Lesson video

Hello, my name's Dr.

Warren.

I'm so pleased that you've decided to join me today for this lesson on forming covalent bonds.

It's part of the structure and bonding unit.

We're going to work through this lesson together.

I'm here to support you all the way, especially through the tricky parts.

So let's get started.

Our learning outcome for today's lesson is, I can use dot and cross diagrams to explain how electrons are shared to form covalent bonds.

We have some keywords, molecule, covalent bond, dot and cross diagram, empirical formula, and displayed formula.

Now we're going to look at the keywords in some sentences.

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

So let's have a look at them.

We have, a molecule is a particle consisting of a fixed number of two or more non-metal atoms covalently bonded together.

A covalent bond is a strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms. A dot and cross diagram is used to show how chemical bonds are formed between atoms. The electrons from one atom are shown as dots and the electrons from the other atom are shown as crosses.

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

A displayed formula is a 2D model of a covalent substance showing all atoms and bonds.

Lines are used to represent the shared pair of electrons.

So pause the video and then press play when you are ready to start.

In our lesson today, we have two learning cycles.

The first learning cycle is about covalent substances and the second learning cycle is about different types of formula.

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

So a covalent substance consists of two or more non-metal atoms. That's really important.

There's only non-metal atoms in a covalent substance.

And in a covalent substance, atoms increase their stability by sharing electrons with other atoms in order to achieve a full outer shell.

So you remember from previous learning that atoms increase stability by achieving a full outer shell, and in covalent substances, they do this by sharing electrons.

So let's have a look at example.

Hydrogen gas is a simple covalent substance.

It exists as a diatomic molecule made by two hydrogen atoms bonded together by sharing a pair of electrons.

And you might often see a hydrogen molecule just drawn like this with two spheres overlapping, and that overlapping is where we have got the shared pair of electrons.

So zooming in a little bit more into the hydrogen atom, remember from previous learning that a hydrogen atom has one electron in its outer shell.

The hydrogen molecule is formed when the outer shell electron is shared.

So it looks a bit like this.

We have a shared pair of electrons, so we have an overlap in between the nuclei of both of our hydrogen atoms. So a covalent bond is a strong electrostatic force of attraction between the shared paired of electrons, so that negative charge in the middle, and the nuclei of the bonded atoms. So that force is going in both directions to kind of hold the hydrogen molecule together.

So let's just have a quick check of understanding about covalent bonds.

Which statements are correct about covalent bonds? So A, a pair of electrons are shared between atoms. B, they form between non-metal atoms. C, they form between a metal and a non-metal element.

D, electrons from all shells can be used to form a covalent bond.

Okay, make your choices.

Well done if you chose A, covalent.

A pair of electrons is shared between atoms in covalent bonding, and it only ever forms between non-metal atoms. So well done if you've got those two answers correct.

Okay, so we'll look at another example.

So chlorine is in Group 7 of the periodic table, so we know from previous learning that there are seven electrons in its outer shell.

The chlorine molecule is formed when two chlorine atoms share a pair of electrons from their outer shell.

So we can draw this using a dot and cross diagram.

So one chlorine atom, we show with the dots.

The second chlorine atom, we show with the crosses.

And you can see that overlap between the two chlorine atoms where there is both a dot and a cross.

This is the shared pair of electrons.

So dot and cross diagrams are used to show how covalent bonds are formed between the atoms. Moving on to another example, water is a simple covalent substance and very familiar molecule formula H2O.

So we take a closer look.

Oxygen is in Group 6 of the periodic table, so it has six electrons in its outer shell and we've drawn it here with dots.

Now it doesn't matter which way round you choose, as long as one atom has dots and the other has crosses.

So we've got six electrons in our outer shell for oxygen shown as six dots, and hydrogen has one electron in its outer shell shown as a cross.

So we know from the formula H2O, for every one electron needed to complete the outer shell of an oxygen atom, one covalent bond will form.

So if we think about that for a moment, we are really gonna be expecting two covalent bonds to form here.

Each hydrogen atom will form a covalent bond with an oxygen atom.

And there, you can see the dot and cross diagram.

You can see that each hydrogen has a cross and a dot overlapping, so the oxygen now has eight electrons in its outer shell, and each hydrogen atom has two electrons in its outer shell.

Okay, so we're gonna do a quick check for understanding, true or false.

The hydrogen sulphide molecule H2S contains two covalent bonds, is that true or false? Well done if you chose true.

That is the correct answer.

So let's justify your answer.

Why? Is it A, there are two hydrogen atoms in the molecule and each hydrogen forms a covalent bond with a sulphur atom? Or is it B, the sulphur atom forms only a covalent bond with one of the hydrogen atoms? Well done if you chose A.

That is the correct answer, and the giveaway is in the formula H2S.

Each hydrogen atom must form a covalent bond with the sulphur atom.

So atoms can share multiple pairs of electrons, and oxygen is a good example of this.

So the oxygen exists in nature as the diatomic molecule O2.

Now we know from previous learning and the example already given this lesson that each oxygen atom has six electrons in its outer shell.

And we've got a dot and cross diagram here for the oxygen molecule.

So one oxygen is shown by dots, and the other one by crosses.

Each oxygen atom shares two pairs of electrons from its outer shell.

And you can see that in the overlap part on the diagram, we have two dots and two crosses.

What this means is two covalent bonds are formed between the atom.

So in a oxygen molecule, there is what we can call a double bond, two shared pairs of electrons.

Now another example of atoms sharing multiple pairs of electrons is nitrogen.

Now nitrogen exists as the N2 molecule.

It's a diatomic molecule like oxygen.

But nitrogen is in Group 5 of the periodic table and has five electrons in its outer shell.

If we look at the dot and cross diagram, we can see that one nitrogen has five crosses and the other nitrogen has five dots.

So each nitrogen atom shares three pairs of outer shell electrons.

And in the overlap, we can see there are three dots and three crosses.

We've made a triple bond.

There are three covalent bonds formed between the atoms. And this is one of the reasons why nitrogen gas, which makes up almost 80% of our atmosphere, is so stable.

There are three shared pairs of electrons in the nitrogen molecule.

Carbon is in group four of the periodic table, so it has four electrons in its outer shell.

So this means that each carbon atom can share four pairs of electrons with other atoms. And I think you can see that depending on the group number, we're starting to get the idea that different elements and different groups will share different number of electrons.

An example is methane.

Methane gas is that gas that comes out of the Bunsen burner, and we'll probably use gas for our cooking at home or central heating.

It is CH4.

And you can see from the diagram that carbon has four electrons in its outer shell and it shares those electrons with four hydrogen atoms. So each hydrogen atom has now two electrons in its outer shell.

Carbon dioxide, another example that we're all very familiar with, CO2.

Well, we've already learned that the oxygen has the ability to share two electrons.

And the carbon has four, so actually what we've got here is CO2.

Each carbon atom is sharing two electrons with each oxygen atom.

So overall carbon is sharing four of its electrons, and carbon has now got a full outer shell, so it's stable, as are the oxygen atoms, and carbon dioxide gas is a stable molecule.

Now diamond, we've all heard of diamond.

Diamond is that lovely gemstone.

It is slightly different, it's a form of carbon, but it exists as a giant covalent structure, and we're not gonna go into much of that in this lesson.

But the key point here is we've already said that each carbon atom can share up to four electrons, and this is what happens in diamond.

A carbon atom, and if I'm just, look at the diagram, we point to a carbon atom, can share with four other carbon atoms. So there are four covalent bonds.

Each carbon atom is covalently bonded to four of the carbon atoms, so there are four shared pairs of electrons in the giant structure, which means that diamond is a very, very strong material and very stable.

Okay, let's just have a quick check for understanding about dot and cross diagram.

So a dot and cross diagram is useful because it, A, explains how a covalent bond holds atoms together, B, it shows the electrons being shared by each atom, C, it allows electron pairs around and between atoms to be counted, and D, it shows the 3D shape of a molecule.

Which are correct? Well done if you chose B and C.

So a dot and cross diagram is useful because it shows the electrons being shared by each atom, and it allows the electron pairs around each atom to be counted, so we know how many covalent bonds are there.

What it doesn't do, it doesn't explain how a covalent bond holds atoms together, and it doesn't really show the 3D shape of a molecule.

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

You're doing great.

That brings us to our first task.

So for our first task in question one, what we'd like to do is just draw a ring round the covalent substances that are shown.

Question two, water exists as a covalent molecule.

What does the word molecule mean? And then can you draw a dot and cross diagram to show how the covalent bond forms in hydrogen sulphide, H2S molecule? Pause the video, answer the question, and then when you're ready, restart, and we'll look at the answers together.

Let's have a look at the answers.

So question one, draw a ring round the covalent substances.

What we've gotta remember here is covalent substances are just non-metal elements.

So hydrogen H2, bromine, Br2, hydrogen sulphide, H2S, and water, H2O, are all covalent substances.

So very well done if you got those correct.

The others are incorrect because they all contain a metal atom.

Right, question two, what does the word molecule mean? A molecule is a particle consisting of a fixed number of two or more non-metal atoms covalently bonded together.

Keyword there is nonmetal atoms. And part B, our dot and cross diagram where sulphur is in group six of the periodic table, so we can show that with six dots or six crosses.

Hydrogen only has one electron in its outer shell.

So each hydrogen will bond with one of the electrons from the sulphur.

It doesn't matter if your hydrogens are dots or crosses as long as each type of element has the same thing.

So very well done if you got that hydrogen sulphide dot and cross diagram done.

So we move on to our next question.

We've got some dot and cross diagrams here.

You need to look at them really, really carefully, because some are right and some are wrong.

So which is the correct dot and cross diagram for fluorine? Give a reason for your answer.

And for carbon dioxide, again, give a reason for your answer.

So look at them really carefully.

Okay, we're gonna have a look at the answers now.

Which one is correct? Well, the first one is correct.

The reason is in the shared pair of electrons, one electron comes from each fluorine atom.

So both atoms now have a full outer shell.

If we look at the other one, which is incorrect, the problem we have here is the fluorine atoms either have eight electrons all the same, so that means they're coming from the same atom, or six electrons all the same.

They should have seven because they are in group seven of the periodic table.

And then we should have an electron from each atom in the shared pair.

So very well done if you've got the diagram right, and if you've got the explanation to go with it, that is fantastic.

Okay, part B.

Again, how have we done? Which one is correct for carbon dioxide? Well, the first one is incorrect.

Why? Well, all the shared electrons come from the same atoms. So it's a bit like the previous example.

We can see all the carbon atoms, all the crosses are just shared with one oxygen, and all the oxygen electrons are sharing with the carbon.

So each oxygen atom should have six electrons in its outer shell, one shows eight and the other shows four.

So it's really important that we get the right number of electrons for the right atom shown by either a dot or a cross.

So let's have a look at the other one.

It is correct.

Why? Well, if we look, each shared pair of electron shows an electron coming from the carbon.

So we only have four electrons in that carbon atom, which are shown by crosses, and they're all being shared two with one oxygen atom and two with the other atom.

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

You're doing great.

So that completes our first learning cycle on covalent substances.

We're now going to move on to look at different types of formulae.

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

And this is something we've come across before when looking at ionic compounds.

So we're gonna look at an example of a covalently bonded substance, and that's ethane.

We can see from the ball and stick model that ethane has two carbon atoms and six hydrogen atoms, and they're all covalently bonded together.

Its formula therefore is C2H6, and the simplest ratio is found by dividing the number of carbon atoms into hydrogen atoms, and that is one carbon for every three hydrogen atoms. So its empirical formula is CH3.

In some covalent molecules, the empirical formula is the same as the number of actual atoms in the molecule, and our example of water, H2O is a good one.

We can see here that the water molecule has one oxygen, shown by the red circle, and two hydrogen atoms. So its formula is H2O.

That is the simplest ratio.

There are two hydrogens for one oxygen.

It's empirical formula is H2O.

So that's something that we need to remember that sometimes we can just look at the formula and it is the empirical formula, as well as being the molecular formula.

So which of the following are empirical formulae, A, CO2, B, C2H4, C, HCL? Well done if you chose A, CO2.

That's the simplest ratio, and so HCL is as well.

So well done if you got C.

B is incorrect.

Empirical formula for B would actually be CH2.

A displayed formula can be used to model a covalent molecule.

So it's slightly different.

It's a 2D model.

It shows all the atoms and bonds in a molecule, and there are lines to represent the shared pair of electrons.

So we've got some example here.

So for hydrogen we know it's H2.

We draw it H line H, and that line tells us that there is one shared pair of electrons in the hydrogen molecule.

Water, H2O, is written H line O line H, and that line again tells us that there is one shared pair of electrons with each hydrogen atom and so on.

For the oxygen molecule, O2, it's written O two lines O, and this is because there are two shared pairs of electrons between each oxygen atom.

Likewise with carbon dioxide, O double line C double line O, showing that each oxygen atom shares two pairs of electrons with the carbon atom.

So let's just check our understanding for displayed formulae.

The displayed formula of carbon dioxide is C with two oxygens drawn to it.

Which statement is correct? The carbon atom shares two pairs of electrons.

The carbon atom shares four pairs of electrons.

Each oxygen atom shares one pair of electrons.

Each oxygen atom shares two pairs of electrons.

Well done if you chose B.

The carbon atom that is essential shares four pairs of electrons.

And well done if you've got D, each oxygen atom shares two pairs of electrons.

Excellent work.

It's good to compare the different types of models, 'cause remember in chemistry in particular, we use a lot of models, but none are perfect and they tell us different things.

So we're gonna use hydrogen cyanide as our example.

The dot and cross diagram shows from which atoms the electrons in the covalent bonds are from.

Okay, so we've got our hydrogen cyanide molecule, and you can see that some of the electrons are coming from the nitrogen, some are coming from the hydrogen, and some are coming from the carbon to form those shared pairs of electrons.

With a displayed formula, what we see here is where the covalent bonds are in the cyanide molecule, and you can see there's a single bond between the hydrogen and carbon, and a triple bond between the carbon and nitrogen.

So in one respect we can say the dot and cross diagram gives us a bit more detail about the electronic structure, because we can see where the electrons are coming from.

But the displayed formula is easier to draw and easier to understand how many shared pairs of electrons there actually are.

So if you wanted to show where the covalent bond is in fluorine, which way would you write it? Okay, which would be the best way to write it just to show where the covalent bond is? Is it A, B or C? Well done if you chose A.

Showing the displayed formula just shows you that that covalent bond is between the two fluorine atoms. We come now to our second task, and we've got some questions for you.

So one, the displayed formula of ethane is shown.

You need to look at that diagram really carefully and then answer the questions, what do the lines represent? Explain why each carbon atom can form four covalent bonds.

What is a covalent formulae? And then we've got a couple of sentences to complete.

So pause the video while you have a go at that question.

Then when you're ready, we'll look at the answer together.

So with the displayed formulae, the lines represent covalent bonds or shared pairs of electrons.

Explain why carbon atom can form four covalent bonds.

Well, the carbon atom has four electrons in its outer shell.

The outer shell can hold up to eight electrons, so carbon can share each outer shell electron with an electron from another atom.

Each pair of shared electrons forms a covalent bond.

So very, very well done if you got that correct.

Moving on to part C, the empirical formula shows the simplest whole number ratio of atoms of each element in a compound.

So when we move on to apply this, the ratio of carbon atoms to hydrogen atoms in ethene is two carbons for every four hydrogens.

We've got that answer from the formulae we had.

Meaning the empirical formula of ethene is CH2.

This is because the simplest ratio of C to H is one to two.

So very, very well done if you got that correct.

We move on to our next question, carbon tetrachloride, CCL4 is another example of a molecule.

We've got a dot and cross diagram for you to complete, so just add the outer shell electrons to complete that diagram.

Then when you've done that, go on and draw the displayed formula for CCL4, and then we have a spot the errors question for you.

And look at these diagrams carefully in part three, part A, B, and C, and then give your reason for any errors that you might spot.

So pause the video, have a go at these questions, and then we'll look at the answers together.

Okay, so carbon tetrachloride, we've got a dot and cross diagram, and hopefully you have drawn something that looks like this.

Now I've chosen to have carbon with the crosses, but equally that could be the dots.

So we have our central carbon showing four electrons, and then we have four chlorines right the way round it.

And each chlorine I've chosen dots, but you could of course have chosen crosses, has seven electrons.

The key part here is though on the overlap you have a dot and cross for every carbon that is sharing with the chlorine.

So there is four shared pairs of electrons.

So very, very well done if you've got that all correct.

Excellent work.

So the displayed formula is gonna look a bit like this, a carbon in the middle and a line drawn to each chlorine atom.

Well done if you got that right.

Question three, so spot the arrows in the dot and cross diagrams and give a reason for your answer.

Okay, so part A, what is the error? Well, only one chlorine atom is sharing in the electron, and we see that from the overlap.

The single cross should be drawn in the overlap.

So at the moment we have one chlorine atom with eight electrons and one with seven electrons, and that is incorrect.

So that is our error for that one.

Part B, we have our oxygen molecule.

What is the error here? The single cross on the left oxygen atom and the single blue dot on the right oxygen atom are drawn in the place.

They should both be in the overlap.

The oxygen molecule should show two shared pairs of electrons, not one.

So the single electron should be drawn in the overlap showing the two shared pairs.

So well done if you spotted that.

It's quite hard when you're looking at diagrams looking for errors, but it really helps your understanding if you can find them.

So excellent work.

Then finally, it's actually a displayed formulae that we are looking at.

Can we find the error there? Well, there should be only one line drawn to each hydrogen atom because a hydrogen atom only has one electron, so it can only form one shared pair of electrons.

The oxygen atom can only share two pairs of electrons, not three.

And that's the other thing, okay.

The oxygen has got three lines coming out of it, so you can look at it kind of both ways around.

Excellent work.

Well done.

So that brings us to our final part of the lesson where we're just gonna summarise our learning.

In a covalent bond, two non-metal atoms share a pair of electrons from their outer shells.

For every one electron needed to complete the outer shell of an atom, one covalent bond will form.

Atoms can share multiple pairs of electrons.

Displayed formulae of covalent substance are a model showing lines to represent shared pairs of electrons.

Thank you for learning with me today.

I hope you've enjoyed this lesson and I look forward to seeing you very soon.