Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you've decided to join me today for our lesson on forming ions for ionic bonding.

It's part of the "structure and bonding" unit.

I'm here to work with you today, and support you all the way through the tricky parts as we learn together.

Our learning outcome for today's lesson is, "I can use electron configuration diagrams and notation to explain how positive and negative ions are formed for ionic bonding." We've got some key words for you.

Ion, cation, anion, dot-and-cross diagram.

And now we're going to have a look at these keywords in some sentences.

An ion is an atom or molecule that has lost or gained one or more electrons, so has a charge.

A cation is a positively charged ion.

An anion is a negatively charged ion.

A dot-and-cross diagram is used to show how chemical bonds are formed between atoms. Now you may wish to pause the video and note down these definitions of the key words so that you can refer to them later on in our lesson.

So there are two learning cycles in today's lesson.

The first is forming positively charged ions, and the second is forming negatively charged ions.

So let's get started with our first learning cycle about forming positively charged ions.

Now, you remember from previous learning that noble gases such as helium are unreactive.

This is because the atoms are stable, and the outer shell of electrons is full.

So let's just remind ourselves about the helium atom.

First of all, we can see from its symbol, there are four at the top, which is the relative atomic mass, and two at the bottom, the proton number.

So each helium atom has two protons and two neutrons in its nucleus, and two electrons in its shell.

And it's drawn like this.

Most atoms achieve increased stability when they have a full outer shell.

So we need to remember that helium can only take two electrons in its outer shell, which is now full, so it's unreactive.

Metal atoms lose electrons in their outer shell in order to become more stable.

So the first shell can take up to two electrons, as we've just mentioned with helium.

The second shell can take up to eight electrons, and the third shell can take up to eight electrons.

So this is really important to remember when we're thinking about how metal atoms lose electrons.

So they form positively charged ions known as cations to get a full outer shell.

So remember that two, eight, eight.

It's really important for what we're doing in this lesson.

Okay, so have a quick check of understanding, true or false? All noble gases have eight electrons in their outer shell.

Well done if you picked false.

So what is the reason? A, there are always eight electrons in the outer shell of the noble gases.

B, there are two electrons in the outer shell of helium, but other noble gases have eight electrons in their outer shell.

Excellent if you went for B.

That is the correct answer.

Okay, moving on.

Let's have a look at another example.

Lithium.

Lithium is in group one of the periodic table.

It has one electron in its outer shell.

And again, remember we always go to the periodic table to work out the structure of the atom.

We can see here that there are four neutrons in the nucleus, three protons in the nucleus, and three outer shell electrons.

And when it loses an outer shell electron, it becomes a lithium ion, and you can see from this diagram here that there are now only two electrons left.

That outer shell electron is lost.

And because of this, it now means that there is a charge.

So we draw an ion with its outer shell of electrons full, and then we put some square brackets around it with a plus, showing it is a positive ion.

It's really important to get this notation right.

So the lithium atom has formed a lithium ion, with one charge.

It's lost one electron.

So we can write it.

Li goes to Li+, plus an electron, which we can write as e-.

So in order to work out the charges, it's really important that we understand and recall our previous learning about the structure of the atom.

A proton is always positive, and it has a relative charge of +1.

An electron is negative, and it has a relative charge of -1.

And the information we've got in this table can be used to calculate the charge on the atom or ion.

So the overall charge in an atom is the difference between the number of positive and negative charges.

So that's the number of protons, which are positive, and electrons, which are negative.

And as there's the same number of protons in an atom as there are electrons, atoms have no overall charge.

They are neutral.

And that's a really important part to remember, because that is our starting point when we are working out the charges on ions.

So let's have a look at the lithium atom as an example.

We can see here from its periodic table symbol, we have three protons in the nucleus.

So that's three positive charges.

And we have three electrons in the shell.

So that's three negative charges.

So for a lithium atom, the overall charge is zero.

Three pluses take away three minuses, and you get zero.

So like all other atoms, the lithium atom is neutral.

So what happens during bonding? Well, when a lithium ion is formed, it loses an electron.

So it now has three protons in its nucleus, which has a charge of 3+, and two electrons in its shells.

The outer shell only has two electrons, so that's 2-.

So for the lithium ion, we have three pluses.

We take away two minuses, and that leaves us with one plus.

So the overall charge on a lithium ion is 1+.

Okay, so the size of the positive charge in an ion formed by metals in group one, two, or three, is equal to their group number on the periodic table.

So this is an important tip to remember because it then makes it easier for us to work out the charge.

So let's have a look.

Group one, the relative charge is 1+, because it loses that outer shell electron.

Couple of examples.

Lithium, 1+, and sodium, +.

In group two, there are two electrons in the outer shell, so it loses two electrons.

It has a relative charge of 2+.

For example, Mg2+ and Ca2+.

Magnesium ion and calcium ion.

Group three, there are three electrons in the outer shell.

An example of this is aluminium.

It loses its three electrons, and forms the aluminium 3+ ion.

So the transition metals, remember, those are the bits in that block at the bottom.

They can form ions as well.

It's a bit harder to use the periodic table to work these out at this stage, because they can have different charges on the ion.

For example, iron, Fe, can be Fe2+, or it can be Fe3+, and you will come across both of them in your learning.

Copper, for example, could be Cu+ or Cu2+.

And for the moment, we're just gonna have to learn those ones.

It's much harder to work it out at the moment.

Hydrogen is the only non-metal atom that forms a positive ion, and that's H+, and on some periodic tables, you see that H hydrogen floating, because it can't quite make its mind up where it's going to be, because it's a non-metal, but it also forms a positive ion.

So let's have a quick check of learning.

Which of the following positively charged ions, cations, are written correctly? So you've got aluminium +3, and remember, aluminium is in group three of the periodic table.

Magnesium 2+, magnesium is in group two of the periodic table.

Calcium +2.

Calcium is in group two.

And you have got potassium 1.

Potassium is in group one of the periodic table.

And you'll notice for the first three, there are superscripts, but for the fourth one, D, it is a subscript.

So which one is correct? Well done if you got B! It's really important that these are written the right way round.

The charge on the ion is written in a superscript.

That is at the top, with the number first, followed by the charge symbol.

Okay, so magnesium, it's got 2+, and they are superscripts.

Really important to get that right.

So well done if you did.

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

Magnesium is in group two of the periodic table, and we've drawn the electronic configuration diagram, and you can see there are two electrons in its outer shell.

We know this from the information of the symbol we get from the periodic table.

There are two in the first shell, eight electrons in the second shell, and two electrons in the third shell of the magnesium atom.

So when it forms an ion, what actually happens? Well two electrons are lost, those two in the outer shell, and we form the magnesium 2+ ion.

As before, we have to put square brackets around it.

And then we put a superscript at the top of 2+.

Magnesium goes to the magnesium 2+, and two electrons are lost.

So again, we can see that we've got increased stability now.

It's got an electron, a configuration, equal to a noble gas configuration, because that outer shell now is complete.

Okay, let's have a look at this in a bit more detail.

In the magnesium atom, we have 12 protons with positive charges and 12 electrons with negative charges.

So the overall charge on a magnesium atom is zero.

12 pluses take away 12 minuses.

So what happens when we come to form the ion? Well, there are now still 12 protons in the nucleus, because nothing changes there, but we only have 10 electrons now, in the ion, so the overall charge is +2.

12 pluses take away 10 minuses, is two.

The magnesium ion has a charge of 2+.

Okay, let's have a quick check for understanding.

A copper ion has a charge of 2+.

So it's in group two of the periodic table.

True or false? Well done if you chose false.

Okay, let's have a look at why.

A or B? Well done if you got A.

We've got to remember, copper is a transition metal.

It can form ions with different charges.

It can do the copper 1+, or the copper 2+.

Right, we've come to our first task now, and to answer the first question, we'd like you to use a periodic table.

The first example is done for you.

Sodium.

It has a charge of +1.

We know that.

It's in group one of the periodic table.

We write its ion as Na+.

If you can complete the rest of that table.

And then for question two, why do metals form positively charged ions, or cations? Pause the video while you answer those questions on your worksheet, and then when you're ready, press play, and we'll look at the answers together.

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

First of all, completing the table.

So calcium, well calcium is in group two of the periodic table.

It has 2+ charges.

It forms the Ca2+ ion.

Magnesium.

Well that is also in group two of the periodic table.

It forms a charge of 2+, and it forms Mg2+ ions.

Aluminium, well we'll find that in group three of the periodic table.

It has a charge of 3+.

It forms the aluminium 3+ ion.

Potassium is in group one of the periodic table.

It has a charge of 1+ and forms a K+ ion.

Well done if you got all of those correct.

That's great.

Now let's have a look at question two.

Why do metals form positively charged ions, or cations? Well, the answer needs to be something a bit like this.

Metals form cations to increase their stability, by having a full outer shell of electrons.

They lose one or more electrons, so are positively charged overall.

It's important that you get both of those points in your answer, so well done if you did that.

Let's go on to the second part of the task, questions three and four.

Beryllium, Be, is in group two of the periodic table.

What we'd like you to do is draw an electronic configuration diagram of the beryllium ion.

And question four, during a chemical reaction, lithium ions are formed.

Want you to compare the electronic configuration of a lithium atom with a lithium ion, so remember, a compare question is you need to do both of those.

Pause the video, answer the questions, and when you're ready, press play and we'll look at the answers together.

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

What we should have is Be, beryllium.

It's in group two of the periodic table, and it only has two electrons in its outer shell.

So we should have the beryllium atom, the first shell showing two electrons, the square brackets, and the 2+ superscript.

It's really important that it's written the way you can see it now.

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

Okay, for question four, during a chemical reaction, lithium ions are formed.

Compare the electronic configuration of the lithium atom with a lithium ion.

So this is what I've put.

A lithium atom, Li, has a total of three electrons.

So when we're talking about electron configuration, we talk about electrons.

There are two in the first shell, and one in the second shell.

A lithium ion, Li+, only has two electrons in its first shell, as it has lost its outer shell electron, so has a charge of 1+.

It's really important when you're comparing something that you actually talk about one thing, and then the other.

So in this case, start with the lithium atom, and move on to the lithium ion.

So well done everybody, if you got those right.

Okay, so that completes our first learning cycle about forming positively charged ions.

And we're now going to move on to our second learning cycle, and think about negatively charged ions.

Okay, so non-metal atoms become more stable when they gain electrons to fill their outer shell, and become negatively charged ions, or anions.

So that's a slight difference.

They still want to have that full outer shell.

They still increase stability, but this time, electrons are gained.

Let's have a look at an example.

So we've got fluorine here.

Remember, we can always get that periodic table information.

Fluorine is in group seven of the periodic table.

It has seven electrons in its outer shell.

To become more stable, the fluorine atom gains an electron, and a negatively charged fluoride ion, F-, is formed.

During ionic bonding, the gained electron is transferred from a metal atom.

So that's where that extra electron actually comes, 'cause remember, this happens during a chemical reaction when the atoms are becoming more stable.

Now we use something called a dot-and-cross diagram to show how chemical bonds are formed between atoms, and the electrons from one atom are shown as dots, and the electrons from the other atom are shown as crosses.

So we've drawn our fluorine atom here.

Fluorine has its two electrons in the first shell, and seven electrons in the outer shell, making nine overall.

And during a chemical reaction, it takes an electron or it gains an electron from a metal atom.

And in this case, we've just shown it as a green dot.

So that electron has come from somewhere else.

That's why we call it a dot-and-cross diagram, to distinguish where the electrons have come from.

So now we have the fluoride ion.

It has eight electrons in its outer shell.

It's stable, so it's now unreactive.

As with the positively charged ions, we need to put some square brackets around it, and we put the negative charge as a superscript.

We can write this as an equation, F + e-, so that that's fluorine plus an electron, produces the fluoride ion, F-.

So let's have a look in more detail.

Remember, we've got our symbol from the periodic table.

We know there are nine protons, so that's nine positive charges in the nucleus, nine electrons, that's nine negative charges in the electron shells.

So for the fluorine atom, nine pluses take away nine minuses, is zero.

So the overall charge on the fluorine atom is zero.

So what happens when the ion is formed? Well in this case, we still have nine protons, nine positive charges in the nucleus, but we have gained an electron.

So we have 10 negative charges in the outer shells.

So for our fluoride ion, we have nine pluses.

We take away 10 negatives, which gives an overall charge of -1.

So the overall charge on the fluoride ion is 1-.

Really important that we get the signs the right way around.

So the size of a negative charge on the ion is formed by non-metals in groups five, six, and seven of the periodic table, is equal to eight, minus their group number on the periodic table.

So let's just have a look.

In group five, we have a relative charge of -3, or 3-.

And the example is the nitride ion, N3-.

In group six, we have a charge of eight minus six, which is 2-, and examples of the oxide and the sulphide ion, O2- and S2-.

In group seven, we have a relative charge of 1-.

A couple of examples are the chloride ion, Cl-, and the fluoride ion, F-.

Although hydrogen is a non-metal, it generally does not form a negative charge.

That's something we learned about in our first learning cycle.

It forms a positive charge, H+.

So hydrogen is the exception to the rule.

Okay, let's have a quick check of understanding.

Chlorine is in group seven of the periodic table.

So the charge on the chloride ion is 1-.

True or false? Well done if you picked true.

That is correct! So why? Well, have a read of A and B and make your choice.

Well done if you chose A.

The chlorine atom gains one electron, so the ion has an extra negative charge compared to the number of protons, positive charges, in the nucleus, giving it an overall 1- charge.

So very well done if you got that right.

Let's have a look at another example.

Sulphur is in group six of the periodic table, so it has six electrons in its outer shell, and you can see the electronic diagram drawn here, with six electrons in its outer shell.

So it's going to gain two electrons from some metals.

Again, we're showing them as those kind of green dots, and it produces the sulphide ion.

It's gained two electrons, so there's two extra negative charges.

So it's the S2-.

And remember to put the square brackets on, and the superscript 2-.

We can write this sulphur plus two electrons, or plus 2e-, produces the sulphide ion, S2-.

Okay, another quick check.

Which of the following negatively charged ions or anions are written correctly? Remember to look where the charges are.

Bromine is in group seven, Br-.

Oxygen in group six, O2+.

Chlorine is in group seven, Cl-.

Sulphur is in group six, S2-.

Well done if you got A.

Br- is correct.

Bromine is in group seven of the periodic table, and the minus has been written as a superscript.

Well done! Also, if you got S2-, sulphur is in group six of the periodic table, so that's eight minus six is 2-, and it's gained the S, the sulphur 2- is written correctly.

We now come to our second task, task B, which is all about forming negatively charged ions.

Question one, we'd like you to complete the table.

Use a periodic table to help.

We've done the first one for you.

Fluorine has a charge of 1-.

We know that because it's in group seven of the periodic table.

Eight minus seven is one, so it forms an F- ion.

If you can complete the information for oxygen, chlorine, sulphur, and bromine.

Question two, why do non-metals form negatively charged ions? Pause the video, work through the questions, and then when you're ready, press play and we'll look at the answers together.

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

So oxygen, it's in group six of the periodic table.

Eight take away six is two.

So our charge is 2-.

Our ion is O2-.

Chlorine is in group seven.

Its charge is 1- because it's eight minus seven.

And the chloride ion is written Cl-.

Sulphur, bit like oxygen, is in group six.

So its charge is 2-.

The ion is written S2-.

Bromine is in group seven.

So it's similar to chlorine.

It has a charge of 1-, it's written Br-.

So if you got all of those right, well done.

That's really, really good.

Excellent.

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

Why do non-metals form negatively charged ions? Well, there's a couple of things we need to say.

First of all, non-metals form anions to increase their stability by having a full outer shell of electrons.

So you really do need to make that point.

They gain one or more electrons, so they are negatively charged overall.

That's the second point they need to make.

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

Excellent work.

Okay, so the next part of the task, we've got two more questions for you.

Question three, it's about dot-and-cross diagrams. So a rule when drawing a dot-and-cross diagram is that the outer shell of each ion should be full.

And this is something that we've talked about in this lesson.

So some pupils are discussing this rule.

What I'd like you to do is to explain whether each pupil is correct or incorrect.

So Lucas says, "This rule explains why atoms react." Andeep says, "This rule helps work out the charge of different ions." Sofia says, "This rule works because atoms want a full outer shell." And finally, for question four, we'd like to answer this question.

Oxygen is in group six of the periodic table.

What we want you to do is to draw a dot-and-cross diagram to show the formation of the oxide, the O2- ion.

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

Right, let's have a look at the answers.

So, who was correct? Well, if you said Andeep was correct, excellent work, because Andeep was correct.

For an atom to become an ion, it loses or gains electrons, so that it has a full outer shell.

The number of electrons lost or gained determines the charge on the ion.

Lucas, well, I'm afraid Lucas wasn't right.

So if you got that, well done again.

The reason being, atoms react to increase their stability, because the outer shells are not full.

Ions have a full outer shell.

So well done if you got that reason.

And what about Sofia? Well, I'm afraid Sofia wasn't right either.

Atoms do not have feelings.

They are not people, they are not living things.

Atoms react to increase their stability, because their outer shells are not full.

And it's important that you don't give atoms feelings.

So if you got all of those right, well done, because that's quite a tricky question to answer.

Excellent work.

Okay, what about question four? Well, we need to draw a diagram.

Here we have our oxygen atom.

It has two electrons in the first shell, and six electrons in the outer shell.

Two electrons are gained from metal atoms. So as a dot, we can add those two electrons.

We can put some square brackets around the ion, and give it a charge of -2.

So overall, there are now eight electrons in the outer shell of the ion.

So if you got that right, well done.

And especially if you remembered to use the dot-and-crosses, it doesn't matter which way round you do it.

You could have put the dots on the oxygen, and then put the crosses showing the electrons coming from metal atoms. It doesn't matter which way it is, as long as we are consistent.

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

Excellent work.

Now, let's have a look at what we've learned today.

Let's summarise our learning for forming ions for ionic bonding.

First of all, most atoms increase stability when they have a full outer shell of electrons.

Most atoms become more stable when they lose electrons in their outer shell and become positive ions, which are also known as cations.

Non-metal atoms become more stable when they gain electrons, to fill their outer shell, and become negative ions, or anions.

Ions form through the transfer of electrons.

And the standard ion notation uses superscript charge values, where the charge follows the number.

So for example, we have Ca, and it's the superscript 2+.

Not Ca superscript +2.

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