Lesson video

Hello, my name's Dr.

Warren and I'm so pleased that you can join me today for our lesson on "Drawing Ionic Diagrams For Ionic Substances." It's part of the "structure and bonding" unit.

We are gonna work through this lesson together and I'm here to support you and help you through all the tricky parts.

The learning outcome for today's lesson is, "I can draw dot and cross diagrams for binary ionic compounds." Let's have a look at our keywords for today's lesson.

Binary ionic substance, empirical formula, ratio, dot and cross diagram, and magnitude.

So let's have a look at these in some sentences to try and understand their proper meaning.

A binary ionic substance is made up of two different elements, one of which is a metal and the other, a non-metal element.

An empirical formula shows the simplest whole number ratio of atoms of each element in a compound.

A ratio can be used to determine the size of one quantity in relation to another.

A dot and cross diagram is used to show how chemical bonds are formed between atoms, and the magnitude is the size of a quantity.

For example, the size for charge on an ion.

So why don't you just pause the video, copy down these definitions so you can refer to them later on in the lesson.

And then when you're ready, press Play and we'll move on to the next part together.

There are two learning cycles in today's lesson.

The first is simple binary ionic substances.

And then once we've mastered those, we'll move on to more complex binary ionic substances.

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

A binary ionic substance is made up of two different elements, a metal and a non-metal.

So here's an example, sodium chloride, NaCl.

It's a binary ionic substance because it has the elements, sodium and chlorine.

Sodium oxide is another example.

It contains the elements, sodium and oxygen.

Doesn't matter what the ratio is.

The key point here is there are only two elements, a metal and a non-metal.

So let's just check our understanding to make sure we have understood what binary ionic substances are before we move on.

So it's copper sulphate, potassium bromide, iron oxide, or calcium carbonate a binary ionic substance? Well done if you chose B, potassium bromide.

It's made up of the potassium and bromine.

And well done if you chose iron oxide, it's made up of the elements iron and oxygen.

If you look at the other two, they're both contain three elements.

So copper sulphate has a copper, a sulphur, and an oxygen, and calcium carbonate has a calcium, a carbon, and an oxygen.

So they are not binary ionic substances.

So a dot and cross diagram can be used to show how an ionic bond is formed between a sodium atom and a chlorine atom.

So here we have just the outer shell drawn of a sodium atom.

It's in group one of the periodic table.

It has one electron in it's outer shell.

Chlorine is in group seven of the periodic table.

It has seven electrons in its outer shell.

So what's actually happens, the electron from the outer shell, the sodium atom transfers to the outer shell of the chlorine atom.

And this results in two ions being formed.

So the sodium ion, and we can now see the next shell inwards has now got eight electrons in its new outer shell and a positive charge.

And the chloride ion has got eight electrons in its outer shell.

And we can see that one of those electrons is coming from sodium as it's marked with a cross.

So the sodium atom forms a positive sodium ion when it's outer shell electron is transferred to the chlorine which forms a negative chloride ion.

And it's really important that we understand that it's during this transfer of electrons that the ions are formed.

So when those ions are formed, they're then held together by electrostatic forces of attraction.

So the positive and the negatives are held together.

And the ionic bond is the electrostatic force of attraction between the oppositely charged ions.

It's not the transfer, that's the process.

The actual ionic bond is when you get those sodium plus ions being attracted to those chloride minus ions.

So let's just have a look at a bit more detail of a dot and cross diagram.

First of all, we only show the outer shell of the electrons.

It makes it easier to draw them and less messy.

So our chloride, we're now showing the outer shell of the chlorine atom, but it has an extra electron, which we're showing by a cross.

Once we've done that, we show the ions with the full outer shells.

We then put a square bracket around it and a charge in the top right hand corner.

So you can see those square brackets and you can see the minus charge.

We then show the imperial or the empirical formula or the ratio of ions in the giant ionic lattice structure.

So in the case of sodium chloride, we have one sodium plus ion for every chloride minus sign, it is in a ratio of one to one.

The empirical formula is sodium chloride.

So these dot and cross diagrams are great for showing how the electrons transfer, are great for showing how the ions are made up.

But what they don't do is show that giant 3D structure that we have learned about in previous lessons.

So let's have another check for understanding which dot and cross diagram is correct.

They are all potassium chloride, but only one is drawn correctly, A, B or C? Well done if you chose B.

B is written correctly, we can see the full outer shells.

We've got our square brackets and we've got the charge in the top, at the top right hand corner.

If we look at the first one, there's no charges on it in part A, so that is incorrect.

And if we look at part C, we can see that the charges are inside the square bracket, so that is incorrect.

So whenever you are doing the dot and cross diagrams of ions, you need to write it as shown in answer B.

Okay, let's have a look at another example.

Magnesium is in group two of the periodic table.

So it's outer shell has two electrons as shown here, and oxygen is in group six of the periodic table.

So as we can see here with the dots, there are six electrons in its outer shell.

So what actually happens during the formation of ions, the magnesium atom loses both of its outer shell electrons and those electrons are transferred to the oxygen atom.

So we can see here by an arrow, the transfer of one electron to the oxygen atom and the transfer of the other electron to the oxygen atom.

So the outer shell on the oxygen atom is filled forming the oxide ion.

And that will have now two electrons shown as crosses because they are the ones that have transferred from the magnesium atom.

Those ions are then held together by electrostatic attraction in the giants lattice.

So you can see here we have drawn the ions.

You can see that the oxide ion has the two electrons from the magnesium shown as crosses, and a charge of two minus, the magnesium ion has a charge of two plus.

And we're showing the full electron shell, which is now got eight electrons in it.

The empirical formula is magnesium oxide, MgO, because for every Mg two plus ion there is 102 minus ion.

Okay, so to draw a dot and cross diagram what we need to do, so we'll think about the example of lithium fluoride.

We need to draw our lithium ion, which has two electrons, and it's outer shell now 'cause it's already become an ion.

We have to shore our fluoride ion, which has the eight electrons in its outer shell.

Now we can see one of them has come from the lithium, it's marked as a cross.

We add our square brackets, bound the lithium and the fluoride, and then we add the charges a plus and a minus going up in the right hand side.

Okay, your turn.

See if you can draw the dot and cross diagram to show the ionic bonding in potassium chloride.

You might want to pause the video while you do it.

Okay, so hopefully you've drawn something like this.

Your potassium iron shown eight electrons in its outer shell with a square bracket and a plus charge in the top right hand corner and the chloride ion showing its outer shell full with seven electrons from the chlorine atom and one electron from the potassium atom making the ion, and again, square brackets and a negative charge in the top right hand corner.

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

It's important that you get all the electrons drawn correctly, the square brackets and the charge.

Excellent work.

So that brings us to our first task on simple binary ionic substances.

To start with, we're gonna check that we understand everything by completing these sentences.

So the empirical formula.

A dot and cross diagram shows ionic compounds are neutral overall.

So pause the video and have a go at completing those sentences.

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

Okay, let's see what the answers are.

The empirical formula shows the simplest whole number ratio of the atoms of each element in a compound.

Well done if you got that right.

A dot and cross diagram shows how the chemical bonds are formed between the atoms. So the electrons from one atom is shown as dots and the electrons from the other atom are shown as crosses.

So well done if you got that right.

Ionic compounds are neutral overall because the positive and negative charges must balance in order to cancel each other out.

So what we've got to remember here is that a compound has, is neutral, it has no charge.

So the positives and the negatives must always balance out.

And we've seen that in all the examples we've done so far.

Right, we're gonna move on to our second question now.

You've got some outlines to draw dot and cross diagrams and we, this time we'd like you to do it slightly differently and show the electrons in each shell of the ion.

So for the first example, it's calcium sulphide.

So it's a calcium ion and a sulphide ion.

And for the second part B, it's lithium chloride, it's the lithium ion and the chloride.

So pause the video while you have a go at completing these diagrams. Okay, so let's have a look at the answer.

So for calcium sulphide, we have two electrons in the first shell, eight electrons in the second shell, and eight electrons in the third shell.

Calcium is in group two of the periodic table.

We must show a charge of two plus because it will have given those two outer shell electrons away.

Sulphur has two electrons in the first shell, eight electrons in the second shell, and six electrons in the third shell.

But it's gained two electrons from the calcium, which we have shown as crosses.

So it's outer shell now has eight electrons as well.

And it's got a charge of two minus.

So really well done if you've got all of those electrons in the right places, the square brackets and the charges.

Excellent work.

Now let's have a look at B, lithium chloride.

The lithium ion only has two electrons in its outer shell, so we draw those on our square brackets and a plus.

The chloride has two electrons in the first shell, eight electrons in the second shell.

And with that electron from the lithium, it has eight electrons in the third shell as well.

So all of its shells are full.

We've got our square brackets and the minus.

So really well done if you've got all of those electrons and charges and square brackets in the right place.

Excellent work.

Right, now, question three.

So often you might go onto the web and you might see some examples.

Sometimes they're not always drawn right.

So a website shows a dot cross diagram for sodium chloride and this is what they have shown.

So part A, just write down what a dot and cross diagram shows.

And then for part B, what I want you to do is look really, really carefully at this diagram and spot all the errors because I think you'll find one or two.

So pause the video and when you are ready, we'll have a look at the answers together.

Okay, so what does a dot and cross diagram show? Well, it shows the empirical formula or the ratio of sodium ions to chloride ions.

Now let's see if we can get all the errors that are in this diagram.

Well first of all, both ions are drawn with crosses.

So the electrons and sodium crosses and electrons on chlorine crosses.

So that is not very helpful.

When we're drawing dot and cross diagrams, it doesn't matter which way round you do do it, but one atom must be dots.

Otherwise we don't see the electron transfer.

So no dots are being used.

You can put it down that way.

We'll also see there's no square brackets around the ions and there's no charges on the ions.

So that's quite a lot missing.

Also, if you have a look where this arrow is, it makes it look like the electron transfer is the ionic bond.

There is nothing to say we've got a plus or a minus.

So again, I think that is very confusing.

And also the arrow suggests that the sodium ion becomes a chloride ion because when we are writing normal chemical equations, that's what we do.

We use an arrow to show the products.

So I think there's rather a lot wrong with that diagram and well done if you spotted all of those mistakes.

So hopefully when you come to write your own, you won't actually make those mistakes.

Excellent work.

So that brings us to the end of our first learning cycle where we've looked at some simple binary ionic substances.

So we're gonna move on now to look at a few more complicated binary ionic substances.

Sometimes we have a compound in which the positive ions do not have the same magnitude of charge as the negative ions.

And this makes it a little bit more complicated.

The overall charge on the compound is always neutral.

So if we have positive ions of two plus and we have negative ions of one minus, then we're gonna have multiples of the minus ions to make it neutral.

So here is an example.

We're showing the giant ionic lattice of sodium oxide.

Now sodium oxide has Na plus ions and O2 minus ions.

And in this diagram, this image that we can see, the O2 minus ions are the red spheres and the sodium plus ions are the purple spheres.

And if we count them, the giant ionic lattice shows that there is a ratio of Na plus to 02 minus of two to one.

So for every two Na plus ions or every two purple spheres, there is one red one or one oxide ion.

So this means that the empirical formula of sodium oxide is Na2O and it's really important that the two is written as a subscript.

So the empirical formula is always showing that ratio of ions.

Okay, let's just check our understanding before we move on.

The formula of potassium oxide is K2O.

Which of these statements are correct? A, the ratio of K plus to O2 minus is one to two.

The ratio of K plus to 02 minus is two to one or C, the overall charge on potassium oxide is neutral? Okay, let's have a look at the answer.

If you picked B, well done.

What that formula is saying K2O is that there are two potassium plus ions for every O2 minus ion.

So that is a ratio of two to one.

And well done if you also put C, the overall charge on the potassium oxide is neutral.

So the two pluses cancel out the two minus, excellent work.

Remember, ionic compounds are neutral, so the positives and negatives must balance out.

So when we're doing a dot and cross diagram for lithium oxide for example, we have to show those ratio of ions.

So we could draw it like this, we could draw out two lithium ions and put it next to one oxide ion showing the square brackets and the charges as before and they cancel each other out.

Or an alternative way of writing it is just using a large two in front of the lithium ion.

And this means that we are multiplying just the lithium ion by two.

So two pluses will cancel out two minuses.

So important points there to make sure that we draw our diagrams correctly.

Okay, so let's just check our understanding.

Which dot and cross diagram is correct for magnesium fluoride? Look very carefully at the charges and the electrons.

Okay, very well done if you chose C, that is correct.

We have the magnesium two plus ion and there are two fluoride minus ions and you can see where the electron is transferred.

If you look at the other examples given in A, the fluoride is incorrect 'cause it has a charge of two minus, and in B, the magnesium is incorrect because it has a charge of plus.

Okay, so we come to our next task.

What we'd like you to do is use the outlines to draw a dot and cross diagram for sodium sulphide and also beryllium chloride and show all the electrons in each shell of the ion.

So you may need to have a periodic table handy to work out all the electrons.

Pause the video, have a go, and when you are ready, we'll look at the answer together.

Okay, so for the first one, sodium fluoride, you should have drawn two sodium plus ions with the square bracket and the one charge, sodium showing that outer shell with eight electrons and that inner shell with two electrons.

And for the sulphide ion, you should have the outer shell with six electrons shown from the sulphur atom and two crosses that have come from the sodium atoms. The overall charge is two minus and you've got your square bracket.

So if you've got all those bits right, well done.

Excellent work.

So what about beryllium chloride? Well, the beryllium only has, ion only has two electrons in its outer shell.

It has a charge of two plus because it's given two electrons away to chlorine atoms. The chlorine atoms have taken that electron and form chloride ions.

We need two of them to balance out the charges.

So it should be written the beryllium two plus and then two of the chloride minus ions.

So make sure you've got your square brackets, your charges, the big number in front, and all the electrons drawn correctly.

If you've got that right, well done because there's a lot of detail that needs to go into these answers.

Excellent work.

Right, so we're gonna move on to our second question.

Sodium reacts with oxygen to form sodium oxide.

So in the first part what we'd like you to do is describe how sodium and oxide ions are formed, and then in the second part, explain how the ions bond together in sodium oxide.

So we've got to describe for the first and an explain for the second.

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

So let's have a look at the answer to Part A, describe how sodium and oxide ions are formed.

So the first thing I would write down is that the sodium atoms lose an electron to form sodium plus ions.

And then I'd go on to say that the oxygen atoms gain two electrons transferred from the sodium atoms to form O2 minus ions or oxide ions.

And then to make it perfectly clear that I understood what was going on, I would draw a dot and cross diagram to show this.

So we'd start off by showing our sodium atoms, two sodium atoms with one electron in its outer shell and an oxygen atom with six electrons in its outer shell.

Then we would show the ions formed where we have two sodium ions formed, two sodium plus ions now showing eight electrons in its outer shell and the oxide ion showing two minus.

So there's only one oxide ion and it must show the two electrons that have been transferred from the sodium.

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

It's important to think through these answers in a logical way.

So start with the metal, go onto the non-metal and then give a dot and cross diagram.

Excellent work.

Right, now we come to explain how the ions bond together in sodium oxide.

So your answer should include these points, not necessarily in this order.

So first of all, the sodium plus or the Na plus ions attract to the oxide or O2 minus ions because of their opposite charges.

The ions bond to the closest oppositely charged ion.

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

The ratio of Na plus to O2 minus is two to one.

Regular repeating patterns of Na plus to O2 minus are seen, and the lattice is held together by ionic bonds.

And remember, the ionic bonds are the electrostatic forces of attraction acting in all directions.

So if you've got all of those points, that is fantastic work because that's drawn on some previous learning from previous lessons.

So well done.

And when it comes to exams, you often get questions that ask you about the dot and cross diagram and how the ions have formed and then move on to think about how they actually bond together.

So excellent work.

Okay, so that brings us to towards the end of this lesson.

So we're just gonna summarise what we have learned.

First of all, dots and cross diagrams can be used as a model for binary ionic substances.

Dots and cross diagrams show the empirical formula, which is the ratio of atoms or ions to show how electrons are transferred.

Ions are shown with full outer shells, square brackets, and a charge in the top right corner.

Ionic compounds are neutral overall, so the positive and negative charges must balance out.

When positive ions do not have the same magnitude of charge as negative ions, multiples of some ions will be required.

I hope that you have enjoyed this lesson and I look forward to learning with you again very soon.