Lesson video

Hello, I'm Dr.

Warren, and I'm so pleased that you can join me today for this lesson on metal alloys.

It follows on from the lesson on giant metallic structures, and is part of the Structure and Bonding unit.

I'm going to learn about this topic together and I'm here to support you all the way through all the tricky parts.

By the end of today's lesson, we should be able to evaluate the metallic structure model in terms of its ability to explain the physical properties of metals and metal alloys.

Now we've got some key words for you; Alloy, physical properties, delocalised, and molten.

And now I'm going to give you these words in some sentences to really help understand what they mean.

So an alloy is a mixture of two or more elements, where at least one element is a metal.

And that's really, really important, because we need to have a metal there for its structure.

A physical property is a characteristics of substance that can be observed or measured.

For example, the temperature at which a substance melts.

Electrons are said to be delocalised when they're free to move through the structure of a metal and can carry an electrical charge.

Molten is a term used to describe a liquid substance formed by heating metals, glass, or rocks.

You may want to copy down these definitions so you can refer to them later on in the lesson.

So if you do pause the video, and when you're ready to start, when you've done that, and ready to move on, just press play.

So in today's lesson we have three learning cycles.

First of all, we're going to look at the metallic structure model, then we're going to look at alloys.

And finally we're going to look at physical properties of metals and metal alloys, and refer back to that metallic structure model.

So let's get learning with our first learning cycle, metallic structure model.

I'm gonna start by asking some big questions.

Why do metals and metal alloys have high melting points? Why are metals and metal alloys ductile and malleable? And why are metals and metal alloys good electrical and thermal conductors? Well, these are the questions that we're gonna try and answer and use the metallic structure model to help us.

You see scientists try and explain physical properties using models, because we can't see what's happening at the atomic level.

The atoms and the ions are just so tiny, we can't observe what's happening.

So we come up with a model and we test out our model, and that's what we're going to do today.

So we've come across this in previous learning, but we're gonna have a little bit of a recap.

A pure metallic is made up of only one type of atom, and they form a giant 3D structure.

You can see you've got layers of positively charged metal ions, which are cations, surrounded by a sea of delocalised electrons.

Those are the electrons that are able to move freely throughout the structure.

So we've got an animation here showing the model.

The outer shell electrons are delocalised, they're no longer bound to a single metal atom.

They can move throughout the structure, and it's this that gives the metals a lot of their properties.

The metallic bond, it's a strong electrostatic force of attraction between those positively charged metal ions and the delocalised electrons.

And that's a really, really important point.

It is a strong electrostatic force of attraction.

So just looking at an example here, we've got all lithium, it's a metal, it's in group one of the periodic table.

It forms a lithium plus ion.

It loses the electron, and it goes into the sea of electrons.

Those electrons can move freely between the positive metal ions right the way through the structure, and that's why we call it a sea, because it can move a bit like the ocean.

So let's just check our understanding in the metallic structure model, what is the term sea referring to? Is it the delocalised electrons? The positive metal ions also known as cations? Or is it the metallic bonds between the positive metal ions and the delocalised electrons? Well done if you chose A, well remembered.

It is the delocalised electrons.

So the metal atoms give the outer shell electrons into the delocalised sea of electrons.

And depending on what atom there is, a different number of electrons are given.

So for example, sodium is in group one of the periodic table, so it will give one electron, it's got one electron in it's outta shell.

Magnesium forms a magnesium two plus ion because it's in group two of the periodic table, and gives up two electrons from its outer shell.

Aluminium is in group three of the periodic table, and will give up three electrons from its outer shell.

Copper and iron for example, are in the transition metals, they're a bit harder to work out, so we'll just have to learn those ones for the moment.

The important point here is that the size of the charge on the ion depends on the number of outer shell electrons that have been lost.

Now you'll notice from the table, we have the ionic radius.

That is telling us how large or how small the metal ion is, and it's measured in nanometers.

And nanometer is one times 10 to the minus nine metres.

So it's a very, very small number.

And generally ions with larger charges like aluminium have a smaller ionic radius than those with smaller charges like sodium.

So you can see it goes from 0.

054 for aluminium to 0.

95 for sodium.

And the reason for this is that the nucleus with a positive, larger positive charge on the atom has a stronger pull on the other electrons in that actual ion.

So it can be a little bit confusing because we've got electrostatic forces of attraction within the metal ion, and also within the metal structure.

So the differences in sizes and chargers are also shown in our metallic structure model.

And you can see here we've got lithium with one plus, and one electron into the sea of electrons from each metal ion, and calcium, we've got twice the number of electrons that are in the sea of electrons.

So the size of the positive charge increases from one to two.

The number of the delocalised electrons doubles.

A really important point if you are drawing some of these structures.

So let's pause now and have another check to see what we've understood.

Which of the following statements about metal cations are correct? A, B, C, or D? Well done if you chose B.

The size of the charge on the cation depends on the number of outer shell electrons that are lost.

And well done if you chose C, cations always have a positive charge.

We know from our previous, from our previous discussion, those all metal cations are not the same size, and they don't always have the same charge.

So very well done if you picked B and C.

Okay, we're going to look at our first task.

So we've got three questions we'd like you to have a go at.

First of all, we'd like you to draw a diagram to show the metallic structure model.

Then tell us what delocalised electrons are.

And you're gonna need a periodic table for question three, but use a periodic table to work out the charges of the ions formed by metals in groups one, two, and three.

So pause the video, have a go at the worksheet, and then when you're ready, we'll look at the answers together.

Okay, when we draw our model, we can choose any ion that we want.

Here is the model of lithium.

What is important is we have our regular structure in rows and columns of the positive metal ion, that's a cation.

We show the electrons, and there must be the same number of electrons as there are positive charges.

So here we've got six positive charges and six electrons shown, and we show the sea of delocalised electrons, that area where they can go.

So very well done if you've got that diagram correct and you've got all the labels correct.

What are delocalised electrons? Well, delocalised electrons are those electrons that are free to move through the structure of the metal.

And that's the key word there, free to move, they are not associated with a single atom.

Right, so let's have a look here.

If metals are in group one, they form ions with a charge of one plus.

So you may have chosen the lithium plus, sodium plus, or potassium plus.

Remember to use the symbols.

Metals in group two form ions for the charge of two plus.

Beryllium two plus, magnesium two plus, and calcium two plus.

And metals in group three form ions with a charge of three plus, for example, aluminium three plus.

So very, very well done if you manage to get all of those ions with the charges written correctly.

Remember, the charges of an ion are always written up as a super script.

We've now finished our first learning cycle, and we know all about the metallic structure model.

So we're gonna move on now to think about alloys.

So let's get learning about alloys.

Now we need to remind ourselves about the pure metal structure.

We've got our metal ions, they are arranged in a regular lattice.

We can see the sea of delocalised electrons.

It's really important that we have that model in our heads while we move on to think about the alloys.

Now, we wrote down that definition earlier in the lesson, an alloy is a mixture of different elements, where at least one element is a metal.

And this means that the regular structure that we can see gets disrupted.

We've got another diagram here, and we can see we've got a larger atom or ion right in the middle of all those lovely regular rows, which means they're disrupted.

And we have an irregular lattice in the structure of an alloy.

And that's the thing that makes it different, and that's the thing that changes the properties of an alloy compared to its pure metal.

When we make a mixture, what we normally do, is mix together some solids or some liquids.

And as we've already said, an alloy is a mixture of elements where at least one of them is a metal.

Now the problem we have is metallic bonds are very, very strong.

So in order to get another ion or another element into that metallic lattice, what we need to do is melt it so the ions can move around and they can be mixed.

So in reality, what happens is the metals are heated up until they turn into the liquid state, and then they are mixed together.

Once they're mixed together, they're poured into a mould, and the mixture is left to cool to room temperature and you have that solid alloy.

Now, alloys are really important, because the structure is not as regular as a pure metal and it affects its properties.

And that's why scientists are so interested in using alloys.

If we look at an example, pure iron, it's relatively soft and it rusts quite easily.

And I'm sure you've all seen things that have been left outside in the winter when it's cold, and it's wet, it's rainy, and you might have a rusty chain, it might be something on your bike, or an iron, or a padlock, and it basically rusts, and it's not so good for that material.

So what scientists often use instead is steel, and that's an alloy, it's a mixture of iron and carbon.

It's harder than pure iron, and it's less likely to rust.

And so steel is a really good metal to use for building, and you may have seen it in building bridges and other constructions.

So alloys are widely, widely used to improve their physical properties over the individual atoms that they're made from.

And that's what is the great advantage of it.

So glasses frames, for example, you know, we want glasses frames that are flexible so that if you sit on them, they don't break.

Well, we can use an alloy of nickel and titanium that has those super elastic properties that it returns to its original shape.

And it basically means that the glasses frames are less likely to break.

Or for example, if you're musical, you might be interested in brass instruments.

A saxophone, for example, is made from brass, brass is an alloy of copper and zinc.

And it's basically stronger and easier to shape than zinc or copper.

And it's also got very good acoustic properties, which is why we get a lot of brass instruments.

So now let's have a quick check of understanding.

Which of the following is the most accurate definition for the word alloy? A mixture of two or more elements, a mixture of two or more elements where at least one element is a metal, or a compound existing of at least one metal elements? Well done if you chose B, that is the best and most accurate definition of an alloy.

Yes, it's a mixture of two or more elements, but we must say that one of them is a metal.

We come to our second task now, and we've got a couple of questions for you.

First of all, you want to write down what the word molten means.

And then we're gonna be looking at bronze as an alloy of copper and tin.

So why should both metals be molten when the alloy is made? The second question, brass is an alloy made from copper and zinc, can you draw a labelled diagram to show the structure of this alloy? Pause the video while you work through these questions and then when you're ready, start it again, press play to look at the answers.

Okay, so let's have a look at the answers.

What does the word molten mean, first of all? Well, molten is a term used to describe a liquid substance formed by heating solid metals, glass, or rocks.

So very well done if you've got that definition right, because it's something you've had to recall from earlier in the lesson.

What about part B, why should both metals be molten? Well, there's a few points that we need to make here.

First of all, metallic bonds are very strong.

And in pure copper, all the cations and the giant lattice are in a regular pattern.

So I'd start by writing that down in my answer.

Then I'd go on to say that in order to add the tin cation to the lattice, the copper ions must be able to move.

And to do this, both metals need to be heated up until they melt and turn into the liquid state so that we can mix them together.

So again, very, very well done if you've got all of those points in, that is a fantastic answer.

Now we'll move on to question two where we had to draw a labelled diagram to show the structure of the alloy.

These are the important things, we need to show the sea of delocalised electrons, we need to show our copper ions, and we need to show our zinc ions of a different size.

The other thing that's important to show is a disrupted or irregular lattice structure.

So again, well done if you've got all of those points and those things labelled, that's great.

So this brings us to the end of our learning cycle about alloys.

And now we are going to move on to look at the physical properties of metals and metal alloys.

And try and apply our learning so far to these different situations.

So let's get started with the physical properties.

So the physical properties of metals and metal alloys are directly influenced by the metallic bonding.

And we can use the model that we've been looking at to explain these physical properties.

Just remember we mentioned this before, that it's important that scientific models are evaluated.

So when a model can no longer explain the observations, scientists need to change the model.

A model is only as good as what it can actually tell us.

So we have metals, and our metal alloys have a high melting point.

That's something that we know from the data.

We have our model here.

And in our model we can see the metallic bond is a very strong electrostatic force of attraction between the positively charged metal ions and the delocalised electrons.

We've got that 3D structure, we've seen it before.

The key point here is a lot of energy is needed to overcome all of these attractive forces.

And when it does break that metallic bond, the ions are free to move and move into the liquid state.

That it is why alloys have high melting points.

So our model can explain why alloys and metals have high melting points.

Alloys are often less ductile and malleable than pure metals.

Well, this is all down to the different size ions or atoms in the lattice that disrupts that regular pattern.

So when you apply a force, it makes it harder for the metal ions to slide over each other.

So the metallic structure model can explain that metals and metal alloys are often ductile and malleable, but often why we find with the alloys, they are less ductile and malleable than pure metals.

Now let's have a quick check of understanding.

What we'd like you to do is match the physical property of a pure metal to its explanation.

So high melting point, is that due to the reason A or B? Ductile and malleable, is it due to A or B? Okay, well done if you said ductile and malleable is due to A.

That's because the atoms are arranged in layers that can slide over each other.

And also very well done if you've got the other part right.

High melting points are due to B, a lot of energy is needed to break that strong metallic bond between the positive metal ions and the delocalised electrons.

So we're gonna move on and have a look at some other properties now.

Metals and metal alloys are good electrical conductors.

We know this because we have seen it in our experiments.

That if we put a metal into electric circuit, it can light up a light bulb.

Now what's the reason? An electric current is a flow of charged particles such as electrons.

And in our metallic structure, we have delocalised the electrons, they can move throughout the structure of the positive ions.

This means they can carry an electrical charge.

So because they can carry an electrical charge, we know that the metallic structure model can explain why metals and metal alloys are good electrical conductors.

We know from our experience that metals and metal alloys are good thermal conductors.

We often use metals as sauce pans where we are wanting to heat up something.

Again, it's down to our structure.

Very similar to the electrical conductors.

We have the movement of electrons, they can allow energy to be transferred through a substance.

So if we've got our delocalised electrons moving throughout the structure carrying energy, then we can say that the metallic structure model can explain why metals and metal alloys are good thermal conductors.

So, so far it seems to be a really good model to explain our observations.

Quick check for understanding again, metals and metal alloys are good electrical conductors? True or false? Well done if you picked true, that is correct, but why? Have a look at A and B and then decide the reason why are metals and metal alloys good electrical conductors? Well done if you chose A, they contain delocalised electrons, which are free to carry charge throughout the metallic structure.

Great, we're doing really, really well.

Okay, so this brings us to task C, our final task of today's lesson.

And we've got a question for you.

Steel is an alloy of iron and carbon.

So I want to start by drawing a label diagram to show its structure.

It's got a high melting point, I want you to suggest why.

It's also stronger than iron, and we want you to use your knowledge of the metallic structure model to explain why.

So if you would like to pause the video while you answer these questions, and then we'll have a look at the answers together.

Okay, so now let's have a look at the answers.

First of all, that labelled diagram, you want to have something that looks like this.

It's important to highlight the, and label the iron ions, and show that disrupted or irregular lattice structure.

You also want to have some carbon atoms as a different colour and a different size atom in the structure, and also label the sea of delocalised electrons.

So really well done if you got that correct and you've got all those labels, that's absolutely fantastic work.

Okay, so part B, the melting point of steel.

It's a high, why? Well, things that we need to include in the answer here is it takes a lot of energy to break all the strong metallic bonds between the positive metal ions and the delocalised electrons.

You also might have said what, instead of strong metallic bonds, if you'd said the strong electrostatic forces of attraction, that would be fine as well.

So again, really well done if you got that right.

Moving on to part C, steel is stronger than iron.

We need to use our knowledge to explain why.

We need to make the point that we have carbon atoms. They are a different size, and it makes it harder for the layers of iron cations to slide over each other.

Because what the carbon atoms do is they disrupt the regular structure found in pure iron.

Therefore, steel is stronger than iron.

So again, if you've got that argument put forward, well done, you've done some great work on this.

We're gonna move on to question two now, which is all about evaluating how good the metallic bonding model is.

So we're gonna start by doing a bit of reflection.

Want you to put a tick in the right column.

So does the metallic bonding model A, does it explain why the physical properties of pure metals and alloys are different? B, does it explain why metals have a high melting point? And C, does it explain why metals and metal alloys are good thermal conductors? When you've done that, you wanna move on to part B, and this evaluates the metallic structure model.

So when you're doing an evaluation, you need to make sure you put both sides of the arguments.

So start off about thinking what is good about the model, what is not good about the model, and what your overall opinion is about the model.

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

Okay, so part A, how good is metallic bonding model? It's pretty good, it explains all the points given in the question.

So well done if you've got that.

B, evaluate the metallic structure model.

Well your answer might be slightly different to mine, also might be similar, but this is what I wrote.

I think the model is good because the general physical properties of the metals and metal alloys can be explained.

I think the model is not good because it doesn't explain specific properties.

For example, why one metal is better conductor than another.

Overall, in my opinion, I would say the model is good, because it can explain more properties than it can't.

So well done if you manage to put together some sort of argument like that, that's brilliant.

So let's just summarise our key learning points from today's lesson.

In pure metals, ions are arranged in a regular pattern with delocalised electrons between them.

If a metal is stretched or forced out of shape, it's ions are able to move position without the metallic bonds breaking.

An alloy is a mixture of different metal elements or metals with non-metal elements such as carbon.

Ions of different metal elements may be different sizes and have different electrical charges.

The structure of an alloy is not as regular as a pure metal, and this affects its physical properties.

I've hoped you enjoyed this lesson, and I look forward to meeting you again very soon.