid sphere, so you should have just tried to draw a sphere.

So draw a circle with a bit of shading and label it up a solid sphere.

Then JJ Thomson's model, here, we've got a ball of positive charge.

So you should have a big positive in the middle representing the ball of positive charge.

And in it, you should have embedded negative electrons.

So you should have labelled those up.

So well done if you've got those correct.

Here's part two of task A.

Models are often used in science, and a chocolate chip cookie could be used as a model for JJ Thomson's theory of the structure of the atom.

What I want you to do is assess the model, and I want you to write two reasons why it's a good model for JJ Thomson's theory and one reason why it's not a good model for JJ Thomson's theory.

Pause the video while you carry out the task, then press play and we'll go through the answers.

Well done for completing that task.

Your answers might not be exactly the same, but they should be similar.

So the model is good because the cookie dough could represent the sphere or ball of positive charge, and the chocolate chips could represent the embedded negative electrons.

Then it's not a good model because the cookie is flat and not spherical.

So he talked about a ball of positive charge and the cookie's flat, so it's not a good model because of that.

But well done if you've managed to get those correct.

We've now completed our first learning cycle, learning about the early atomic models and the plum pudding model.

Now we're going to look at some experiments that were important in developing the model of the atom, the gold foil experiment and nuclear model.

In 1909, Ernest Rutherford and his research group performed the gold foil experiment.

Here, positive alpha particles were fired at very thin gold foil.

The gold foil was surrounded by a detector, and that could detect the path of the alpha particles.

So see which path they took, whether they went straight through or whether they came out at an angle.

Here were Rutherford's findings from the gold foil experiment.

So he expected that all the alpha particles would go straight through.

That was his prediction.

That's what would happen.

However, the actual results show that the alpha particles did one of three things.

Most of them went straight through, a few rebounded straight back, and some were deflected at an angle.

And these results proved that the plum pudding model was incorrect.

Rutherford's conclusion from the gold foil experiment led to the development of the nuclear model, and you can see that pictured here.

You can see it's got a positive nucleus with a cloud of electrons surrounding it.

So most of the alpha particles went straight through.

And the conclusion for this was that most of the atom is actually empty space, so those positive alpha particles could just pass straight through.

Very few of them rebounded, so some of the alpha particles rebounded, and that meant that there must be a tiny, dense, nucleus in the centre of the atom.

Some of the alpha particles were deflected, and the conclusion that Rutherford said from this was, that the nucleus must be positively charged.

So the positive alpha particle was sent on a different direction, so deflected from the positive nucleus, and there must be a cloud of negative electrons orbiting it.

Here's a quick check for understanding.

What happened to the alpha particles in Rutherford's gold foil experiment? A, all the alpha particles went straight through the gold foil.

B, all the alpha particles were deflected back from the gold foil.

C, some alpha particles rebounded and some were deflected.

Pause the video for some thinking time.

Press play and I'll tell you the answer.

Well done if you said C, some of the alpha particles rebounded and some were deflected, which meant that some went straight through.

So well done if you got that correct.

Which image shows Rutherford's nuclear model? Is it A, we've got a nuclear cell with a cloud of positive charge.

Is it B, we've got embedded negative electrons into a ball of positive charge.

Or is it C, we've got a cloud of negative electrons with a nucleus in the centre.

Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said C, there's a nucleus there which is positive and a cloud of negative electrons surrounding that.

Not very much later in 1913, Niels Bohr adapted the nuclear model.

Scientists were puzzled because they wanted to know why the atom didn't collapse if there was a cloud of negative electrons orbiting the positive nucleus.

So they felt that that cloud of negative electrons will be attracted to the positive nucleus.

So Bohr said, electrons orbit the nucleus at fixed distances from the nucleus in different energy levels or shells.

And theoretical calculations agreed with these experimental observations.

So here's Bohr's model there.

You can see we've got a positive nucleus.

So very much like Rutherford's nuclear model.

However, the electrons now are in fixed distances orbiting, some moving around the nucleus.

So fixed distances in shells from the nucleus.

So it's almost just like a little small tweak to Rutherford's model.

Which image shows Bohr's model? Is it A, electrons in shells at fixed distance from the nucleus? Is it B, the nucleus with a cloud of electrons.

Or is it C, a ball of positive charge with embedded negative electrons.

Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said A.

A shows Bohr's model.

You can see those electrons in shells at fixed distances from the nucleus.

B is Rutherford's model, so that's the positive charge with a cloud of electrons surrounding it.

And C is JJ Thomson's plum pudding model, so that was a ball of positive charge with negative electrons embedded in it.

Rutherford continued working on his atomic model, and in 1917, he showed that the nucleus could be subdivided.

So instead of one positive charge, he said it was divided into a whole number of smaller particles.

And each of these particles have the same mass and amount of positive charge.

The name proton was given to these particles.

So you can see here, here's a nucleus with protons in it.

What did later experiments show about the nucleus? Did it show that the nucleus had no charge? Did it show that the positive nucleus was made up from smaller positive charges within it? Or did it show that the positive nucleus was made up from smaller negative charges within it? Pause the video and then press play when you've got an answer.

Well done if you said the positive nucleus was made up of smaller positive charges within it.

Remember, these positive charges were called protons.

Just like Rutherford, Jun is writing a conclusion from the gold foil experiment.

Help him explain the findings.

Jun needs to write the three different things that the positive alpha particles did when fired at the gold foil, a conclusion for each of these findings, and a description of the model of the atom that this new evidence provided.

So here's the diagram to help Jun.

Pause the video, complete the task, and then press play and we'll go through the answers.

Well done for completing that task.

So here's what Jun wrote.

When the alpha particles fired, most went straight through the gold foil, some were deflected a little in a different direction, and some were rebounded straight back.

So you should have made a note that we've got three options.

They went straight through, some were deflected, and some rebounded straight back.

Next thing, there was a positive charge in the centre of the atom, and he called this the nucleus.

Most of the atom was empty space.

And there was a cloud of negative electrons surrounding the nucleus.

So well done if you've got all of those parts correct.

You've done really well.

Part two of task B now.

What I want you to do for this one is I want you to compare the plum pudding model to the nuclear model.

So I want you to sort the true statements into the Venn diagram.

So here's a Venn diagram.

You are going to draw yours much larger than this.

And you're going to label it up plum pudding model, Rutherford's nuclear model, and both.

So we've got the statements that you're going to sort.

So we've got electrons moving or electrons embedded, so they're not moving, contains protons, contains electrons, ball of positive charge, ball of negative charge, no nucleus, positive nucleus, negative nucleus, made up of mostly empty space, contains opposite charges, contains only positive charges, contains only negative charges.

So you're going to start these statements, and you're going to either put 'em in the plum pudding model side, the nuclear model side, the both.

And if they don't fit in any of those three categories, so if they're incorrect, you're going to put them on the outside of that Venn diagram.

So pause the video while you complete that task, press play and we'll go through the answers.

Well done for working really hard and completing that task.

Let's go through the answers now.

So, electrons moving.

So remember, Rutherford's model, the electrons were moving.

They were going around the nucleus.

The plum pudding model, they were embedded, so they were not moving.

Contains protons, neither of these versions of the model contains protons.

That wasn't suggested until after the nuclear model.

Contains electrons, both of them contained electrons.

A positive charge or ball of positive charge, that's a plum pudding model.

A ball of negative charge, well, that doesn't belong to any of them, so we sit that outside of the Venn diagram.

No nucleus, so there was no nucleus in the plum pudding model.

Wasn't until after the gold foil experiment that the nucleus was suggested.

So that is a positive nucleus in Rutherford's model.

A negative nucleus doesn't exist, so we've put that outside.

Made up of mostly empty space, that's what Rutherford's nuclear model suggested.

And contains only positive charges, both of the models contained negative and positive charges.

Contained opposite charges, so that's for both of them.

They contained positive charges and negative charges, so that's opposite charges.

Contains only negative charges.

That was incorrect, so that sits outside of it.

Well done if you got all that information correct.

You've worked really hard.

So we've now completed two learning cycles.

We've done the early atomic model and the plum pudding model, the gold foil experiment and the nuclear model, and now we're going to look at the current model.

Scientists had carried out some experiments and they noticed a difference in the atomic number and the mass of an atom.

So the experimental work of somebody called James Chadwick in 1932 provided the evidence to show the existence of another subatomic particle within the nucleus.

So here's the current model and the current understanding of it.

We've got protons and neutrons in the nucleus.

And this particle had a similar mass to a proton but no charge, and that's why it was called a neutron.

What subatomic particle did James Chadwick suggest existed? Was it the electron, the proton, or the neutron? Pause the video now.

Press play and I'll tell you the answer.

Well done if you said it was a neutron.

The neutron was the last subatomic particle to be discovered and it was James Chadwick that did that in 1932.

Scientists noticed a difference in the atomic number and the mass of an atom.

Is that true or is that false? Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said true.

Scientists did notice a difference in the atomic number and mass of an atom.

Why was that though? So, Chadwick proved the existence of the neutron with a mass similar to a proton and no charge, or was it that Chadwick proved the existence of neutron with no mass and charge? Pause the video now and press play when you've made up your mind.

Well done if you said A, Chadwick proved the existence of the neutron with a mass similar to a proton and no charge.

So for part one of task C, what I want you to do is sort the atomic models from the oldest to the newest.

So put the oldest on the left and the newest on the right.

And then match each model to the scientist listed below that proposed it.

So there's the scientists, Rutherford, Dalton, JJ Thomson, Chadwick, and Bohr.

Pause the video while you complete this task.

Press play when you've got the answers.

So well done if you said that the first one was Dalton with his solid spheres.

The next one was JJ Thomson with the plum pudding.

Followed by that, very similar, but the Rutherford nuclear model, and then Bohr's Bohr model, and then finally, Chadwick, and that's our current model that we use now.

For part three of task C, what I'd like you to do is compare the nuclear model to the current model, and I want you to give both similarities.

So what's the same about them and what's different about them? So give the similarities and differences.

And this time, we are going to look at the nuclear model and the current model that are pictured here.

Pause the video while you compare them, and then press play and we'll go through the answers.

Well done for completing that task.

So what is the same or what is similar about both models? So they both contain a nucleus, they both contain a positively charged centre, and they both contain electrons, and finally, the electrons couldn't move, so they can orbit around.

So only in the nuclear model, there was no neutrons, no protons, and no shells or energy levels, which means in the current model we've got contains neutrons, contains protons, and it contains shells or energy levels.

So well done if you've managed to get all of that correct.

Well done.

Here's a summary for today's lesson.

The model of the atom has changed over time.

Each change resulted from new evidence gained from experimentation.

So firstly, we started with Dalton's solid spheres.

Then JJ Thomson suggested the plum pudding model.

Rutherford suggested the nuclear model.

Bohr suggested Bohr's model.

And Chadwick developed the current model.

So, JJ Thomson proposed the plum pudding model of the atom, and that comprised of electrons embedded in a ball of positive charge.

Rutherford's gold foil experiment then led to the nuclear model of the atom with a positive central nucleus orbiting electrons and the atom being understood to be mostly empty space.

Further evidence by Bohr and Chadwick led to the current model of the atom, and that now includes electron shells and neutrons.

Well done for working really hard this lesson.

I'm sure you feel more confident now about the history of the atomic model and how it's been developed over time through experiments.