Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased that you can join me today for this lesson on giant ionic structures.

It's part of the structure and bonding unit.

We're going to work through this lesson together, and I'm here to help you all the way, especially through the tricky parts.

The learning outcome for today's lesson is I can describe how metal and non-metal ions bond together to form giant ionic structures.

So we've got some key words for you.

Ionic bond, ball and stick model, and lattice.

And now we're gonna look at those words in some sentences.

An ionic bond is the electrostatic force of attraction between oppositely-charged ions acting in all directions.

The ball and stick model is used to represent the atoms and bonds in a chemical compound.

And the lattice is the regular arrangement of atoms or ions in a 3D space.

So you may wish to pause the video now and copy down those words and definitions so that you can use them later on in the lesson.

Then press play when you're ready to start.

We have two learning cycles in today's lesson.

The first is on ionic structures, and then when we've learned about that, we're gonna move on to look at how we can model giant ionic structures.

So let's get started with the first learning cycle on ionic structures.

So all salts are ionic structures because they contain a metal ion and a non-metal ion.

And an example of this is sodium chloride.

That's table salt, the stuff that we put on our chips.

It contains sodium plus and CL minus ions.

Now, large sodium chloride crystals occur naturally.

And in many parts of the world, there are salt mines, including in the UK.

And in the salt mines, large crystals of sodium chloride are mined, and brought out, and then used.

The other big source of sodium chloride is sea water.

And if you've ever been swimming in the sea and got a mouthful, you'll find it very, very salty.

It contains a lot of sodium chloride.

And in many parts of the world where it is warm and hot, they use salt pans for mining sodium chloride.

So what happens is when the tide comes in, the pan is filled.

When the tide goes out, the water, the sea water, which contains sodium chloride, is left in the pan.

The sun comes out.

It evaporates off the water, and the crystals of sodium chloride are left behind as you can see in this photograph.

Now, a really important principle in this topic is electrostatic attraction.

We need to understand what that is.

So electrostatic attraction is when a positive charge is attracted to a negative charge.

And when that happens, they are held together by this force.

So to show this and illustrate this, what we can do, and you may have done this before, is a balloon could be charged by rubbing it on a jumper.

And once it's charged, the balloon is attracted to the jumper.

And because what you can see is a negative charge on the balloon is attracted to the positive charge on the jumper.

And we're showing this in this little animation that's going on.

When you rub a balloon, you actually transfer some electrical charge, and you can see that the negative is attracted to the positive.

Well, that is the principle on which giant ionic structures are formed and held together.

So it's really important to understand this quite simple idea.

Positive charge is attracted to a negative charge.

So in an ionic structure, each positive ion attracts to the negative ion, and each negative ion attracts to the positive ion.

They bond to the closest ions, and they form a crystal lattice.

Now, this diagram here shows a crystal lattice.

It's a 3D model, and you can see all the Sodium plus ions, which are drawn in purple, are alternate to the chloride minus ions, which you can see in green.

It forms a regular pattern of green, purple, green, purple in rows and columns or sodium plus, chloride minus, sodium plus, chloride minus.

That is again something important to understand.

The lattice consists of alternating positive and negative ions, and these ions are held together by the strong ionic bond.

An ionic bond is that electrostatic force of attraction between oppositely-charged ions acting in all directions, and it's really good to see the model in 3D so we can understand that these ions are spherical.

So the charge is going up, it's going down, it's going left, it's going right, it's coming out diagonally, in all directions.

Everywhere it touches another particle, we have that strong electrostatic attraction.

So let's check our understanding of how we're doing so far.

Copper II chloride is an ionic compound.

Which of the following statements describe copper II chloride? A, it contains Cu2+ and Cl- ions, B, it forms a giant crystal lattice, C, it forms simple molecules, or D, one copper II plus ion is attracted to only one CL- ion.

Well done if you chose A, copper II means it's a Cu2+ ion and chloride ion is always minus one.

Well done if you've got B.

It forms a giant crystal lattice where we have got copper II plus ions and CL- ions alternating next to each other.

So it does not form simple molecules and it is not just attracted to only one chloride ion because all the coppers are attracted to many in all directions.

Excellent work.

So we'll move on and have a look at a different example.

Calcite or calcium carbonate is an ionic compound.

It contains metal ions, calcium two plus ions, and also non-metal ions, carbonate ions, and they are attracted to each other.

And calcite is found underground in many caves.

And you can see a picture here.

The white colour that you can see on the rock is calcite.

The giant lattice is held together by ionic bonds.

These are the electrostatic forces of traction this time between calcium two plus ions and carbonate two minus ions.

And that electrostatic force acts in all directions within the lattice.

Okay, so let's just check our understanding about ionic bonds.

So ionic bonds are.

Is it A, the electrostatic forces of attraction between oppositely-charged ions acting in one direction, B, when ions are formed by the transfer of electrons, C, the electrostatic forces of attraction between oppositely-charged ions acting in all directions, or D, the magnetic forces of attraction between oppositely-charged ions? Excellent work, well done if you chose C.

That is the correct answer.

So we'll just go through what was wrong with the others.

So A, the electrostatic forces of attraction act in all directions, not just one.

That is really important.

B, when ions are formed.

Ions are formed by the transfer of electrons.

This is not the ionic bond.

And D, magnetic forces are very different.

You may be used to using magnets where the opposite poles, the north pole and the south pole attracts, but it's not an electrostatic force.

So we mustn't confuse electrostatic forces with magnetic forces because they are different.

So excellent work if you've got C.

Ionic compounds form regularly shaped crystals such as copper II sulphate, which are those beautiful blue-colored crystals that you can see in the photograph.

So during the process of crystallisation, the positive copper II metal ions are attracted to the negative SO42- non-metal ions.

The electrostatic forces of attraction between the oppositely-charged ions, that's the Cu2+.

The SO42- minus are the ionic bonds that hold together that giant lattice structure.

And that's why it has a lovely regular shape because you've got a regular pattern of copper II plus and SO42-.

And if you've ever grown copper II sulphate crystals, you'll see that they have a lovely shape.

So, to grow copper II sulphate crystals in the lab, and this is something that you might be able to do, an easy way to do it is to make up a saturated solution of copper sulphate, and then gently evaporate off the water and leave to crystallise.

Now, there are a couple of ways of evaporating off the water.

One way is to use a Bunsen burner.

We've got to be very careful if we do that because if we heat it up too quickly, and it boils, and all the water evaporates off very quickly, the crystals will not be formed well, and also they'll loose what we call their water for crystallisation, and they will end up looking pretty dull really, and it won't be that beautiful colour.

So I'm going to show you a very short video clip of an alternative way of evaporating off the water.

So we're gonna have a beaker of water, water bath, which is heated on the Bunsen burner.

And then on the top of that, place our evaporating basin, and you will be able to see the water evaporating off.

Once quite a lot of the water has reduced and gone, you will be able to see the crystals starting to form.

So it's a very short clip.

Look out for those crystals.

(liquid boiling) (basin rattling) When the crystals have started to form, we can remove the heat supply and leave the evaporating basin on its own to cool.

And eventually, you'll see crystals form a bit like what we've got in the image here.

So which of the following compounds have ionic structures? Is it magnesium carbonate, calcium chloride, carbon dioxide, or sodium sulphate? So well done if you've got A, magnesium carbonate, B, calcium chloride, and D, sodium sulphate.

The only one that doesn't is carbon dioxide, and we know that because there is not a metal in carbon dioxide.

Remember an ionic structure must have a metal and a non-metal element present.

So well done if you've got that correct.

That brings us to our first task.

For question one, we'd like you to define a, ionic bond, and B, lattice.

In question two, we'd like you to look at, explain why potassium chloride is an ionic compound.

And then in question three, please, can you sketch a diagram to show how the ions are arranged in an ionic compound? Pause the video, work through the questions.

And then when you're ready, we'll look at the answers together.

Okay, let's have a look at the answer to question one.

An ionic bond, well, it's the electrostatic force of attraction between oppositely-charged ions acting in all directions.

And it's important that you have the all directions part included in your definition.

A lattice, well, a lattice is the regular arrangement of atoms or ions in a 3D space.

In an ionic compound, the lattice consists of alternating positive metal ions and negative non-metal ions held together by strong electrostatic forces of the ionic bonds.

And it's important to include detail in this definition, talk about the alternating positive metal ions and negative non-metal ions.

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

Excellent work.

Okay, let's move on and have a look at the answer to question two.

Why is potassium chloride an ionic compound? Well, it contains potassium plus ions and chloride minus ions, and these ions are electrostatically attracted to each other.

It has a giant 3D lattice structure with a arrangement of K+ ions and CL- ions or potassium ions and chloride ions.

Again, to get the answer complete, you need to put in all those bits, talk about the ions present and how they are attracted together.

So very well done if you've got that correct.

Now, what about the sketch? We asked you to sketch a diagram to show how the ions are arranged in an ionic compound.

Well, you may have attempted to draw a 3D structure.

Well done if you have.

You must give a label, so show the negatively charged ion and the positively charged ion.

But if, like me, you're not very good at drawing, you might just want to draw a 2D diagram, that is fine too, in which case you need to show that alternating pattern in a layer.

And again, you need to label the negatively charged ion and the positively charged ion.

So well done if you manage to show a structure with those alternating ions.

Excellent work.

Right, we've now got one more question we'd like you to have a look at in this task.

Some students are discussing their ideas about ionic bonding.

And in a moment, I'll show you what the ideas are.

Who do you agree with and why? So we need to look carefully at these answers and see who is right.

Okay.

Izzy.

An ionic lattice is held together by a mixture of ionic bonds and other forces of attraction.

That's what Izzy thinks.

Is she right? Sofia says, "A positive ion bonds to all its neighbouring negative ions." Lucas says, "An ionic lattice contains small molecules." And Andeep say, "A positive ion forms an ionic bond with the negative ion when it transfers an electron." What do you think? Who do you agree with and why? Pause the video, have a go at the question, and then we'll look at the answer together.

Okay, so what did you make of that? Who was right? If you chose Sofia for the correct answer, well done.

An ionic bond is a positive ion bonding to all of its neighbouring negative ions.

So Sofia did a good job.

Excellent work if you chose her.

What about the others? Well, Izzy, Izzy's not quite right.

An ionic lattice is held together by ionic bonds, which are the electrostatic forces of attraction.

That is not what Izzy said.

What Izzy said was an ionic lattice is held together by a mixture of ionic bonds and other forces.

You don't get that.

It's only one type.

Okay? So that's what Izzy needs to learn.

Excellent work if you said Izzy was wrong and you got a reason.

So what about the others? Well, Lucas, I'm afraid he is incorrect.

Ionic lattices do not contain small molecules.

An ionic lattice contains positive and negative ions, and that's it.

No molecules.

So be really careful when you use the term molecule.

So what about Andeep? He said a positive ion forms an ionic bond with the negative ion when it transfers an electron.

Well, I'm afraid Andeep is almost there but not quite yet because the electron transfer occurs when the ions form.

It is not the ionic bond.

This is a common mistake people often make in exams. The ionic bond is a force of attraction between ions.

The electron transfer occurs only when the ions are formed.

So if you've got all of that right, well done because you really did need to think to answer that question.

Excellent work.

So that brings us to the end of our first learning cycle.

We're now gonna move on to look at how we can model giant ionic structures in our second learning cycle.

So we're gonna have a look at a ball and stick model first of all.

So a ball and stick model shows alternative pattern of positive and negative ions in the 3D lattice structure.

So you can see here we've got a ball and stick model.

We have our sodium ions, which are purple, and our chloride ions, which are green.

And we can also see very clearly there is one Na+ for every Cl-, or there is one purple for every green.

It gives us our ratio of one to one, and the formula, the empirical formula being NaCl.

Now, we use a lot of models in science, especially in chemistry, to try and explain those things we can't see.

So that's why we use models, but sometimes models have limitations.

And we're gonna have a look at some of those limitations now and think about it.

So the first limitation is it shows gaps between the ions.

So you can see where these ions are, the purples and the greens, but there's lots of gaps in those previous models that I showed you.

They were all stuck together closely.

There are no gaps between ions.

It's not to scale.

We actually can see in this particular model that they're all the same size.

And also, this is a giant model 'cause we're talking about stuff on the submicroscopic level.

It doesn't show how the electrons are transferred either.

So what we are modelling is just the lattice.

And the other thing is the ionic bond appears to be a physical connection between the ions.

And you can see that label on the ball and stick model showing where the ionic bonds are.

And perhaps we could say it's only acting in some directions, not every single direction.

So the ball and stick model does have limitations, and we've been through some of those.

So let's just check our understanding on the ball and stick model.

So in a ball and stick model of the lattice structure of sodium chloride, what do the sticks represent? Is it A, ionic bonds only? Is it B, forces of attraction? Is it C, ionic bonds and forces of attraction, or D, a physical connection? Well done if you chose A, it's the ionic bonds only.

So that is what the stick is representing.

So well done if you've got that.

So 3D models can be used to represent the ions in a lattice structure.

And we've got an example of a 3D model here.

This time it is a model of lithium bromide.

The bromide ions are shown as a large sort of rusty colour ion, and the lithium plus ions are shown in purple.

In this model, we can clearly see there is an alternative pattern of positive ions and negative ions.

You can see the purple and the browns sort of alternating between each other, and it is also 3D as well.

So there's some really good features of this model.

We can also see very clearly that there is one lithium ion for every bromide ion.

That gives us a ratio of one to one.

And it tells us the formula is lithium bromide, LiBr.

So that's another good point of the 3D model that is representing the ions in this lattice structure of lithium bromide.

But again, it's got its limitations.

So let's have a look at the limitations.

There is no information about the forces of attraction between the ions.

It doesn't show how electrons are transferred.

And again, it's not to scale.

So some of the limitations of this giant 3D model are similar to the ball and stick model.

Now, sometimes we might use a 2D model to represent the ions in an ionic compound, in which case we're kind of just showing one layer of the lattice.

So we've got a 2D model here of lithium fluoride where we can clearly see lithium plus ions and fluoride minus ions.

Again, the model is showing that alternative pattern of positive, negative, positive, negative in one layer of the lattice.

Again, it has its limitations.

So when we're looking at models and thinking about models, we do need to get into the way of evaluating them, thinking about what is good about the model and where the limitations are.

So what are the limitations for this 2D model? Well, first of all, there is no information about the forces of attraction between the ions, it's not to scale, and it does not show where the ions are located in other layers as it's not in 3D.

So perhaps this one is the most limited of the models that we've looked at so far.

Okay, so let's just check our understanding.

True or false.

The 3D model of sodium chloride is a true representation of the structure.

Is that true or false? Well done if you chose false.

Why? Is it A or B? A, it shows the repeating pattern of positive and negative charges.

B, it provides no information about the forces of attraction between the ions.

Well done if you chose B.

If it was a true representation, it would tell us something about the forces, but it can't tell us anything about the forces.

Okay.

So well done if you've got that question right.

So we come to our next task now.

Our first question is about a ball and stick model that's used to show the structure of potassium iodide.

The diagram that you can see of the model is slightly different to the one we looked at before, but it's clearly got balls and stick.

And what we want you to do is to complete the sentences to evaluate that model.

So the model is good because.

Then pick out some good points about the model, what the model can do.

The model is limited because.

Pick out some of the things that the model can't do.

And then this is the key part when you're doing an evaluate type question.

You need to say, "In my opinion, I think the model is good because," or, "In my opinion, I think the model is bad or poor because." It's your opinion.

You need to weigh up the positives and the negatives or the good and the limited.

Then if you can move on to question two, and please explain how NA+ and CL- ions bond together to form solid sodium chloride.

So pause the video, have a go at the questions.

And then when you're ready, we'll look at the answers together.

Okay, let's have a look at the answer to the first question.

So that ball and stick model, we're gonna do our evaluation.

So the model is good because it shows a 3D lattice structure.

It shows the potassium plus and the I- alternating a regular arrangement.

It shows a one-to-one ratio of K+ two I- ions.

So those are all really good features of the ball and stick model, but it's limited because it shows gaps between the ions.

It's not to scale.

It looks like there's a physical connection between the ions and the bonds appear only to act in some directions.

So really well done if you've got all of those points.

And if you didn't get all of them, just make a note of the ones that you missed.

So next time that question comes up, you'll be able to answer it.

Now, the final part of the evaluation, in my opinion, I think the model is.

You may have put I think it's good because it shows the 3D lattice, or you might have put it's bad because the stick does not explain the ionic bond.

You may also have come up with some different answers, but the key point here is it can be good or it can be bad.

It's your opinion, but you must give an explanation as to why you think it's good or why you think it's bad.

So really well done on that question, especially if you got the last part and you gave your opinion and you backed it up with some evidence.

Excellent work.

Okay, so we'll move on to our next question.

Explain how sodium plus and CL- ions bond together to form solid sodium chloride.

Well, your answers should include these points.

So when you're writing an explain answer, you need to try and put them into a logical order.

So here we go.

The sodium plus ions attract the chloride minus ions because of their opposite charges.

That's a really good place to start.

The ions bond to the closest oppositely-charged ions.

The ions packed close together, arranged in a giant 3D lattice structure.

The ratio of Na+ to Cl- is one to one.

There is a regular repeating pattern of Na+ to Cl-.

And the lattice is held together by ionic bonds or the electrostatic forces of attraction acting in all directions.

So those are the main points that should be included in your answer.

When you're answering it, try it and do a logical order.

If it's slightly different to my order, that doesn't matter, but you do need to get the points included.

So very well done if you've got all of those points included.

Great work.

Right.

We'll move on to the third question in this task.

So lithium metal reacts with chlorine to form lithium chloride.

And Alex and Laura have drawn some particle diagrams to represent the lithium chloride.

We've also got a bit of a symbol equation here.

We've said that two lithium solid reacts with chlorine gas to give two lithium chloride solid.

Now, Alex has drawn his diagram, which you can see there, and Laura has drawn her diagram.

So you need to look at them really closely.

And then what I'd like you to do is A, describe a benefit of Laura's diagram.

What does it help to explain? B, what misunderstandings could Laura's diagram cause? And C, describe how Alex's diagram could help avoid the misunderstandings.

So remember the particle diagram is representing lithium chloride.

Pause the video, have a go at the question.

And then when you're ready, we'll look at the answer together.

Okay, so let's have a look and see what Laura did.

So Laura has just drawn a purple and green kind of looks like a molecule twice.

So what can we say? Well, what Laura has drawn, it shows the empirical formula or the ratio of Li+ ions to Cl- ions, and it will help us balance that equation.

So that is definitely a benefit of drawing that diagram from Laura, but it could also lead to misunderstandings.

So in part B, well, what might be the problem there? It looks like the structure is made of small molecules or atoms. And we know from our learning that ionic structures are not made from small molecules.

So although it's helpful in showing the empirical formula, it's not so helpful in showing the actual structure.

Part C, describe how Alex's diagram could help avoid this misunderstanding.

Well, in Alex's diagram, we see that giant lattice structure and we still have that one-to-one ratio, which will give us the lithium chloride formulae.

So, not an easy question.

If you've got all those answers correct, really, really well done.

Excellent work.

So let's think about what we have learned in today's lesson on giant ionic structures.

First of all, salts form ionic structures because they contain metal and non-metal ions.

In an ionic structure, each positive ion attracts the negative ions and vice versa.

Metal ions attract all non-metal ions and vice versa.

They bond the opposite charged ions that are closest.

An ionic bond is the electrostatic force of attraction between oppositely-charged ions acting in all directions.

Ionic substances are 3D lattices with regular arrangements of ions in a repeating pattern of positive and negative charges.

I hope that you have enjoyed today's lesson, and I look forward to learning with you again very soon.