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- Hi folks, my name's Mrs. Farring.
For today's session, you're going to need a pen and a piece of paper and it's a good idea to turn off or silence your devices in order to help avoid distractions.
If you need to take a moment to sort those things, then please do so now and press play when you are ready to resume the video.
So today's session, we're going to be talking about isotopes.
We're gonna define what an isotope is and compare isotopes of different elements.
We're then gonna use the isotopic abundances to calculate the relative atomic mass of different elements.
Some of the keywords that you can expect to see as we go through today's session include isotope.
An isotope is the version of an element with a different number of neutrons.
Isotopic abundance.
This is a fancy way of basically saying how many of each isotope there is.
So it's a measure of the average amount of a given isotope that's naturally occurring on Earth.
And then lastly, we have atomic mass of an element.
This is the number of protons added to the number of neutrons which are all found in the nucleus of the atom.
So you can look out for these as we go through today's session.
We've broken today's lesson down into three different cycles.
We're going to be looking at what is an isotope, comparing isotopes of the same elements, and then looking at the atomic mass of isotopes using abundance.
So let's jump straight in and take a look at what is an isotope.
So here we have magnesium, okay? And I'm just gonna take this opportunity to recap a little bit of knowledge that you should know from previous studies.
So, on the periodic table we see that there is a symbol, Mg, which represents magnesium, and then we have two numbers.
We have the larger number, in this instance at the top of the box, and this is the mass number, the number of protons added to the number of neutrons, okay? We then have the smaller number, in this case at the bottom of the box, and this is our atomic number of the element.
Now, this is the number of protons found in nucleus.
It's really important to remember that it's our number of protons which determines what an element is, okay? So, any atom that has 12 protons is always going to be the element magnesium, okay? So, both protons and neutrons have a relative mass of one and so both contribute towards that mass number.
That's why it's the number of them added together.
If we wanted to work out the number of neutrons in the nucleus, we would take the mass number of 24, deduct the number 12, which is the proton number, and that would give us the number of neutrons.
The electrons, because they have negligible mass, which means very little, are not included within either of these two numbers.
So, what's this got to do with isotopes? Well, an isotope is a version of an element with the same number of protons but a different number of neutrons.
So if you take a look here at the diagram, we can see that this would in fact be hydrogen, in both instances the atom diagrams show that it has one proton, so both would be termed hydrogen.
But if we look at the diagram on the left there are no neutrons present in the nucleus, whereas the diagram on the right shows that there is one neutron present in the nucleus.
This means that we have an isotope of hydrogen.
Many elements have isotopes but often you tend to find there's one version which is much more abundant, there's much more of it, and this is the one that tends to be quoted on the periodic table.
So it's only the mass number that's changing when we add neutrons, the atomic number is very much staying the same, okay? So if we have a look here, one proton means an atomic number of one and a mass number of one.
If we look at the diagram on the right hand side, we have got one proton and so an atomic number is still one.
Only in this instance, because we now have the addition of a neutron, we can see that the mass number is now two because we have one proton and one neutron contributing to that mass number of two.
So the number of protons has not changed and this means the element has not changed.
Now, there's a bit of a clue on the periodic table to the existence of isotopes, some of you may have noticed if you have been sat just flicking through the periodic table, that chlorine has a mass of 35.
5 and you've possibly wondered, how? What, has it got like half a neutron? How is that possible? Well, it doesn't, you can't have half a neutron.
It's to do with the abundance of its isotopes, which in the case of chlorine are fairly even.
So let's take a look at some other isotopes, here we have some isotopes of carbon.
We can see that the mass number is increasing, this is how we know it's an isotope.
The atomic number remains the same so it's still carbon, okay? We have six protons in the carbon that's denoted on the left hand side of the screen and there's six protons in the carbon on the right hand side of the screen.
The mass number is going up by one.
So the element will remain the same, its reactivity stays largely the same in terms of the electrons, but the mass number has changed and that's a really important learning point that we need to take on board, okay? The mass number changes, that's the number of protons, and so the atomic number remains the same.
Here are some isotopes of oxygen, again, we can see the mass number has increased, okay? But the atomic number has remained the same.
So the reason the mass number has gone up is nothing to do with a change in the number of protons, it has to solely be down to an increase in the number of neutrons found in that nucleus.
So again, we can say that the number of protons is the same but the mass number has increased, in this case by two.
There are in fact three isotopes of hydrogen and we can see them here.
We've got hydrogen, which has one proton in the nucleus and so is shown as H with an atomic number of one and a mass number of one.
We've got deuterium, which is an isotope of hydrogen.
And so denoted by a H again, it's the same symbol but it has a mass number of two, atomic number still one.
And we have tritium, which has a mass number of three.
Still has that one proton, still a hydrogen atom, okay? But it has an increase in mass each time 'cause we are gaining those neutrons shown here in the green.
We can also see that the number of electrons is remaining the same throughout each of the isotopes and so not affecting its chemical reactivity.
So let's do a quick knowledge check and see what you can recall.
True or false, isotopes have the same number of neutrons, but a different number of protons.
Isotopes have the same number of neutrons but a different number of protons.
Feels like the sentence should be correct, but if you look carefully, the two key words are switched.
So this sentence is in fact false.
It should read, isotopes have the same number of protons but a different number of neutrons.
Well done if you got that correct.
The next question asks, why is the mass of chlorine on the periodic table 35.
5? Is it A, it has half a neutron, B, it shows there are isotopes, or C, it's a halogen.
The correct answer is B, it shows that there are isotopes.
Now it's time for you to have a go at a practise task, you're being asked to complete the sentences by filling in the gaps with the most appropriate word.
You'll want to pause the video and give this task your full attention and when you're ready to see the correct answers, please press play.
Shall we take a look at how you've got on? So let's take a look.
The number of protons in an atom is called the atomic number.
The relative mass of neutrons and protons is one.
The total number of protons and neutrons in an atom is called the mass number.
Atoms of an element which have different numbers of neutrons are called isotopes.
Well done if you've got all of those correct.
If not, do make some corrections as this will give you a handy summary paragraph for the first cycle of our learning.
The next practise task we're going to take a look at asks you to answer some questions using the symbols of chlorine taken from the periodic table and one of its isotopes.
Look at the symbols on the right hand side of the screen representing two forms of chlorine and then have a go at the questions.
When you're ready to go through the answers, please press play.
Ready to see how you've done? Let's take a look.
So, why do the atoms have different mass numbers? So if we look at the chlorine on the top it says it has a mass of 35 whereas the chlorine on the bottom has a mass of 37.
What does this show us? Well, it suggests that they have different numbers of neutrons and therefore that they are isotopes.
Question number two, what two things does the number 17 tell you about the structure of a chlorine atom? The number 17 is the smaller number and what we would call the atomic number.
The atomic number tells us the number of protons in the nucleus of the atom.
Being as atoms have an overall neutral charge, if we have 17 positive protons in the nucleus, we must also have 17 negative electrons in this atom.
And so it tells us there are 17 protons and it has 17 electrons.
And finally, part three, calculate the number of neutrons in an atom of chlorine-35.
So we're going to be only using the top image.
In order to calculate the number of neutrons we need to use the mass number and subtract the atomic number.
So in this instance the sum would be 35 minus 17, which gives us the answer of 18.
And so the number of neutrons for chlorine-35 is 18.
Well done if you got these right.
It's worth remembering that the atomic number is the number of protons and the mass number is the number of protons added to the number of neutrons.
This will make things much easier when it comes to studying chemistry.
Well done if you managed to get all of those correct.
If not, don't worry, you can always go back and take a look at this section of the video again.
So we've taken a look at what is an isotope.
Next we're gonna compare some isotopes of the same element.
Different isotopes of the same element always have the same chemical properties because their reactions depend on their electron structure, not the formation of their nucleus.
However, the physical properties of isotopes, things like their melting or boiling points, so how much you need to heat them in order to get them to change from solid to liquid or liquid to gas, their density, the number of particles per given volume, so whether it's likely to float or sink, are all different due to the change in mass.
For example, ice made with water with two hydrogens and one oxygen floats.
However, if we make ice from two deuterium atoms, the isotope of hydrogen we've already seen, and one oxygen, it actually has much greater density and therefore the ice sinks.
Isotopes generally fall into two categories, they are either stable or unstable which means they become radioactive.
The larger the nucleus becomes, the greater the chance the isotope has of being unstable or radioactive.
Tritium, which is isotope of hydrogen, so having one proton but two neutrons, is an example of a radioactive isotope.
The nucleus has become very large and therefore is unstable and means it's likely to break down and emit radiation.
We also see numerous isotopes of carbon.
Here we have the familiar carbon-12 with six protons, six neutrons, and six electrons.
There are also isotopes such as carbon-13.
This still has the six protons but now has seven neutrons.
And then carbon-14, still having the six protons, but now with eight neutrons.
As we can see here.
Only the number of neutrons is changing and so only the mass number is going to change.
Notice how we refer to isotopes when talking about them as either carbon-12, carbon-13, or carbon-14.
This would be their mass number.
The ratio of these isotopes in material can be used to work out how old something is.
You may have heard of it, it's called carbon dating.
Isotopes of nitrogen include nitrogen-14, which has seven protons and seven neutrons making up the 14, nitrogen-15 which has seven protons and eight neutrons, and nitrogen-16, which has seven protons and nine neutrons.
Isotopes of nitrogen are often used in medical scanning.
They make images showing how the inside of the body works.
You may also have heard of isotopes of oxygen.
We have oxygen-16, oxygen-17, and oxygen-18.
Again, in each case, it's only the number of neutrons which is changing.
The number of protons remains eight for each one.
Eight protons and eight neutrons makes oxygen-16, eight protons and nine neutrons gives us oxygen-17, and eight protons and 10 neutrons gives us oxygen-18.
Each one being an isotope.
Isotopes of oxygen have been studied to find out global temperatures dating back millions of years from inside ice cores.
So, isotopes can in fact be incredibly useful.
Let's do a quick knowledge check and see what we've learned.
What causes the difference in the physical properties between isotopes of the same element? Is it A, mass, B, the number of protons, C, the number of electrons, or D, the atomic number? The correct answer is A, mass.
Increasing the mass affects the physical properties of the isotopes, including their density.
Now it's time for a practise task.
In this task we're asking you why is the density of hydrogen different to deuterium and what effect does this have? And then part B, hydrogen reacts with chlorine to form hydrogen chloride.
Would you expect the same reaction for each of the isotopes of hydrogen, and why? You'll wants to take a moment to consider your answers and write them down, so pause the video and when you're ready to hear a model answer, press play.
How did you find those? Not too difficult, I hope.
Question A, why is the density of hydrogen different to deuterium, and what effect does this have? Well, let's take a look at a response.
Deuterium has more neutrons than hydrogen.
This gives it a greater mass but in the same volume, and so increases its density.
This means substances like ice made with deuterium are more likely to sink due to their greater density.
Well done if you got that right.
Remember, with this style of question, you don't have to write word for word exactly what I have so long as your answer follows the same general themes.
Part B of the task states hydrogen reacts with chlorine to form hydrogen chloride and would you expect the same reaction for each of the isotopes of hydrogen, and why? The correct answer to this question is yes, you would expect the reaction to be the same.
This is a chemical reaction and it occurs due to the electrons present.
The electron number is unchanged between hydrogen and each of its isotopes and so the reaction should be the same.
So now we know what an isotope is and we've taken a look at some isotopes of different elements.
How can we use that to work out an accurate atomic mass using the isotopic abundances? And why does chlorine have a mass number of 35.
5? We can use the percentage abundance and the masses of isotopes to work out the relative atomic masses of elements.
This sounds really complicated but actually it's much more simple than it sounds.
Chlorine we know has two isotopes.
It has chlorine-35 and chlorine-37.
In the environment, they're found in the following proportions.
75% of all chlorine atoms are chlorine-35 and 25% of all chlorine atoms are chlorine-37.
So how does that give us this slightly unusual 35.
5 mass number? Well, I'll show you how to do the calculation.
We can work out the relative atomic mass using the following formula.
The atomic mass is equal to the atomic mass of isotope 1 times by its abundance, added to the atomic mass of isotope 2 times by its abundance and then dividing the whole lot by 100.
So let's plug this one in and take a look at what we get.
If we look at chlorine-35, it has a mass number of 35 and 75% of the atoms are of that type and so we have 35 times 75.
We then need to add that to the atomic mass of isotope 2, the atomic mass being 37, and times that by the abundance of 25.
We then divide the entire amount by 100 and it gives us 35.
5.
And this is a mass number quoted on the periodic table.
We're going have a little bit more of a practise at this.
I'm going to show you an example and then you're going to use that as a way of practising writing your own.
Let's take a look at copper.
Copper also has two isotopes.
Copper-63 of which 70% of atoms exist, and copper-65 of which 30% of atoms exist.
The atomic mass is equal to the atomic mass of isotope 1 times by the abundance of isotope 1 plus the atomic mass of isotope 2 times the abundance of isotope 2 and then dividing the entire figure by 100.
So let's substitute in those numbers and see what comes out.
So, 63 times 70 plus 65 times 30 divided by 100 would give us the answer 63.
6.
And so if we were to accurately quote the atomic mass of copper on the periodic table, it would be 63.
6.
Often on periodic tables used within schools and colleges, we find these numbers are rounded to the nearest whole number.
So, we'd like you to have a go at calculating the relative atomic masses for antimony.
Antimony has two isotopes, it has antinomy-121 and antimony-123 and the proportions of each are given here.
Pause the video now, grab yourself a pen and a piece of paper, and a calculator if you want, and have a go at working out the atomic mass for antimony.
Let's see how you've got on.
So your sum should look something like this.
The atomic mass is equal to 121 times 57 added to 123 times 43 and then divided by 100.
You should've got the answer 121.
86.
Now, often the issue here is when using a calculator to help you with these, instead of placing the correct items into brackets or doing the sums a section at a time, you type it in as one long sum and the calculator uses bodmas for what it thinks the answer should be rather than the actual answer.
Sometimes it helps if we lay the sum out slightly differently.
An alternative way of laying the sum out looks something like this, and if you prefer it, it's completely okay to use this method instead.
So let's have a look.
I'll model for calculating the relative atomic mass for lithium and then you're going to have a go too.
So lithium exists as two isotopes, isotope 6 and isotope 7.
There's 8% of atoms exist as lithium-6 and 92% of lithium atoms exist as lithium-7.
So, I prefer to do these sums separately.
6 times 8 gives me 48 and then 7 times 92 gives me 644.
And then add these two numbers together to get 692.
I need to divide this total by 100 and I get the atomic mass of 6.
92.
You may find it slightly easier to lay your working out like this and you're less likely to make mistakes when typing it into your calculator.
In order to have a practise at this, we'd like you to calculate the relative atomic mass for gallium, which has two isotopes, and bromine, of which there are two isotopes.
Laying them out as I've shown you on the left hand side of the screen.
You'll need to pause the video now, and don't forget, you can use a calculator so long as you are remembering to press equals at the correct time.
When you're ready to hear the correct answers, press play to resume.
How did you find it? Was it easier? I hope so.
So let's take a look.
Gallium then has gallium-69, which is 60%.
So we take 69 and times it by 60.
This is 4,140.
We then take gallium-71 and times that by 40, so 71 times 40 equaling 2,840.
We add those two figures together and get a total at 6,980.
We must divide this figure by 100 and we get the answer 69.
8, which is the accurate relative atomic mass for gallium.
Well done if you got that right.
I know this is tricky and requires multiple stages, but you're doing really well.
Let's see how you got on with bromine.
Bromine-79 exists as 51% of atoms, so we do 79 times 51.
This equals 4,029.
We then have bromine-81 and we times that by 49.
So 81 times 49 is 3,969.
We need to sum those two figures to reach a total of 7,998.
This is then divided by 100 and gives us an accurate relative atomic mass for bromine of 79.
98, which is rounded on the periodic table to the closest whole number of 80.
Well done if you're able to do these, it is tricky, but it's important that we practise.
Let's do a quick knowledge check.
In order to calculate the relative atomic mass, you need which two pieces of information? Is it A, the electronic configuration, B, the percentage abundance, C, the number of protons, or D, the isotopes' masses? So the correct answer is B, the percentage abundance and D, the isotopes' mass.
We were using both these numbers in the last task in order to calculate the relative atomic mass.
Sometimes relative atomic mass of isotopes are presented on a graph.
You need to be able to use a graph in order to get the isotopes' mass and percentage abundance in order to complete the calculations.
This adds an extra layer of complexity and is usually seen on more challenging exam questions.
But actually, once you've seen how to do it and had it explained, it's not that tricky.
If we have a look, then at the graph we can see we have percentage abundance going up the side and the masses of isotopes along the bottom.
We have two isotopes with a mass of 204.
We then have another isotope with a mass of 206 and there are 24 of them.
Each of the numbers at the top of the bar should add up to 100, so 52 add 22 add 24 add 2 gives us 100%.
So if we've got the masses of each of the isotopes and we've got their percentage abundance by reading the numbers off of the axes, we can then go on to calculate the relative atomic mass for this particular element, whatever it may be.
So let's take a look.
We've got two lots of the isotope 204, which gives us 408.
We've got 24 lots of the isotope which has a mass of 206, which gives us 4,944.
We need to times 207 by 22, which gives us an answer of 4,554.
And then we have 52% of atoms existing with a mass of 208.
So it's 208 times 52, which equals 10,816.
If we sum all of those together, we get a total of 20,722.
We then divide this by 100, the same as we have done in all of our other questions and we get a relative atomic mass for this particular element of 207.
22.
This is slightly trickier, but hopefully you can see where we've got each of the numbers from and how it corresponds to the graph.
And it means that we're able to calculate the relative atomic mass using these figures.
The relative atomic mass of this element would be 207.
If you wanted to know what it was, you could now take this number and look it up on the periodic table.
It's time for you to have a practise.
Now, you may use either method of laying these out.
When I go through the answers I'll be using method two which shows the multiple steps of the calculation.
So for each of the following elements, you're going to work out the relative atomic mass having been given the mass of each of the isotopes and the percentage abundance.
You'll need to pause the video at this point and I would suggest using a pen and paper and a calculator to support you in completing these.
When you're ready to see the answers, press play to continue.
Ready to take a look and see how you did? Okay, let's start with nitrogen.
Nitrogen-14 exists as 99.
64.
We times those two numbers together and get 1399.
44.
We then have nitrogen-15, which is 0.
36%.
We times the 15 by 0.
36 and we get 5.
4.
Let's add those two together, giving us a total of 1,404.
84.
We then need to divide our total by 100, which would give us the relative atomic mass of nitrogen as 14.
0484.
Again, this will be rounded in order to fit onto our periodic table.
Well done if you've got this one right.
If not, you may wish to go back to our I Do We Do slide and have another practise.
Let's take a look at number two.
This one is for carbon.
Carbon-12 exists as 98.
93.
So, 12 times 98.
93 is 1,187.
16.
We then have carbon-13, which exists as 1.
07.
We times these two numbers together, giving us a total of 13.
19.
Add them and we get a total of 1,201.
07.
This is then divided by 100 and we get the relative atomic mass of carbon at 12.
0107.
Well done if you got it.
Next on potassium, potassium isotope 39 has an abundance of 93.
2.
Timesing those two numbers together gives us 3,634.
8.
Potassium-41 exists as 6.
8% of atoms. And so, by timesing those two together we get 278.
8.
The sum of these two numbers is 3,913.
6.
Let's divide those by 100 and it gives us 39.
136.
And so on the periodic table we see the relative atomic mass of potassium rounded to 39.
Boron.
Boron also has two isotopes, boron-10 with 19.
9% and boron-11 with 81.
1.
Let's plug them into the same equation.
10 times 19.
9 is 199 and 11 times 81.
1 is 892.
1.
Add them together to form 1,091.
1.
This then needs to be divided to give us the relative atomic mass of boron as 10.
911.
And then lastly, we have silicon.
Silicon, we can see, has three isotopes.
Silicon-28 at 92.
2, so we times those together to give us 2581.
6.
Silicon-29, which has an abundance of 4.
68% and gives us a total of 135.
72.
And then silicon-30, so we do 30 times the 3.
08% that exists as that isotope.
This equals 92.
4.
The sum total of these three figures is 2,809.
72.
We divide this by 100 and we get the relative atomic mass for silicon as 28.
0972.
Really is a fantastic effort if you managed to get all of those correct.
It's important to always show your working so that it might be that you've made a calculation error whilst typing it into the calculator and missed a digit off.
However, if you've written down the working, then your teacher or an examiner would still be able to credit you with marks as you've shown that you understand the process and what to do.
For your final practise task today, I'm gonna ask you to work out the relative atomic mass of this element in which the masses of the isotopes and their abundance is shown on a graph.
Remember, you'll need to read off the masses from the bottom axes and the percentage abundance above the lines in order to help you reading off of the Y axis.
You'll want to pause the video, give yourself some time, and remember to write down your workings so you can see how you came to the answer you got.
When you're ready to see the correct answer, please press play.
How'd you do? Let's see.
So, it's been a very mathematical day, but we're nearly there.
We've got the mass of an isotope of 90 and it exists as 50%.
So 90 times 50 is 4,500.
We then have the next isotope with the mass of 91 and it exists as 10% of all atoms. So 91 times 10 gives us 910.
Hopefully you're with me so far.
The next isotope has a massive of 92 and exists as 20% of atoms. So 92 times 20 gives us 1,840.
And then finally we have an isotope with a mass of 94, again, existing as 20%.
Which gives us the figure 1,880.
If we add all of those together, we get a total of 9,130.
This needs to be divided by 100 and gives us the relative atomic mass of 91.
3.
Huge congratulations if you managed to get that figure.
This is a really challenging task and you deserve a massive pat on the back if you were able to get the correct answer.
If you're still struggling with this topic, then please go back and watch this section of the video again.
It can be quite tricky.
However, I think you've worked incredibly hard today and you should be really proud of yourselves.
So that brings us to the end of this lesson.
What are our key takeaway points about isotopes? Well, isotopes are versions of an element with the same number of protons but a different number of neutrons.
Isotopes have the same chemical properties because the number of electrons is unchanged.
However, the physical properties of isotopes, such as their density and melting and boiling points differ because there is a change in the mass.
Differing physical properties of isotopes include sometimes being radioactive and this can make them useful for different things.
And then lastly, it's possible to calculate the relative atomic mass of an element from the isotopic abundances and the masses of each isotope.
I really hope you've enjoyed this lesson today.
I know it's been one of the more challenging but I really think that you've got to grips with it.
I'd like to thank you for choosing to use Oak National Academy today, and we hope to see you again soon.
Bye for now.
