Lesson video

Hello, my name's Dr.

Warren and I'm so pleased that you can join me today for this lesson on bonding models.

We're gonna be looking at all types of bonding.

You've come across many of these models before in previous lessons in the structure and bonding unit.

This lesson brings them all together.

We're gonna work through this lesson together and I'm here to support you all the way, especially through the tricky bits.

Our learning outcome for today's lesson is, I can compare bonding models, describe which models would be useful to show different aspects of bonding, and describe the limitation of these models.

And here are today's key words, model, ball and stick model, dot and cross diagram, space filling model, and displayed formula.

We're now going to look at these key words in some sentences and you may wish to pause the video and note down their definitions so that you can refer to them later on in the lesson.

So let's have a look at them.

First of all, a model is used by scientists to represent parts of the natural world that are too difficult to observe or explain directly.

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

A dot and cross diagram is used to show how chemical bonds are formed between atoms. A space filling model is a 3D molecular model where the atoms are represented by spheres.

A displayed formula is a 2D model of a covalent substance showing all its atoms and bonds.

In today's lesson, there are two learning cycles.

The first is on 2D bonding models, and the second on 3D bonding models.

So let's get started with our first learning cycle on 2D bonding models.

So just as a quick reminder, scientists use models to represent parts of the world that are too difficult to observe or explain directly.

And this is certainly true when we come to talk about chemical bonding, we can't see the electrons that are involved, so we need to use models.

And for this topic, chemists use a range of different 2D and 3D models to explain how atoms bond together and why the physical properties of different substances are different.

But like with all models, there are their limitations.

And this is something that we will focus on during this lesson.

So as a quick reminder, and you may not have seen all of these before, we have a ball and stick model, we have a dot and cross diagram, we had a good go at those in the ionic and covalent lessons.

We have the space filling model, which may be new to some of you, and displayed formula.

We're gonna be looking at all of these in turn during this lesson.

So now we're going to have a closer look at a dot and cross diagram.

Now remember this diagram is used to show how chemical bonds are formed between atoms in an ionic or covalent bond.

So we'll start by looking at the sodium chloride example.

So we have a sodium atom with one electron in its outer shell marked by a cross, add a chlorine atom with seven electrons in its outer shell marked by dots.

And during the formation of ions, the sodium gives its electron to the chlorine, and we end up with a sodium plus ion, and a chloride minus ion represented by, the outer shell being shown with the dots and crosses and square brackets and a charge around it.

For a covalent bond, electrons are shared, and carbon dioxide is our covalent example.

You can see here that carbon in the middle has four electrons in its outer shell represented by crosses.

Each oxygen atom has six electrons in its outer shell represented by dots.

And we can see that in the overlap we have two shared pairs of electrons between the carbon and each oxygen atom so that each atom has eight electrons in its outer shell.

With dot and cross diagrams, it doesn't matter which atom has the cross and which atom has the dot, as long as you are consistent throughout the diagram.

So we're going to have a look at some advantages of this model.

Okay, first of all, the real advantage is it shows which atoms the electrons come from.

It helps us to work out the empirical formula of ionic substances or the molecular formula of simple covalent substances by counting the atoms and the ratios.

It shows the charges on the ions, which is important, to work out if they're positive or negatives.

And it shows the shared pairs of electrons as we can see in our example of sodium chloride where we've got the ions and water where we've got the shared pair of electrons.

So what about limitations of this model? Well, one of the things is it doesn't actually show the shape of the structure, it's a 2D model and it doesn't actually tell us that the ions are part of a giant 3D lattice.

So that's something we need to be aware of and make sure it doesn't lead to the misconception that ions are molecules.

There's nothing to indicate that a chemical bond is an electrostatic force of attraction between charged particles.

And this is something that is really important when we come to any type of bonding, that there are these forces between charged particles that we've learned about in previous lessons.

Our dot and cross diagram doesn't show that.

So looking again, it's our example of sodium chloride and water.

Even though we have drawn the water like that, it doesn't give us a 3D shape.

So those are the limitations of the dot and cross diagram.

So let's have a quick check of understanding, which statements about the dot and cross diagrams are correct? A, they show which atoms the electrons come from during chemical bonding.

B, the ionic bond is shown.

C, the covalent bond is shown.

D, all shared pairs of electrons in a molecule are shown.

So well done if you've got A, C and D, they are the correct answers.

So another type of 2D model you may have come across, if not it's quite an exciting one to do, is using pipe cleaners and beads of two different colours because they can show a model of covalent bonding and there's an example here of a water molecule.

So the red pipe cleaner is representing the oxygen atom and the white pipe cleaner is representing the hydrogen atom.

This model is really good at showing the idea of shared pairs of electrons because if you look carefully at that image, you'll be able to see that the blue and the orange beads are threaded through both the white pipe cleaner and the red pipe cleaner showing that they are sharing a pair of electrons.

And if you count them up, each hydrogen atom has two electrons in its outer shell, one coming from the oxygen atom and one coming from the hydrogen atom.

And the oxygen atom now has eight electrons in its outer shell.

One of the limitations of this model is that it can only show the outer shell, and there's nothing to represent any other parts of the atoms such as the nucleus or charges.

So that will make it much harder to be used to represent ions with charges.

But it's quite a fun way to actually learn about covalent molecules.

But you've got to remember, only the outer shell is shown.

Another 2D model that we can use for ionic models, is chemical jigsaws.

They're really good at working out the empirical formula.

And to do work out the formula, all you need to do is complete the jigsaw, and your jigsaw has positive pieces and negative pieces.

And you can see from this diagram here that the plus has a big hole in it, that is where one of the electrons has disappeared, so there's two holes in that one, so it's two plus.

And the blue negative charges have a bump on it, and that is meant to be representing where the electron has come.

So for example, magnesium iodide, magnesium has two plus ions and iodide one minus ions.

So we could actually use this model, put them together to work out the formula of magnesium iodide, which is MgI2.

So it works really, really well for working out empirical formula of ionic compounds.

That is why it is good, the limitation is it does not show that ions are part of a giant lattice structure, so could create the wrong idea that it's a molecule.

Remember, we do not get ionic molecules.

Ions are always part of a giant lattice structure.

Now a displayed formula can be used to model a covalent molecule.

And here are some examples.

Hydrogen, which is H2, two hydrogen atoms joined together, is the displayed formula is shown by a hydrogen, an H with a line, and another H.

Water, H2O, is shown by H with a line to an oxygen, with a line to an H and so on.

In carbon dioxide we have double bonds, i.

e.

two shared pairs of electrons and they're shown by two lines.

So the advantage of the displayed formula is it shows all the atoms and bonds in the molecule.

It uses lines to represent the shared pairs of electrons.

And compared to a lot of other models, it is very easy to draw and write down.

However, there are some limitations.

One of them being the electronic structure is not shown, so we don't see the actual electrons.

There's nothing to indicate there's any forces of attraction in that molecule.

Only models covalent molecules, you can't use it for other bonds such as ionic.

And it doesn't always show the shape of the molecule.

For example, water here is drawn in a straight line.

So quick check of understanding, a displayed formula is a 2D model of an ionic substance.

True or false? Well done if you chose false.

Now let's justify the answer.

The lines are used to represent shared pairs of electrons in covalent bonds, or B, the lines are used to represent the transfer of electrons between atoms in covalent bonds.

Well done if you chose A, the lines are used to represent shared pairs of electrons in covalent bonds.

That is a really important point for displayed formula.

We're gonna move on now to think about metallic bonding.

So 2D models can be used to model bonding in metals.

And again, this is something you'll have seen before.

We have our lattice of sodium ions, the positive metal ions all lined up in neat rows and columns.

We have electrons and a sea of delocalized electrons.

So you know, in many ways this is a good model because it shows that regular arrangement of positive metal ions.

It shows the delocalized ions, and it can be used to explain why metals are malleable and ductile, because those lines or rows of ions slide over each other.

It could be used to explain why metals are good electrical and thermal conductors, because the sea of delocalized electrons can move through the structure carrying electrical charge and carrying thermal energy.

So it's very, very good because of all of those things and really can explain a lot of the properties of metals.

But as with all models, there are some limitations.

And in this diagram, our electrons are static.

We'd have to know that they actually move.

We can only see one layer of the lattice, so that doesn't really indicate that it is a giant lattice.

It doesn't show how metals conduct.

It doesn't show the actual metallic bond, which is again a force, electrostatic force for traction between the positive metal ion.

So this 2D model can be easily extended to explain why the properties of alloys are different to that of a pure metal, which is a positive thing.

So if you look at the diagram we've got now, we can see that purple atom or ion is larger than all the other ions in the structure.

And what this does is it changes the properties.

It explains why, for example, some metal alloys are less malleable and ductile, because those larger atoms or ions disrupt that regular structure and make it harder for the ions to move over each other, making the alloy less malleable or less ductile.

So let's think about some of the disadvantages, the limitations.

So what are the limitations of the 2D metallic bonding model? A, it shows a regular arrangement of metal ions.

B, the delocalized electrons are not moving.

C, the forces of attraction in the metallic bond are not visible.

And D, it explains why metals are malleable.

So we're looking for limitations of this 2D model.

So well done if you wrote B, the delocalized electrons are not moving in this particular model.

So excellent work, if you've got that right.

The forces of attraction in a metallic bond are not visible, well done if you've got that right, they are both the correct answers.

The other two things are advantages of the model.

So they weren't the correct answers for this question.

So that brings us to our first task.

Answer the following questions about the models below.

So, one, why do scientists use models? Two, magnesium metal reacts with oxygen to produce magnesium oxide.

Describe a 2D model that could be used to show the formation of magnesium oxide.

Include a diagram in your answer, and then write down the limitation of your chosen model.

When you've done that, go on to question three.

Chlorine gas exists as a diatomic molecule, CL2.

A, draw the displayed formula for CL2.

B, the dot and cross diagram for CL2.

And then complete the sentences.

I would use the displayed formula if.

I would use the dot and cross diagram if.

So, pause the video, answer the questions, and then when you're ready, press play, and we'll have a look at the answers together.

So let's have a look at the answers.

Why do scientists use models? Well that's to represent parts of the natural world that are too difficult to observe or explain directly.

So well done if you've got that answer.

Question two, magnesium metal reacts with oxygen to produce magnesium oxide.

So what 2D model are you going to explain? Well, dot and cross diagrams show how bonds are formed.

The dots are used to show the electrons for one atom, and the crosses for the electrons form another atom.

So that's the model I would do.

And here is a diagram that hopefully you've drawn showing the magnesium with two electrons, the oxygen with six electrons, and then arrows showing how those electrons move.

And then having your magnesium oxide ions.

So to get this answer right, you need to make sure you've got show how the electrons are moving.

And then when you draw your magnesium oxide, make sure you've got your square brackets and the charges two plus and two minus.

Now if you chose to use dots on the magnesium and crosses on the oxygen, that's absolutely fine, it doesn't matter which way around you did it.

So if you've got all of that right, well done.

Okay, so part B, write down a limitation of your chosen model, and you only need to put down one limitation.

So you could have said, the giant lattice is not shown, so that will be one.

You might have said something around the ionic bonds are not shown and they act in all directions.

Or you might have said it looks like an ionic molecule.

So well done if you picked any of those.

Right, question three, chlorine gas exists as a diatomic molecule.

Draw this displayed formula.

So what you should have done is a CL with a line and then another CL.

Remember that line represents the shared pair of electrons.

For the dot and cross diagram, you want your two chlorine atoms both with seven electrons in the outer shell, one shown by dots the other shown by crosses and the overlap having a shared pair of electrons.

So excellent work if you got that right.

So part C, I would use the displayed formula if.

I just wanted to show all the atoms and bonds in a covalently bonded molecule.

So if that's all I was interested in, that's what I would do.

I would use the dot and cross diagram if.

I wanted to show how the chemical bond was formed and the shared pair of electrons.

So remember that's the main difference between the two.

So very, very well done if you got that correct.

Excellent work.

So that brings us to the end of our first learning cycle on 2D bonding models.

And we're now going to move on to our second learning cycle and have a look at 3D bonding models.

And we're gonna start by taking a look at the ball and stick model, which is used to represent atoms and bonds in chemical compounds.

We have an ionic one here, the sodium chloride ionic lattice that we can see, and also a covalently bonded molecule, the methane molecule, CH4, where we can see our carbon and hydrogen atom.

Now you've probably seen some of these models before in the classroom and use modelling mods, which basically are ball and stick models.

So what are the advantages of this model? Well, the first thing is we can actually use them to model both ionic and covalent substances, which is really, really good.

Taking a look at ionic substances first, well, you'll see from that previous slide we have that alternative pattern of positive and negative ions in a 3D lattice structure.

And that's really important 'cause we get the shape of the crystal and we also get the idea that it is 3D and the size of it.

The empirical formula can be determined from the ratio of the positive to negative ions.

And in that lattice we saw there was one chloride ion for every sodium ion, so that gives us that one-to-one ratio.

In a covalent substance, one of the really big advantages is the 3D shape, and we can see the bond angles.

We could see that that methane molecule on the previous slide was a tetrahedral shape.

And of course we can determine the molecular formula despite counting the atoms that are present in that molecule.

So lots of advantages for the ball and stick model.

So let's have a look at some of the limitations of the ball and stick model.

So we've got our diagrams there to help us, remind us of what they are like.

First of all, it shows gaps between the ions or the atoms. That is not very good because in practise there are no gaps.

It's not to scale and that's true of the models we've looked at so far as well.

And it doesn't show how the electrons are transferred during the formation of the ionic bond, it doesn't show how the electrons are actually shared, it just shows the results of that.

The bonds appear to be a physical connection between the ions or atoms. And we know from previous learning that actually there's no physical thing there, it is an electrostatic force for traction that holds the ions and atoms together.

And of course the bonds appear only to act in some directions, which is true on the ionic compound, but in practise the ionic bond is a force acting in all directions.

So we've got a few limitations there to this model as well.

So let's just check our understanding, what are the limitations of the ball and stick model? So A, in an ionic lattice it shows gaps between the ions.

B, it shows the shape of the covalent molecules.

C, chemical bonds appear to be physical connections, or D, the formula can be determined from the model.

What are the limitations? Well done if you've got A, that is correct, remember that model of the 3D lattice has gaps.

And well done if you got C, the chemical bonds appear to be physical connections, in practise they are not.

B and D are both advantages of the ball and stick model.

So excellent work if you've got those right.

Now gonna move on and have a look at the space filling model.

And this might be a new one to some of you.

It's a 3D model and there are no spaces shown between the bonded atoms or ions.

So that kind of gets around one of the issues that we had in the previous ball and stick model.

So the radii of each atom is shown in proportion.

So another thing that this one does is it actually helps to show the size of the ions, so the size of the ion, size of the atoms. And this diagram here of ethanol shows we've got a hydrogen atom that's a bit smaller than the carbon atom, and we've also got the oxygen atom at the back being shown a different size as well.

So that is another thing that comes in.

And if we look at the sodium chloride ionic lattice here, we can see clearly that the sodium ions, the purple ones are smaller than the chloride ions.

And this gives us the information again, from previous learning you will have already realised that ions and atoms are very, very small, and the atomic radii is in the order of magnitude of one times 10 to the minus 10, so a very small number, and you can see the actual radius here, and hopefully from looking at the diagrams, and the models, how they are in proportion.

So an advantage of that space filling model is it can be used to represent both ionic and covalent substances.

Looking now at the ionic substances, you've got that alternative pattern of positive and negative ions, and you can see the 3D ionic lattice structure.

So again, really important to get that, good advantage and also the empirical formula can be determined from the ratio of the positive to negative ions present, you can do that by counting.

So some of the advantages of the covalent substances, that 3D shape is seen, the atoms overlap, and this gives the idea of sharing electrons between atoms. So, that point where they overlap is the place where those electrons are shared.

And again, we can count the atoms to determine the molecular formula.

So these are the advantages of that space filling model.

But of course there are some limitations.

So let's have a look at those next.

It provides no information about the forces of attraction between the ions or shared pairs of electrons.

That's similar again to the other models that we've been looking at.

It doesn't show how the electrons are transferred or shared, only shows the result of them.

It's not to scale, even though we've got that idea of proportionality between the different sizes of atoms or ions, it's not to scale.

And again, just looking at those images will help us see what those limitations are.

So another quick check for understanding, which models can be used to represent the ethane molecule C2H6? Is it A, B, or C? Okay, very well done if you got A, and those of you who've looked very carefully at this image will be able to count, there are six white hydrogens, even though the ones at the back are quite difficult to see.

And two black carbons which make C2H6, and the ball and stick model, part C, is also correct.

And that's much clearer to see the two carbons and the hydrogen atoms. B of course is showing an ionic lattice, so it wouldn't be used for the ethane molecule.

So very well done if you got that correct.

So 3D space filling models can be used to model the bonding in metals.

We've not talked much about metals, so let's have a look at this one.

We have got our lattice of positively charged ions there from the metallic lattice, and we have got our delocalized electrons sort of shown on the side.

So this model is good, because it shows that regular arrangement of positive metal ions in that giant lattice structure, it's good because it shows the 3D electrons, and it could be used to explain why metals are malleable and ductile, and it can be used to explain why metals are good electrical and thermal conductors.

So overall it's a good model, but as always there are some limitations on the 3D space filling models.

For metals, now what are those limitations? Well, the delocalized electrons, those shown in blue, are static.

We don't actually get the idea that they are moving.

It doesn't provide any information about the metallic bond in terms of forces, electrostatic forces of attraction between the positive metal ions and the negative electrons.

It doesn't show that idea of a sea of electrons that these electrons can move over quite a range of distance.

So that is another limitation of the model.

And we can't tell from this model how good the metal is at conducting.

We can say it's a good electrical conductor, but we don't know, we can't compare that of different metals.

So that is another limitation of the model.

So another quick check for understanding, what are the advantages of the space filling model? So it's the advantages this time we're looking at.

A, in a ionic lattice, it shows the alternative pattern of positive and negative ions.

B, it does not provide any information about electrostatic forces of attraction.

And C, in covalent molecules the atoms overlap, giving the idea of sharing electrons between atoms. So make your choices.

So well done if you chose A, and well done if you chose C.

In covalent molecules the overlap gives the idea of sharing electrons between atoms. And of course B is basically a limitation.

And this question we were asked for advantages.

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

So that now brings us to our next task.

We've got some questions for you.

So question one, there are several different ways to model ammonia, which is NH3, and you've got some models in front of you, you just need to label each model and then give an advantage and a limitation for each of the 3D models.

Question two, we've got some 3D models used to model sodium chloride, NaCL.

Again, label the model, and for each model give a limitation and an advantage.

So pause the video, have a go at the question, and then we'll look at the answers together.

Okay, so question one, we have a space filling model, that's the first one.

The second one is a ball and stick model.

And the third one is a displayed formula.

So for the 3D models for the space fitting model first, the advantage, well its 3D shape is C, the atoms overlap giving the idea of sharing between atoms. We can determine the molecular formula by counting the atoms. And limitation is, it doesn't show how the electrons are shared and there's no information about the actual bonds.

So very well done if you've got the advantages and limitations.

Now for the ball and stick model Again, the 3D shape is seen, we can determine the molecular formula by counting the atoms. A limitation is the stick suggests that there is a bond is a physical connection between the atoms, which of course it isn't, it's an electrostatic force between the atoms. So very well done if you've got those answers right.

So let's have a look at the answer to question two now.

First of all, the labels.

Well the first one is showing a space filling model.

And the second one, a ball and stick model.

So well done if you've got those two correct.

For question B, the advantages first of all, the space filling model, well, it shows a giant lattice structure.

It shows that regular pattern of positive and negative ions.

We have the empirical formula that can be determined by counting the ions.

So those are all advantages.

Limitations is it doesn't show how the electrons are transferred, and there's no information about the bonds.

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

Now for the ball and stick model, the advantages are again very similar, regular pattern of positive and negative ions are shown.

And we have a 3D giant structure seen, and we can determine the empirical formula.

Limitations, well, the stick suggests that the bond is a physical connection between the ions.

Bonds aren't acting in all directions on this model, and we can see gaps between the ions.

So very well done if you've got all of those correct, that's great work.

So now we come to our next question, which is about the model of a metal.

So for part A, we'd like you to label the diagram.

For part B, give an advantage of the model.

And for part C, explain why dot and cross diagrams are not used to model the bonding in metals.

Pause the video and then when you're ready we'll look at the answers together.

So first of all, the labels.

Well, we've got a delocalized electron, and we've also got the lattice of positively charged metal ions.

So well done If you've got those labels correct.

Part B, given advantages model, well, you could have any of the following.

First of all, it shows the regular pattern of positive metal ions in a lattice structure, and delocalized electrons.

It can also explain the properties of metals, for example, why they're ductile, malleable and good electrical and thermal conductors.

So well done if you've got an advantage of this model.

Part C, well, we can't use a dot and cross diagram to model the bonding of metals because in a dot and cross diagram, the electrons, in particular atoms are shown as dots and crosses.

And of course, in metallic bonding we only have one type of atom.

It also does not model the delocalized electrons.

So that is why we cannot use a dot and cross diagram to show the model of a metal.

So very well done if you got that right.

So that brings us to the end of this lesson on bonding models.

So let's have a look at a summary of the key learning points.

First of all, scientists use models to represent parts of the natural world that are too difficult to observe or explain directly.

There are multiple ways, models, of drawing or showing bonding.

There are multiple advantages and limitations of particular representations and models.

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