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, and dot-and-cross diagram.

Here are the key words in some sentences.

It's really important that we understand what these words mean, so you may wish to pause the video and note down the definitions so you can refer to them later on in the lesson.

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

A cation is a positively charged ion, an anion is a negatively charged ion, and a dot-and-cross diagram is used to show how chemical bonds are formed between atoms. In our lesson today about forming ions for ionic bonding, we have two learning cycles.

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

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

You remember from previous learning that noble gases, such as helium, are unreactive, and this is because they are stable, the outer shell of electrons is full.

So let's just have a little recap.

Remember, we've got all the information we need on the periodic table.

We can see from the symbol of helium here that we have two protons and two neutrons in the nucleus, that's given from the relative atomic mass, the large number at the bottom of the symbol, and two electrons in its outer shell, which is the same as the number of protons, and given by the proton number, which is the smaller number seen at the top of the symbol, and we can write that or draw it by showing the helium atom with two electrons in its outer shell, and within the nucleus, two protons and two neutrons.

So most atoms achieve their stability when they have a full outer shell of electrons, so most metal atoms lose electrons in their outer shell in order to become more stable.

The first electron shell can hold up to two electrons, the second, up to eight electrons, and the third, up to eight electrons as well, and it's really important that you remember these numbers when you're coming to work out the ions and draw them.

So they form positively charged ions, which are known as cations, so they have a full outer shell, and that's what they are doing during the reaction with another atom.

So quick check for understanding.

All noble gases have eight electrons in their outer shell.

Is that true or false? Well done if you picked False.

Now what's the reason why? A or B? B; there are two electrons in the outer shell of helium, but the other noble gases that we've been looking at have eight electrons in their outer shell.

Really important to remember that.

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

Lithium has three electrons, it's in group 1 of the periodic table, it has one electron in its outer shell, and we can draw the lithium atom as shown.

So what actually happens when a positively charged ion is formed? Well, it loses an electron, it loses that outer shell electron, and the lithium ion is formed, that is Li+, and we draw this by showing the full shell of electrons, which is now the outer shell, it has two, we draw square brackets around it, and we put the charge as a superscript at the top.

We can also write as an equation, Li, lithium, produces an Li+ ion, the lithium ion plus an electron, and we can write that as e-.

So let's have a look at this in a little bit more detail.

Why have we got these charges? Well, remember, protons have a positive charge, electrons have a negative charge, and we can use this information to calculate the charge on an ion or an atom, because the overall charge of an atom is the difference between the number of positive and negative charges.

That's the number of protons and the number of electrons.

And in an unreactive atom, there is always the same number of protons as electrons, so atoms are neutral, they have no overall charge.

Again, a really important point to remember.

So going back to our example of lithium, the lithium atom has three protons, which is three plus charges, and three electrons, which is three negative charges.

For a lithium atom, we work out the difference, 3+ take away 3-, the overall charge on the atom is 0, like all other atoms. When it comes to the lithium ion, well, what's changed? We still have three protons in the nucleus, so we've got three plus charges, we have two electrons in the outer shell, that's 2-, so for a lithium ion, we have 3+, we take away 2-, and we are left with 1, so the overall charge on the lithium ion is 1+.

The size of the positive charge in the ion formed by the metals in groups 1, 2, and 3 is equal to their group number on the periodic table, so that is really, really helpful.

Periodic tables are always available for tests and exams, so you don't have to remember everything, we can work it out.

So let's have a look at an example.

In group 1, we have a charge of plus 1.

Some examples are lithium and sodium, Li+, Na+.

Group 2, there's a relative charge of plus 2, and we can have the magnesium 2+ ion and the calcium 2+ ion as examples.

And group 3, the one that we always get in group 3 is aluminium.

It has a relative charge of 3+, it forms the Al3+ ion.

So the transition metals, those are the metals in the part of the periodic table at the bottom, a bit like the rectangular part, they can form ions with different positive charges, and it's important that we learn these at this stage.

It's quite hard to explain why.

That's something you'll do in future learning.

So for example, iron, Fe, can be Fe2+, a 2+ ion, or a Fe3+ ion.

Copper, well, that can form a Cu+ ion and a Cu2+ ion.

And these are just things that you'll have to learn.

Hydrogen is a bit of an oddity, it's the only non-metal to form a positive ion, so we get an H+ ion with hydrogen.

So just have a check of understanding.

Which of the following positively charged ions or cations are written correctly, A, B, C, or D? Remember to have a look at the charges as well as the symbols.

Well done if you got B.

Mg2+ is correct.

So what we have to remember is the charge on the ion is written in a superscript at the top, and the number always comes first, followed by the charge symbol, so Mg2+ is correct.

Well done if you've got that.

Right, I'm gonna move on to look at another example.

This time, we'll consider magnesium.

Magnesium is in group 2 of the periodic table, this means it has two electrons in its outer shell, and we've got the electron diagram drawn there for you.

So what's gonna happen when it forms an ion? Well, two electrons are going to be lost, and it's gonna be those outer shell electrons, so we're going to end up with a magnesium ion which has eight electrons in its now outer shell.

We draw our square brackets around it, we put our charge of 2+, and we have the magnesium ion.

As before with lithium, we can write it: Mg produces an Mg2+ plus 2e-.

Magnesium atoms produce magnesium 2+ ions and two electrons.

Let's just remind ourselves exactly how we came to the answer.

First of all, there are 12 protons and 12 electrons in a magnesium atom, so that's 12 plus charges and 12 minus charges.

For a magnesium atom, we take the difference, 12+ take away 12- is 0, so the overall charge on a magnesium atom is 0, and this is the same with all other atoms. When a magnesium ion is formed, what's changed is the number of electrons.

The protons stay the same, and the nucleus, nothing changes in the nucleus, we still have 12 pluses, but we only now have 10 electrons, 10 minuses, and when we look at the magnesium ion and we do our sum, 12+ take away 10-, it equals 2, so the overall charge on a magnesium ion is 2+.

Okay, let's check our understanding again.

A copper ion has a charge of 2+, so it's in group 2 of the periodic table.

True or false? Very well done if you picked False.

What is the reason? Have a look at A or B.

Well done if you picked A.

Copper is a transition metal and can form ions with different charges, so you can get Cu+ and Cu2+.

Copper is not in group 2 of the periodic table.

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 1 of the periodic table, we write its ion as Na+.

If you can complete the rest of that table.

And then for question 2, 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 2 of the periodic table, it has two plus charges, it forms the Ca2+ ion.

Magnesium? Well, that is also in group 2 of the periodic table, it forms a charge of 2+, and it forms Mg2+ ions.

Aluminium? Well, we'll find that in group 3 of the periodic table, it has a charge of 3+, it forms the aluminium 3+ ion.

Potassium is in group 1 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 2.

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 3 and 4.

Beryllium, Be, is in group 2 of the periodic table.

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

And question 4, 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 3.

What we should have is Be, beryllium, it's in group 2 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 4: During chemical reaction lithium ions are formed.

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

So this is what I 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 brings us to the end of our first learning cycle.

We're now going to move on to our second learning cycle, and we're going to look at forming 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.

So let's have a look at an example.

We've got fluorine.

Fluorine is in group 7 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 is formed.

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

That's where it comes from.

This only happens when a chemical reaction is taking place.

So to show this, we can use what we call a dot-and-cross diagram.

This shows how chemical bonds are formed between atoms. So the electrons from one atom are shown as dots, and the electrons from another atom are shown as crosses.

So here we can see the fluorine atom.

We have two electrons in its inner shell, and seven electrons in its outer shell, and they've been drawn as crosses.

So when an electron is gained from a metal atom, we can draw it as a dot, and you can see here, a green dot has been added into the outer shell.

Now the fluoride ion has eight electrons in its outer shell, it has an overall charge of negative 1.

The fluorine atom has gained an electron, and it now has formed a fluoride ion that has a 1- charge.

So let's have a look in more detail.

In our fluorine atom, there are nine protons in the nucleus, so that's 9+, there are nine electrons in the outer and inner shells, giving a total of 9-, so the fluorine atom, the overall charge is 0, because 9+ take away 9- equals 0, so like all atoms, the fluorine atom is neutral, it has no charge.

So what happens when the fluoride ion is formed? Well, the nucleus never changes, it still has nine positives, but this time, it's gained an electron, so there are now ten electrons in the electron shells for fluorine, and ten negative charges, so the fluoride ion is 9+ minus 10-, giving a negative charge, so the overall charge on a fluoride ion is 1-.

So the size of the negative charge on the ion formed by nonmetals in groups 5, 6, and 7 is equal to 8, minus their group number on the periodic table, and this makes it a nice, easy way to work out what the charge is.

So let's have a look at this table where we summarise the information.

We have the example in group 5, the relative charge is 8 minus 5, which is 3-, and we have an example of the N3-, the nitride ion.

In group 6, the relative charge is 8 minus 6, which is 2-, and we have some examples of the O2-, the oxide ion, and the sulphur, the sulphide ion, S2-.

And group 7, well, that's 8 minus 7, which has a relative charge of 1-.

And a couple of examples of the chloride ion, Cl-, and the fluoride ion, which is F-.

So although hydrogen is a non-metal, it generally does not form a negative ion, it forms a positive ion, H+, and this is something we talked about in the first learning cycle.

Hydrogen is the exception, and that is something that we just need to remember.

So let's check our understanding again.

True or false? Chlorine is in group 7 of the periodic table, so the charge on the chloride ion is 1-.

True or false? Very well done if you picked True.

So what's the reason why? Well, have a look at these suggestions and make your choice of A or B.

Well done if you picked A.

The chlorine atom gains one electron, so the ion has an extra negative charge.

Really important.

It is the chlorine atom that gains an electron, the nucleus never changes.

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

Sulphur is in group 6 of the periodic table, it has six electrons in its outer shell.

We can see the sulphur atom here.

Two electrons are gained from metals, and again, we're showing those in that green colour.

You can see the two dots on the sulphide ion.

We put the square brackets round and the 2- as a superscript.

The sulphur atom gains two electrons.

We show the two that have come from metals as dots, and we make S2-, the sulphide ion.

Right, another quick check for understanding.

Which of the following negatively charged ions, that's anions, are written correctly, A, B, C, or D? When you're looking at this, have a good look at how the charges are written.

Well done if you picked A, Br-, that's great, and really well done if you up the second one, which is S2-.

It's always important to remember that it is a superscript and the number comes first before the charge.

So very well done if you got those right.

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

Question 1, 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 7 of the periodic table.

8 minus 7 is 1, so it forms an F- ion.

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

Question 2, 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 1.

So oxygen, it's in group 6 of the periodic table, 8 take away 6 is 2, so our charge is 2-, our ion is O2-.

Chlorine is in group 7, its charge is 1-, because it's 8 minus 7, and the chloride ion is written Cl-.

Sulphur, a bit like oxygen, is in group 6, so its charge is 2-, the ion is written S2-.

Bromine is in group 7, so it's similar to chlorine.

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

So if you've 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 2.

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 that you 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 3, 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 4, we'd like to answer this question: Oxygen is in group 6 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 is 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've got that reason.

And what about Sofia? Well, I'm afraid Sophia 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, 'cause that's quite a tricky question to answer.

Excellent work.

Okay, finally, question 4.

So for our atom, we first of all need to draw our oxygen atom with two electrons in its first shell and six in its outer shell.

I've drawn them in crosses, but equally, you could have used dots.

Two electrons are gained from the metal atoms, so I've got them as dots and done them in a different colour, and now you can see my non-metal oxygen ion.

It has two electrons added into the outer shell giving a total of eight, so there're now eight electrons in the outer shell of the ion, it is stable.

Well done if you got that right.

Okay, so now for a summary of today's lesson.

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

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

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

Ions form through the transfer of electrons.

And the standard ion notation uses superscript charge values where the follows the number, so for example, it's Ca2+, the calcium ion, not Ca+2.

I hope you've enjoyed your lesson today, and I look forward to learning with you sometime very soon.