Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased that you've decided to join me today for our first lesson in the structure and bonding unit on why chemical reactions occur.

I'm here to teach you, guide you, and support you through all the tricky parts of today's lesson.

The learning outcome for today's lesson is, I can explain chemical bonding in terms of the transfer or sharing of electrons.

We've got some keywords for you, molecule, noble gas configuration, outer shell, cation, and anion.

You can see now these words in some sentences, and you may wish to pause the video and copy down these sentences so that you can refer to them later on in the lesson.

It's really important that we understand the meaning of the keywords.

There are two learning cycles in today's lesson.

The first is on monoatomic substances, and the second on chemically bonded substances.

So let's get started with monoatomic substances.

A monoatomic substance consists of single atoms. Now, let's just think about this word monoatomic.

The first part of the word mon means one.

Now, you might be familiar with the idea of a monocycle, and this is a cycle which has one wheel.

Or a monorail, which is a railway which has one rail, one line.

So a monoatomic is a substance of one atom, and that's really important to remember.

An example of this is helium gas.

Helium gas goes around just as a single atom.

It consists of one atom of helium, not joined to anything else.

However, on the other hand, hydrogen gas, H2, is not an example of a monoatomic substances because it consists of molecules of hydrogen.

And in each molecule, there are two atoms of hydrogen, as we can see in the diagram.

Okay, so let's just check our learning.

Which of the particle diagrams below represents a monoatomic substance? Well done if you've got B.

B is a substance on its own, we can only see one particle.

So well done.

Let's move on.

When an atom has a full outer shell, it's considered to be stable and very unreactive.

So let's have a look at helium as an example.

Now you remember from previous learning that the number four is the relative atomic mass that tells us the number of protons and neutrons in a helium atom.

And the number two is the proton number, or the atomic number, and that tells us the number of protons and electrons.

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

And you can draw it like this.

A helium atom is considered to be stable because it has one electron shell, which is full.

So helium has two electrons in its outer shell that is full.

Helium exists as a monatomic substance, and it's atoms have a noble gas configuration because the outer shell is full.

And this is a really important point that we need to remember.

All of the elements in Group 0 of the periodic table exist as monoatomic substances.

Group 0 is often called inert or noble gases, and this is because they are really unreactive, and their uses show this and make use of this property.

So, in fact, argon is used as a shield in welding.

This means it can stop the hot metal reacting with oxygen in the atmosphere, making the weld stronger and also keeping that metal pure.

The outer shell in most atoms can hold eight electrons.

Let's have a look at the example of argon.

Here we see its symbol.

We know from the information here that there are 18 protons in its nucleus, 22 neutrons in its nucleus, 18 electrons in its shells.

And we can see the picture of the argon atom that is two electrons in the first shell, eight electrons in the second shell, and eight electrons in the third shell.

So this means that argon is considered to be very stable because it has a full outer shell of electrons, in this case, eight.

Argon exists as a monoatomic substance.

It doesn't want to react with anything because the outer shell is full and it is considered to be stable.

We also say it has a noble gas electron configuration.

Let's have a quick check of understanding.

True or false.

All elements in Group 0 exist as a monoatomic substance.

Well done if you picked true.

That is correct, but why? Let's have a look.

They either all have a full outer shell of electrons or only some of the elements have a full outer shell of electrons.

Well done if you chose A.

This is because they all have a full outer shell of electrons.

That's why they are monoatomic substances.

Most atoms increase their stability by forming chemical bonds with other atoms so that they too have a full outer shell of electrons.

So let's consider hydrogen for a moment.

We can see it symbol there.

One, one.

This means that hydrogen has one proton in its nucleus and one electron in its outer shell.

And you can see the diagram of hydrogen there.

A hydrogen atom is considered to be unstable as it does not have a full outer shell of electrons.

Like helium, hydrogen can take up to two electrons in its outer shell.

So therefore, hydrogen atoms will react with other atoms to fill that outer shell and achieve a noble gas electron configuration.

So let's have a quick check of understanding.

All elements in Group 1 and Group 7 exist as monoatomic substances.

True or false? Well done if you chose false.

Now let's have a look at the reason why.

Is it A, they have a full outer shell of electrons, so do not need to react with the atoms to become stable, or B, they do not have a full outer shell of electrons, so need to react with other atoms to become stable.

Well done if you chose B.

That is the correct answer.

So this brings us to our first task, and we've got four questions for you.

One, what is a monoatomic substance? Two, which of the following substances are monoatomic, helium, water, oxygen, argon, or chlorine? Three, neon is in Group 0 of the periodic table.

Explain why it exists as a monoatomic substance.

And four, hydrogen is in Group 1 of the periodic table.

Explain why it is reactive.

So pause the video, have a go at the questions.

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

Number one, what is a monatomic substance? Well, it's a substance that consists of single atoms. They're not bonded to anything.

Question two, if you've got helium and argon, well done.

They are the monoatomic substances, and we can tell because there's no other numbers in the formulae, so excellent work.

Three, neon is in Group 0 of the periodic table.

So neon atoms have full outer shells of electrons, so they are stable.

They have a noble gas configuration.

Well done if you got that.

And finally, question four, hydrogen is in Group 1 of the periodic table.

Explain why it's reactive.

Well, hydrogen atoms do not have full outer shells of electrons, so they are unstable, and they will react to achieve a noble gas configuration.

Brilliant work if you've got all of those right.

Well done.

And so this brings us to the end of our first learning cycle on monoatomic substances.

We are now going to think about chemically bonded substances.

So let's get started with learning cycle two.

Most atoms increase their stability by forming chemical bonds with other atoms, so they have a full outer shell of electrons.

Some atoms will form a molecule.

And a molecule contains a defined number of non-metal atoms, held together by strong covalent bonds.

And you can see the diagram there of two hydrogen atoms held together in a molecule.

Hydrogen is known as a simple covalent substance.

It is that diatomic molecule made from two hydrogen atoms that are bonded together by sharing electrons.

Now another example of this is water.

It's also a simple covalent substance, but this time the molecule is made of an oxygen atom and two hydrogen atoms. The hydrogen atoms and the oxygen atom both achieve a noble gas configuration by sharing electrons, so they all have outer shells that are full.

This makes it stable.

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

Which of the following statements about simple covalent substances are correct? A, electrons are shared between atoms, B, they exist as molecules, C, sodium chloride forms covalent molecules, and D, all atoms in the molecule have a noble gas configuration.

Well done if you picked A, and B, and D, because all of these answers are correct.

Now, diamonds, this is a form of carbon which exists as a giant covalent structure, so you have lots and lots of carbon atoms in the structure.

And you can see from this diagram that each carbon atom is bonded to four other carbon atoms by sharing electrons.

So this means that every single carbon atom in this giant structure has a noble gas configuration.

It can be considered stable because all the carbon atoms have a full outer shell of electrons.

Okay, another quick check for understanding.

Which of the following statements about giant covalent substances are correct? Electrons are shared between atoms. They contain a large, undefined number of atoms. Some atoms gain or lose electrons.

And all atoms in the molecule have a noble gas configuration.

Okay, well done if you chose A, B, and D because all of these statements are correct.

Excellent work.

Now I'm gonna move on to think about some slightly different substances.

Sodium chloride is an example of an ionic substance.

And table salts, the stuff we put on our chips is sodium chloride.

And you see from this diagram that it is, it exists as solid crystals.

So when a metal atom reacts with a non-metal atom, they can increase their stability by gaining or losing an electron, and that's how they end up with a noble gas configuration.

So we have a couple of rules.

Metals lose their outer shell electrons when they react with non-metals to form cations.

A cation is a particle with a positive charge.

So sodium, it's in Group 1 of the periodic table, it loses one electron and forms the sodium plus ion.

Non-metals, on the other hand, gain outer shell electrons when they react and form anions.

An example of this is chlorine.

It gains an electron because it's in Group 7 of the periodic table and makes the chloride ion, which has a negative charge.

An anion is a particle with a negative charge.

So in ionic substances, the positively charged particles, that's the cations, and a negatively charged particles, that's the anions, are held together by strong electrostatic forces of attraction, and these are called the ionic bond, and they form a giant crystal lattice.

So you can see from the diagram that we've got here, the giant crystal lattice has lots of chlorides and lots of sodiums. And the main thing is here, they are alternating and they form a regular structure.

All of the ions in that big regular structure have a noble gas configuration, and they can all be considered stable as they have a full outer shell of electrons.

Okay, another quick check for understanding.

Which of the following statements are correct about ionic substances? They contain positively charged non-metal ions.

They contain positively charged metal ions.

Ionic substances form giant crystal lattices.

Or they form ionic molecules.

So well done if you've got B.

Remember, it's the metal ions that are positively charged.

Well done if you've got C.

Ionic substances form giant crystal lattices.

It's really important that we don't say molecules.

Molecules are only for covalent compounds.

Excellent work.

Right, we've got another structure that we're going to look at now, and that's a metallic structure.

So copper is an example of a metallic structure.

And copper, pure copper, is made up of only one type of atom, and all the atoms are packed closely together in layers, and they form a giant lattice as well.

Now you might be familiar with copper.

It's a great electrical conductor, and it's used in lots of leads, electrical leads that we have.

So let's have a look at the structure.

So in a metal, the outer shell electrons are lost from the individual atoms, and they become free to move within the structure.

So what we can see here is all the metal ions, the positively charged metal ions all lined up in a regular lattice, and the delocalized electrons, so that's their outer shell electrons that are lost to the structure are all free-moving.

And we say that these are free-moving or delocalized electrons because they're no longer attached to individual atoms. So the positivity charged metal ions, or the cations, are more stable because they have a noble gas electron configuration.

Okay, true or false.

In metallic substances, the metal ions, or cations, and delocalized electrons form a giant lattice.

Is that true or false? Well done if you chose true.

Now, let's think about why.

Either metals exist as monatomic molecules or metal ions are packed close together, forming a giant lattice structure surrounded by a sea of delocalized electrons.

Well done if you chose B.

That is the correct answer.

Okay, so we come to our second task now.

What we'd like you to do for this first question is to match the following key terms to their descriptions.

So we've got ionic bond, covalent bond, anion, and cation.

If you would just like to draw a line from the key term to its description.

So stop the video, and then, when you have done this, press play, and we'll look at the answers together.

Okay, so an ionic bond is formed when electrons are transferred between two atoms. So very well done if you've got that.

A covalent bond is formed when electrons are shared between two atoms. An anion, well that's a particle with a negative charge.

And so the only one left is cation.

That's a particle with a positive charge.

So excellent work if you've got all of those right.

It's really important that we understand and remember the meaning of these words.

Okay, question two, which of the particle diagrams below represents a giant covalent structure? So identify the particle, and then please give a reason for your answer.

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

Diamond, well, that is a giant covalent structure because each atom is joined to many other atoms in a giant structure.

Helium only has one atom, so it's a monoatomic substance.

And water, that exists as a molecule with three non-metal atoms sharing electrons, so it is a simple covalent substance.

Again, well done if you've got that right.

It's important to be able to identify these different types of structures from their diagrams. All right, question three.

Calcium chloride exists as an ionic substance.

Describe how it's giant ionic lattice is formed.

So want you to do a little bit of writing here, and in your answer, can you try and include the following terms, anion, cation, electrons, gain, lost, and electrostatic attraction.

Pause the video while you have a go at this question, and then we'll look at the answer together.

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

Well, this is what I wrote.

Calcium atoms lose electrons to form positively charged ions, known as cations, and chlorine atoms gain electrons to form negatively charged ions known as anions, so it's important to make those two points at the start.

It doesn't matter which order you do it in.

Once we've got our ions, there is an electrostatic force of attraction between all of the positive and negative ions that holds them in place, which is known as the ionic bond.

So when you're answering a question like this, it's important to get a good logical order.

So very, very well done if you've got all of that right and all of those points included.

Now we are going to summarise what we've actually learned in our lesson today on why chemical reactions occur.

So there are five different types of substances, monoatomic, simple covalent molecules, giant covalent, ionic, and metallic structures.

The outer shell in most atoms can hold eight electrons, but in hydrogen and helium, it can only hold two electrons.

Most atoms increase their stability by forming bonds with other atoms so that they have a full outer shell.

Some atoms will share electrons with other atoms to increase their stability.

Some atoms will lose or gain electrons to increase their stability, forming cation or anions respectively.

I do hope that you've enjoyed learning with me today, and look forward to teaching you again very soon.