Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you can join me today for this first lesson in the structure and bonding unit on why chemical reactions happen.

We're going to learn together and I'm here to support you all the way.

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 key words for you, molecule, noble gas configuration, outer shell, cation, and anion.

You can see now these words and 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 key words.

So today's lesson on why chemical reactions happen has two learning cycles.

The first learning cycle is monatomic substances followed by chemically bonded substances.

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

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

The first part of the word mon means one.

Now, you might be familiar with the idea of a mono cycle, and this is a cycle which has one wheel or a monorail, which is a railway which has one rail, one line.

So a monatomic 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 monatomic 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 monotonic 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 of electrons, it's considered to be stable and very unreactive.

Let's have a look at the helium atom.

We know from previous learning that the top number, the relative atomic mass, can tell us how many protons and neutrons in the nucleus and the bottom number.

The proton number can tell us how many protons and electrons there are in the atom.

So we need to be able to draw a picture of the helium atom, and here we can see there are two protons and two in the nucleus and two electrons in the outer shell.

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

Helium exists as a monatomic substance, and it's atoms have a noble gas configuration.

That is why it's not very reactive.

All of the elements in group zero of the periodic table exist as monatomic substances, and this is why they're often called inert or noble gases because they're not very reactive.

We have a look at another example, argon.

Argon gas is used in welding as a shielding gas because what it does is it gets around the hot metal, which can react with oxygen in the air, and it stops that reaction taking place, meaning that the metal stays nice and clean.

The outer shell of most atoms can hold up to eight electrons is just hydrogen and helium that hold up to two.

So here we have our argon atom, and we can see from the formula in the periodic table that it can hold 80.

It has 18 protons in the nucleus, 22 neutrons in the nucleus, 18 electrons in its shell.

This means the first shell has two electrons, the second shell has eight electrons, and the third shell has eight electrons.

Remember, an argon atom is considered to be stable when it has a full outer shell of electrons.

Argon exists as a monatomic substance because it has a full outer shell of electrons and atoms who have a full outer shell.

We say they have a noble gas electron configuration.

Okay, let's check our learning.

True or false? All elements in Group zero exist as monotonic substances.

Well done if you chose true.

Now, let's have a look why.

They all have a full outer shell of electrons.

Not only some of these atoms have a full outer shell of electrons.

Well done if you picked A, they all have a fuller outer shell of electrons, which is why they exist as monotonic 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 its 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 one and group seven exist as monatomic 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.

We've got four questions for you.

One, what is a monatomic substance? Two, which of the following substances are monatomic, helium water, oxygen, argon, or chlorine? Three neon is in group zero.

The periodic table explain why it exists as a monatomic substance.

And four hydrogen is in group one 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 monotonic 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 monatomic substances and we can tell because there's no other numbers in the formula.

So excellent work.

Three, neon is in group zero 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 group one at 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.

So that brings us to the end of our first learning cycle on monatomic substances, and we move on to our second learning cycles on chemical bonded substances.

So let's get going with our second learning cycle.

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, it's 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 under 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.

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.

You can see from this diagram that 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 cat ions.

A cation is a particle with a positive charge.

So sodium, it's in group one of the periodic table.

It loses electron and forms the sodium plus ion.

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

An example of this is chlorine.

It gains an electron because it's in group seven of the periodic table and makes the chloride iron, which has a negative charge.

And anion is a particle with a negative charge.

So in ionic substances, the positively charged particles, that's the cat irons and a negatively charged particles, that's the anion, 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 some 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 used, it's 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 a delocalized electron.

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 cat irons are more stable because they have a noble gas electron configuration.

Okay, true or false in metal substances, the metal ions or cat ions 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, and 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, an ion, and cat ion.

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 ion, well, that's a particle with a negative charge.

And so, the only one left is cat ion.

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 monatomic 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, right? Question three, calcium chloride exists as an ionic substance.

Describe how its 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, loss, 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 cat ions and chlorine atoms gain electrons to form negatively charged ions known as an ions.

So it's important to make those two points at the start.

Doesn't matter which order you do it in.

Once we've got an 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.

Okay, so let's summarise what we've learned today in our lesson on why chemical reactions happen.

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

The outer shell in most atoms can hold 80 electrons, but in hydrogen and helium, it can only hold two electrons.

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

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

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

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