Lesson video

- Hello, I'm Ms. Barrett from the Oak National Academy.

Today's lesson is on development of the atomic model from the atomic structure in the periodic table unit for Key Stage 4.

So today we are going to describe the discoveries that led to an increased understanding of the structure of atoms and how we've arrived at the current atomic model.

So you may know what the current atomic model looks like from Key Stage 3, but don't worry if you can't remember 'cause we're gonna go through that today.

Okay, so some keywords that will help you with today's lesson.

First, an atom is the smallest unit of matter.

An electron is a subatomic particle with a negative charge and a negligible mass.

A proton is a subatomic particle with a positive charge and a relative mass of plus one.

And I'm sure you know what's coming next.

A neutron is a subatomic particle with a neutral charge and a relative mass of plus one.

So try and keep these definitions of the subatomic particles in your brain because they're gonna be really useful during today's lesson.

Okay, so the lesson outlined for today, we're gonna start with looking at the early models of the atom so tiny spheres to plum pudding.

That will all make sense in a minute.

And then we're going to look at the Rutherford nuclear model.

And then finally we're going to look at the improvements to the nuclear model that led to today's current accepted model.

So starting with tiny spheres to plum pudding.

So everything in the world is made from atoms. So whether that's the paper that you're writing on, the food that you eat, your hair, everything is made from atoms. And this is why it's so important for us to understand what the structure of the atom is.

So on the right-hand side here, we've got the current accepted model of the atom, which you might recognise.

And the atomic structure that we know today is the result of many, many years of research.

And it's taken a really, really long time for people to actually understand what this atom looks like.

And that's because the atom is really, really tiny.

So if we have a look here on the right, you can see that the atomic radius is one times 10 to the minus 10.

So that is 0.

0000000001 metres.

So they are invisible to the eye.

They're really, really small.

The smallest thing apart from the atomic particles that make them up.

So this is why it's so difficult for us to understand what that structure is.

However, there has been loads of research and loads of models of the atom developed over thousands of years.

So we start off with a very simple sphere.

That's what people thought atoms were to start with.

And over time, we've developed to more complex models with our charges until we get to our current accepted model.

We're gonna go through each individual model today and understand how they were developed.

So starting with Democritus in 400 BC.

"If I chop this piece of wood into smaller and smaller pieces, eventually I'll not be able to cut it any further.

And these indivisible pieces are what I will call atoms." So he thought if he took this log and he chopped it in half and then he chopped it in half again and then he kept going until it was not possible to chop it any further, then these are what atoms are.

So he had the idea that we have these particles that were tiny and could not be divided any further.

He didn't have a model of what they were and he didn't have any evidence to support this but this was just the idea that he had.

So it wasn't for thousands of years later that John Dalton came along in 1803 and he said, "I think Democritus was right.

Atoms are indivisible, they are tiny, invisible spheres and there are different sizes of tiny sphere depending on what the element is." So this was his idea.

Each atom was just a sphere and the spheres would be different sizes for the different elements.

So he was one of the first people to come up with the idea of different elements.

So in 1897, JJ Thomson came along nearly 100 years later and he said, "I think there's more to it.

Atoms are not just tiny spheres, they contain charge.

My experiments have proven the atom is a ball of positive charge with negatively charged particles studded throughout.

These are the electrons, just like a plum pudding." And I'll show you his model in a second.

And so he thought because of things such as statics, when you get a static shock, when you touch some metal or touch an escalator and you get a little shock, he thought that must be due to having differences in charge.

So atoms must have charges within them.

So this is his model on the right.

So you can see here, here's a ball of positive charge, a solid ball of positive charge with electrons just randomly spread throughout.

And like I said, he likened this to a plum pudding.

I prefer a chocolate chip cookie, but there we go.

So as you can see here, the electrons are the fruits, the plum that is studied throughout the pudding.

And then the rest of the dough would be the positive charge.

So that's how he imagined atoms to look like, obviously on a very, very small scale.

Okay, let's see how much we've understood so far.

So which person first theorised the atom as individual particles? Have a think.

Was it JJ Thomson? Was it Democritus or was it Dalton? That one was Democritus.

So in 400 B.

C.

, he came up with the idea that atoms were individual particles but had no evidence to prove it.

Who discovered electrons? Was it JJ Thomson, Democritus or Dalton? Okay, hopefully you got this one right.

That was JJ Thomson.

So the last person that we spoke about, he was the person that discovered those electrons, the negatively charged particles.

Which scientist theorised the atom as a solid sphere? And that one was Dalton.

Well done if you got those ones right.

Which scientist concluded the atom had positive charge spread throughout it? That one was JJ Thomson.

So the most recent model that we spoke about in the plum pudding model.

All right, so your first task for today, I have given you a paragraph with a few mistakes in it.

So I want you to rewrite the paragraph on your sheet or in your book, and I want you to correct the mistakes.

So the atom was first theorised by JJ Thomson.

He said that atoms were divisible particles.

Many years later, John Dalton concluded that atoms were indivisible and were tiny solid cubes.

Different elements were all the same size.

In 1997, JJ Thomson completed experiments and discovered the atom had charges.

He concluded that the atom was a ball of negative charge with positively charged electrons studded throughout it.

So hopefully you noticed there were some mistakes in there.

So have a go, rewrite the paragraph, pause the video and come back when you are done.

Okay, hope you didn't find that too bad and you were able to spot those mistakes.

I'm gonna show the correct answers now.

So have a little check of your paragraph.

The atom was first theorised by Democritus.

He said that atoms were indivisible particles, so cannot be cut down any further.

Many years later, John Dalton concluded that atoms were indivisible and were tiny solid spheres, not cubes.

Different elements were different sizes.

And in 1897, not 1997, this was a very long time ago, JJ Thomson completed experiments and discovered the atom had charges.

He concluded that the atom was a ball of positive charge with negatively charged electrons studded throughout it.

So give those a tick if you got those right.

And if you didn't, make sure you correct any mistakes that you made.

Okay, so part B of task one, you're going to complete this table with a diagram of each scientist's model and a description.

So we've got John Dalton's model and JJ Thomson's model.

I want you to draw a diagram and then describe what that looks like.

Pause the video, have a go and come back when you're done.

Okay, how did you get on? Hopefully that wasn't too bad.

So let's go through the first model, John Dalton's model.

So his diagram, you should have just drawn a very simple circle to show the solid sphere.

And the description of that is a tiny solid sphere.

And then JJ Thomson's model, remember, this was the plum pudding model.

He had a ball of positive charge with negatively charged electrons throughout.

So hopefully your diagram looked a little bit like this.

Okay, give those a tick if you've got those right.

Well done.

So we'll move on to the second part of the lesson, which is all about Rutherford's nuclear model.

So in 1909, Ernest Rutherford and his student Ernest Marsden wanted to test out the plum pudding model.

So Rutherford said, "Let's fire alpha particles at a thin sheet of gold." And I imagine the conversation went, "All right then.

Sounds fun." So this is what they did.

Marsden and another scientist, Geiger, carried out the experiment.

But first, we'll just talk about what that is.

So an alpha particle is a helium nucleus.

So that consists of two protons and two neutrons.

So what charge do you think an alpha particle is? So hopefully you've remembered that protons are positive, neutrons are neutral and a nucleus has a positive charge overall.

So they are going to fire positively charged particles at a sheet of gold.

And they thought if JJ Thomson is correct, the positive charge is spread across the whole atom.

Then all of the alpha particles will go straight through the gold atoms, in the gold foil without deflection, like this.

So they'll just travel straight through.

They're not going to go off course or they might even puncture the gold foil.

So that's what they were expecting to find.

But actually, in the experiment, most of the alpha particles did pass through undeflected as they expected.

But there were some unexpected results.

So some alpha particles were deflected at angles as they pass through the foil.

And then a very small amount of alpha particles bounced straight back off the foil.

So as the alpha particles were firing through, some were bouncing off, a very small amount were bouncing off and some were going through but as they were hitting the gold foil, they were coming off at an angle.

So they thought, well, that's really strange.

There must be something in the atom that is causing them to go off course and to bounce off because that doesn't fit with the plum pudding model.

So how are we gonna interpret those results? So the first thing, the fact that most alpha particles passed straight through the foil.

Well, their interpretation of this was that most of the atom is mostly empty space.

So there's not a lot in the atom.

The majority of the volume of the atom is empty space.

Some alpha particles were deflected off angles.

Well, they interpreted that as there is a concentration of positive charge in the atom because like charges repel.

So if the the plum pudding model was correct, and the entire atom was positive charge, then that positive charge is relatively spread out across the whole atom.

So it shouldn't cause a huge repulsion.

Whereas what they actually considered was that in the middle of the atom, there is an area that is really positively charged and that caused those alpha particles that are also positively charged to deflect off as they were coming through.

So when they were going near that positively charged area, they were going off at an angle because they were being repelled.

And then the small amount of alpha particles that bounced back, well, that's because the positive charge and the mass of the atom are concentrated in a very tiny volume in the centre of the atom.

And this was what was the nucleus.

So they thought because there's only a really tiny amount of particles that were bouncing back, that's because the chance of an alpha particle actually hitting that nucleus was really, really small because the nucleus itself was so small.

So that's where the positive charge and the mass must be.

And this was their conclusions from that experiment.

So they took these results and Rutherford said, "The plum pudding model is incorrect.

I propose a new model.

A small, positively charged nucleus at the centre of an atom surrounded by empty space with a layer of electrons on the outside of the atom." And this is what his model looked like.

So positive in the middle and a ring of negative charged electrons round the outside.

All right, let's see how much we've understood from the Rutherford nuclear model.

So true or false, alpha particles are positively charged.

Have a think and have a look at the justifications.

So alpha particles, they are positively charged.

And why was that? Well, that's because they contain protons, which are positively charged and neutrons that have no charge.

So alpha particles overall must have a positive charge.

True or false again.

Alpha particles bounced back because they were bouncing off the negatively charged electrons.

Have a think.

That one was false and that is because they were bouncing off positively charged nuclei because like charges repel, so positive is going to repel positive.

If they were negatively charged, that would actually attract them towards the nucleus.

So that wouldn't make sense.

Okay, on the next three questions, I'm gonna test your understanding of that experiment, the gold foil alpha particle scattering experiments.

So there is a word that you need to insert and that word is either gonna be most, some or very few.

So something alpha particles bounced straight back off the foil.

That one was very few.

Remember, the chance of them hitting the nucleus because the nucleus is so small was really, really, really small.

So therefore that was unlikely to happen.

And there was very few alpha particles that bounced back off.

Something alpha particles passed straight through the gold foil.

That one was most.

Because most of the atom is empty space, most of 'em were able to just pass straight through with no problem.

Alpha particles were deflected at angles as they passed through the gold foil.

That one was? That one was some.

So some were deflected off at angles due to the positive charge of the nucleus sending them off course as they went through the foil.

But they still were able to travel through.

This one's slightly different.

Which model is the Rutherford nuclear model? We've looked at three so far.

Which is the one that is the Rutherford model.

Can you remember? This one was B.

So a positive charge in the middle with the negative charged electrons around the outside of the atom.

Okay, so we're gonna move on to the second task of the day.

So explain with reference to the atomic structure why most alpha particles pass straight through the gold foil.

Some alpha particles were deflected at angles as they passed through the gold foil, and a small amount of alpha particles bounced straight back off the gold foil.

So it's really important that we understand this alpha particle scattering experiment because it helps us understand how he came up with his nuclear model.

So have a go, pause the video and then come back once you have finished.

Okay, hope you got on all right with those questions on the alpha particle scattering experiment.

Let's go through the answers together.

So most alpha particles passed straight through the gold foil.

Well, that was because most of the atom is empty space.

Some alpha particles were deflected at angles as they passed through the gold foil.

That was because there is a concentration of positive charge in the centre of the atom.

And as alpha particles were also positive, they were repelled as they passed through the foil.

Don't worry if you didn't get exactly those words.

As long as you got words to that effect, that is absolutely fine.

So give those a tick if you've got those right.

And then the third question.

A small amount of alpha particles bounced straight back off the gold foil.

That it was because the centre of the atom has a nucleus, which is where the positive charge and mass is concentrated.

As very few particles bounced off, the nucleus must be very, very tiny.

So that's why very few particles were bounced straight back off because the chance of them actually hitting the nucleus as they passed through was really small.

So give that a tick if you got that right.

I'm gonna move on to the second task of task two.

And what I want you to do is compare the plum putting model and Rutherford's nuclear model.

So I want you to explain the differences or similarities between the electrons, the positive charge and the mass of the atom.

Pause the video and come back once you've had a go.

All right, how did you find that? Let's go through the answers.

So the electrons in the plum pudding model, they were randomly spread out through the atom, whereas in the Rutherford model, they had a layer of electrons on the outside of the atom.

What about the positive charge? So the positive charge in the plum pudding model was spread across the whole atom.

So the entire solid sphere was the positive charge, whereas in the Rutherford model, the concentration, it was, sorry, it was concentrated in a tiny volume in the centre of the atom.

And this was the nucleus.

What about the mass of the atom? So the plum pudding model, the whole atom is a solid sphere, whereas with the Rutherford model, most of the atom is empty space and the mass is in the centre, which is the nucleus.

So as before, don't worry if you didn't get exactly those words.

As long as you've got words to that effect, then that is great.

So give that a tick if you got that right.

Okay, well done.

That is task two finished.

So we're gonna move on to the last part of the lesson, which is looking at an improved nuclear model.

So in 1913, Niels Bohr, who was actually a student of Ernest Rutherford, came along and he said, "Rutherford, you are wrong.

If your structure were correct, the atom would collapse in on itself due to the negative electrons being attracted to the positive centre.

And this would make all the atoms very unstable." So this was Rutherford's model.

Positive charge in the middle with electrons around the outside.

And of course, opposite charges attrack.

So this would happen.

The negative charged electrons are attracted to the positively charged nucleus and the atom would collapse in on itself.

So he thought this can't be right.

So he proposed a new model.

Electrons are in different energy levels, which are called shells, and the electrons orbit around the atom in fixed distances from the positively charged nucleus.

So he completed loads of experiments and this matched his results.

So this was his model, which is more similar to what we see these days.

So you have a positively charged nucleus in the middle with electrons in fixed positions, fixed distances from the nucleus around the outside, which we call shells.

So that was the Bohr atomic model.

However, there was one more improvement to be made.

James Chadwick in 1932.

He said, "It's been over 20 years since the nucleus was discovered.

And he's completed experiments and found evidence for a neutral particle in the nucleus.

And this is the neutron and we should add this to the model." So back when Rutherford was doing his experiments, he did have an idea that there might be a particle that was neutrally charged but he had no evidence that it existed.

So James Chadwick completed these experiments and he proved that the neutron does exist.

So he was the person that discovered the neutron.

So this is now our current accepted model.

We have our nucleus, which is made up of protons and neutrons and the electrons in shells around the outside.

So over many, many, many years, we have come to accept the current atomic model, which is here.

So the structure of the atom is still being researched every single day.

And the nuclear model is the most common one that we still use.

You might see other models but this is the one that is most commonly used.

Okay, so let's see how much we've understood so far.

Which two things were added to improve the Rutherford model? Was it shells, protons, electrons, or neutrons? That was shells and neutrons.

So shells was Bohr, neutrons was Chadwick.

True or false, the neutrons are located in the nucleus.

And have a think about what your justification will be.

So that one is true and that is because the mass of the atom is in the nucleus and neutrons and protons have mass.

They are located in the nucleus of the atom.

Who found evidence for the existence of the neutron? Was it Niels Bohr, James Chadwick, Ernest Rutherford or John Dalton? So that one was the most recent discovery and that was by James Chadwick in 1932.

So your last set of tasks for this lesson on the improved nuclear model.

So match each statement to one of the following names.

Dalton, JJ Thomson, Rutherford, Bohr or Chadwick.

So various different things you need to match up to the scientists.

So discovered electrons, solid spheres, proposed the nucleus, proposed the shells, alpha scattering experiment, found evidence for neutrons, proposed atom had empty space, and the plum pudding model.

So which scientist do those statements correlate to? Pause the video, have a go and then come back and check your answers.

Okay, so let's go through the answers.

Don't worry if you struggle with that a little bit.

This is a bit of a history lesson today.

So a little bit different to the normal science content and trying to just remember some names is quite difficult, so don't worry if that was hard.

Let's go through the answers.

So discovered electrons, that was JJ Thomson.

And the solid spheres, that was Dalton.

And proposed the nucleus, that was Rutherford.

Proposed the shells, that was Bohr.

The alpha scattering experiment.

That was Rutherford and his students.

Found evidence for neutrons.

That was Chadwick.

That was the most recent discovery.

Proposed the atom had empty space, that was Rutherford.

Remember, all the models before that.

Well, Bohr's model and Democritus, they thought they would be solid lumps or spheres.

And finally, the plum pudding model, that was JJ Thomson.

Okay, part B, I want you to write a paragraph to compare the current accepted model of the atom with the plum pudding model.

So we compared earlier the plum pudding model to the Rutherford model.

But this time, I want you to compare the current accepted model with the plum pudding model.

But you can use your previous answer to help you write this one.

Pause the video, have a go and I'll see you when you get back.

Okay, so I'm gonna show you some things that you could have included in your paragraph and I want you to check your answer.

So the plum pudding was a solid atom.

The nuclear model is mostly empty space.

The positive charge is spread throughout the whole atom in the plum pudding model, whereas today's model has positive charge concentrated in the centre.

This is known as the nucleus.

The electrons were randomly arranged in the plump pudding model, whereas today's nuclear model has them in fixed distances from the nucleus in the shells.

And finally, the plum pudding model has no neutrally charged particles, whereas the nuclear model has neutrons in the nucleus.

So they're the main differences or the comparisons.

So they're the main comparisons between the nuclear model and the plum pudding model.

So have a read of your answer and make sure that you have included those.

Okay, so your final task for the lesson, you 're going to create a timeline of the development of the atom.

And you must include a drawing of each atomic model, the names of the scientists, the year of development and the explanation of each structure.

So on your worksheet, you'll have boxes that show you the way you're going to structure your timeline.

And I want you to fill in those boxes with all of the information as detailed as you can for each model.

Good luck and I will show you an example when you are back.

Okay, so let's go through the example timeline that I've done for you.

So the first model, which one was the first model? That one was John Dalton's solid spheres.

And remember, that was in 1803.

So the solid spheres are varying sizes depending on the element.

Very, very simple model.

What came next? Next was JJ Thomson's plum pudding model.

So that was solid spheres of positive charge with negatively charged electrons spread randomly throughout that structure.

And that was in 1897, so nearly 100 years later.

Next, we've got Ernest Rutherford's and his students model.

So we have the mass and the positive charge in a tiny nucleus with electrons surrounding the outside of the atom.

And the rest of the atom is empty space.

And this was in 1909.

And a few years later, Niels Bohr, which was a student of Ernest Rutherford, he came up with this model, so positively charged nucleus with electrons in fixed distances from the nucleus in shells and the rest of the atom is empty space.

And then finally in 1932, James Chadwick came along and he found evidence for the existence of neutrons and he added those to the nucleus.

But otherwise, the model was the same as the Bohr.

Well done for having a go at all those tasks.

There's a lot to learn in this lesson and that there is a lot of history to try and remember here.

The most important thing to remember from this lesson is the five different models on who came up with them and some information about the structure of those models.

So to summarise, the atom was first proposed by Democritus as indivisible particles.

Dalton suggested the atom was a solid sphere with different elements having different sized atoms. Thomsom proposed the plum pudding model, a positively charged atom with electrons randomly arranged throughout.

Rutherford completed the alpha scattering experiment, proving the atom was mostly empty space with a tiny positively charged nucleus and electrons surrounding the outside of the atom.

This was the nuclear model.

And then Bohr had an improved nuclear model by adding electrons in shells.

And then Chadwick discovered neutrons that were located in the nucleus and added those to the nuclear model.

So that is everything for today.

So a really, really big well done.

Have a go at the exit quiz and see how much you've remembered.

And thank you for using the Oak National Academy and I'll see you next time.