Lesson video

Welcome to today's lesson on chemical formula.

It's part of the unit atoms, elements and compounds.

I'm pleased that you've come along to learn with me today.

My name's Mrs. Mitam Smithson.

Don't worry if you don't know anything about chemical formula.

We're going to go through this step by step.

By the end of it, you'll be able to tell me about the chemical formula for elements and compounds.

Here are today's keywords, chemical symbol, chemical formula.

On the next slide there's some definitions for these keywords.

So if you want, pause the video to read them.

There's two learning cycles for today's lesson on chemical formula.

The first learning cycle is representing compounds using chemical symbols, and then we're going to move on to variations of compounds.

So let's get going with representing compounds using chemical symbols.

The chemical formula of a compound contains the symbols and numbers of each atom of the element that's in that compound.

So a compound is made up of two or more elements chemically joined together.

So we're gonna have a look at an example, and this example we're going to have a look at is carbon monoxide.

This contains one oxygen atom bonded to a carbon atom.

Bonded, remember is just a word for chemically joined together.

So it's one carbon atom bonded to one oxygen atom.

The different colours are just used to represent different atoms so you can tell them apart.

So usually we would use black to represent carbon, red to represent oxygen.

That's obviously not the real colour of these elements.

So in this one carbon monoxide, there's one carbon atom and one oxygen atom.

And we say that this is in a ratio of one to one, one carbon atom, one oxygen atom.

Elements contain one type of atom.

If you have a look here, we can see that nitrogen, N2 is represented by these blue circles, and this is showing that there's one nitrogen atom bonded to another nitrogen atom.

And here's another example.

We've got oxygen, which is two oxygen atoms chemically joined together.

However, oxygen is just made up of oxygen atoms and nitrogen is just made up of nitrogen atoms, and that's what makes an element.

We have a look at compounds.

Now these contain two or more different types of atoms. So the atoms for this must be different.

So if we have a look here, we've got nitrogen oxide.

This is one nitrogen atom bonded to one oxygen atom.

We show that they're bonded together by a little overlap, which is very clear to see on the nitrogen oxide.

Another example might be HCL.

This is hydrochloric acid.

So now we've got one hydrogen atom represented by the white circle and one chlorine atom represented by the green circle.

So we've got one hydrogen atom to one chlorine atom.

Let's check your understanding of compounds.

So which of the following represents compounds? There might be more than one answer.

So just select whatever you think is a compound.

So is it A, B, C or more than one? Pause the video for some thinking time, press play when you've got your answer or answers.

Well done if you said A and B.

A and B are made up of two different types of atoms chemically joined together, that's what makes them a compound.

So you can see A is made up of a carbon atom joined to a oxygen atom, and B is made up of a hydrogen atom bonded to a chlorine atom.

If you have a look at C, that's just an element because there's just one atom and it's not bonded to anything else well done if you got that right.

The subscript numbers, so that's the small number that's written next to the element symbol.

They're in a formula and they tell you how many atoms of the element are in a molecule.

So if you have a look at the one on the left hand side, you can see that there are two atoms of hydrogen bonded to one atom of sulphur.

This means that the ratio would be for every two hydrogen atoms, you've got one atom of sulphur.

Then if we have a look, this is methane.

You can see here we've got one atom of carbon bonded to four atoms of hydrogen.

If you can remember, that little four should be a smaller than the text size that you're writing in and it should hang just below the line.

So that's one atom of carbon and four atoms of hydrogen, which diagram shows ammonia.

The formula for ammonia is NH3.

So have a look at these diagrams and see which one of them shows NH3.

Pause the video for some thinking time.

Press play when you've got your answer.

So well done if you spotted that it was C.

C is the correct one because if you have a look, the little subscript three is next to the H, which means we're looking for the structure that's got three hydrogen atoms. So the structure that's got three hydrogen atoms, one nitrogen atom is C.

Well done if you got that right, what's the formula for the compound shown below? How would you write this if you were going to write it using the element symbols and the subscript numbers? So what's the formula for the compound shown below? Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you've got B, C2H6.

Now if you have a look at the difference between the way that B's written and C's written, you'll see that B uses subscript to those little numbers hang just below the line, and C uses super script, which is flying in the air.

So that's up.

Please don't get confused with that.

There must be subscript in science.

Now another check for formula.

So what's the correct formula for carbon dioxide? Here's a picture of carbon dioxide and you want to select whichever you think the formula is.

So is it A, B, C, or D? Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said A, A is the correct answer because it shows that there's one carbon atom bonded to two oxygen atoms. Remember you must have that little number as a subscript.

So that's below the line.

So if you wrote any of the others, that would be wrong.

Some compounds form giant structures, for example, this is sodium chloride.

So this is a salt that you put on your chips.

If you had a microscope that was good enough to zoom in to see the individual atoms, you would see for every sodium atom is one chlorine atom.

And we would write this in a one-to-one ratio.

So this formula, NACL, with no subscript numbers, that shows the ratio of these elements in the compound.

I hope you're feeling a little bit more confident now about representing compounds using chemical symbols.

We're going to complete an independent task now, and you're going to complete this table of information.

It looks a bit complicated, but all you've got to do is look.

So the first one is done for you.

So the compound formula is KBR.

The symbols of the atom in the compounds are K, which is potassium, and BR, which is bromine.

The number of each type of anat in the compound is 1K, one BR, and the ratio of this is one to one.

What I'd like you to do now is pause the video, complete the task, and then press play when you want the answers.

We're now going to take a look at the answers.

So you'll have a look at my first one that I showed you, which is KBR, the symbols of the elements in that compound.

We've got K, which is potassium, BR which is bromine and the number and type.

So we've got 1K for one BR.

So that's in a one to one ratio.

If you have a look at NACL, you'll see that the two symbols of the elements in the compound are NA and CL, so that's sodium and chlorine and the number of each type of atom.

For every one sodium, we've got one chlorine, that makes it that also is a one-to-one ratio.

Now we're going to have a look, and this one obviously has got a little subscript number in there.

So that number relates to the nitrogen as it's next to the nitrogen.

So N2O.

So we've got nitrogen atoms in there and we've got oxygen atoms in there, and we've got two nitrogens for every one oxygen.

And that means it's a two to one ratio, two nitrogens for one oxygen.

You don't get it right if you said that there's two oxygens for one nitrogen.

So the next one down, we're gonna have a look at this and we can identify that there are carbon atoms and hydrogen atoms. For every three carbon atoms, there's eight hydrogen atoms. So the ratio of the elements in this compound are three to eight.

Now we're gonna have a look at copper sulphate.

A little bit more complicated, this one to identify.

So there are three elements in this compound, copper, sulphur and oxygen, and the number and type of each.

There's one copper, one sulphur, and that little subscript relates to the oxygen only.

So that's four oxygens, and then that means that the ratio for this is one to one to four.

Well done if you got this one right.

Now for the final one, it's a little bit of a challenge.

Okay, so we've got a two to one to four, and we've got two hydrogens, one sulphur and four oxygens.

So if we have a look there, two hydrogens, one sulphur, four oxygens.

Really well done if you managed to get that last one, that was very challenging indeed.

So well done for completing that task.

It's brilliant that you're working really hard this lesson.

We've completed our first learning cycle of representing compounds using chemical symbols.

Now we're going to get onto variations of compounds.

So let's get working.

So some compounds contain the same atoms as other compounds, but in a different ratio.

These different compounds are gonna have different properties.

For example, we've got H2O and H2O2.

You might recognise H2O as being water.

So water, you'll know it's a liquid, we can drink it, it's perfectly safe.

However, when we look at H2O2, that's hydrogen peroxide and that's got a totally different set of properties.

That one is very corrosive and very harmful as you can see here by the hazard symbols that we've added to this bottle.

So you wouldn't want to mix those up.

So the only difference is the ratio.

So H2O is two hydrogens for one oxygen and H2O2 is two hydrogens for every two oxygens.

Ethanol and ethanoic acid have got the same elements in them.

You can see that you've got C2H6O and C2H402.

The only difference is the subscript numbers, but because they're different compounds, their physical and chemical properties are going to be different.

So here's ethanol and here's ethanoic acid.

You can see there that the atoms, you've got two carbon atoms for ethanol and you've got one, two, three, four, five, six hydrogen atoms and one oxygen atom.

However, when you look at ethanoic acid, you can see you've still got the same number of carbons, but you've got a different ratio for hydrogen and oxygen.

So if we take a look at ethanol, the boiling point for ethanol is quite low.

It's 78 degrees C, and it's got a density of 0.

8 grammes per centimetre cubed and it's flammable.

Then if we have a look at ethanoic acid, this has got a higher boiling point, so 118 compared to 78, and it's got a higher density.

So 1.

1 grammes per centimetre cubed.

However, they are both flammable.

So not all the properties will be different, but some of the properties will be different, and that's just down to the different ratio of the elements in that compound.

Let's check your understanding now.

So which compound or compounds contain the same atoms in different ratios? So there might be more than one answer.

What I'd like you to do is pause the video for some thinking time and press play when you've got your answer or answers.

Well done if you said B and D, you can see there that B and D both contain carbon, both contain hydrogen and both contain oxygen.

However, they are in different ratios, so there will be different compounds.

Now I've got a quick true or false for you.

Different compounds containing the same elements will always have the same chemical and physical properties.

Is that true or is that false? Have a look at that statement.

Pause the video for some thinking time.

Press play when you've got your answer.

Well done if you said that that was false, it is false.

Now I want you to justify why you said that that was false.

So let's have a look at the two sentences to justify the answer.

Compounds with different ratios of the same elements have properties similar to the individual elements, or is it compounds with different ratios of the same element will behave differently.

Pause for some thinking time.

Press play when you've got your answer.

well done if you said that it was B, compounds with different ratios with the same elements will behave differently.

Well done if you got that right.

Now we're moving on to our independent task.

So I've got three parts of task B.

So which three compounds contain the same elements bonded in different ratios? Name the elements in the compounds you selected and write the formula of the compounds.

So there's three tasks there for you to do.

Pause the video, complete the tasks, and then press play when you're ready to go through the answers.

Well done for completing that task.

Which three compounds contain the same elements bonded in different ratios, so hopefully that you managed to identify the A, C and D all contain the same elements, just bonded differently.

So I want you to name the elements in the compounds you selected.

So O is for oxygen and N is for nitrogen.

Now slightly trickier, but still achievable is writing the formula of the compounds.

So the first one is A, that should have been written NO, it needs to be capital N and it needs to be a capital O.

If you've written N1O1, I will still give you the mark, but because scientists like to do things very quickly, they don't tend to write the number one in the subscript.

If there's no subscript, it's presumed that you've just got one of the atom.

So for C, we've now got NO2, and that number two needs to be written in subscript.

So that's below the line.

If you're written in superscript or normal size, then you're not going to get that one right.

So for D, that is N204.

So there's two nitrogen atoms and four oxygen atoms. So it's N204.

And again, those numbers need to be super script and that letters N and O need to be capitals.

Well done if you managed to get those three parts of the task correct.

The final task for today's lesson, I want you to correct the students' ideas about compounds.

So Aisha, if compounds contain the same elements in different ratios, they will have the same properties.

Sam, carbonic acid, which is H2CO3, glucose C6H12O6, and ethanol C2H6O all contain the elements nitrogen, cobalt, osmium and hydrogen.

So I'd like you to correct Sam's ideas, and then Jun, some compounds contain the same elements that are stuck together in different ways.

Make sure you're writing in a good scientific language and correct all the students' ideas about compounds.

Pause the video now while you do the task and then press play when you're ready for the answers.

Well done for completing this task.

So here we are correcting the students' ideas about compounds.

Aisha was saying the same elements in different ratios had the same properties.

They don't have the same properties, they've got different properties, totally different properties in some cases.

Sam there saying carbonic acid, glucose and ethanol all contain the elements, carbon oxygen, and hydrogen.

Jun has now corrected the way that they're saying it, and this is now saying in more scientific words.

So some compounds contain the same elements that are bonded together in different ways.

So well done if you use that scientific word, bonded.

Well done for completing the task.

I understand that this might not have been very straightforward for you, but I'm really pleased that you put in the effort and gave it a really good go.

Well done for working hard this lesson.

Here's a summary of what you should have learned in today's chemical formula lesson.

The symbols of each element a substance contains appear in its chemical formula.

The subscript numbers in a formula tell you how many atoms of the element are in a molecule.

Each different compound contains a different combination of elements.

Some compounds contain the same elements, but in a different ratio.

So hopefully you'll now understand when you see chemical formula written down, what it means and the implications for the properties of that element.

Well done for working really hard throughout this lesson.

I know that some of the tasks have been quite challenging, so well done for working hard throughout this lesson.