Lesson video

Welcome to today's lesson on atomic number and mass number.

It's part of the "Unit: Atomic structure and the periodic table." My name's Mrs. Mytum-Smithson, and I'm here to help you learn today.

We're going to work through examples step-by-step to learn about the atomic number and mass number.

By the end of the lesson, you should feel much more confident about using these terms. By the end of this lesson, you should be able to relate the atomic number and mass number to the structure of atoms of each element on the periodic table.

Today's keywords are atomic number, mass number, relative atomic mass.

On the next slide, there's some sentences that give definitions for these keywords.

If you wish to read them, pause the video, and then press play when you're ready to learn.

There are three learning cycles for this lesson: Atomic number, mass number, calculating subatomic particles.

Let's start with the first learning cycle, atomic number.

There are three subatomic particles in an atom.

Here's an atom of beryllium, labelled upon the protons, electrons, and neutrons.

They're the names of the three subatomic particles.

So we find the protons in the nucleus, they would be positive, and they would have a relative mass of 1.

Electrons are going to be orbiting the nucleus, so going around the nucleus in shells, they're negative, and they've got a relative mass of 0.

0005, so very small indeed.

The neutrons are found in the centre of the atom in the nucleus.

They're neutral, sometimes written 0, and they've got a relative mass of 1.

Now a quick check for understanding, true or false, all subatomic particles in an atom have a relative mass that equals 1? Pause the video now for some thinking time, decide if this is true or false, and then press play when you want the answer.

Well done if you said that that was false.

Now I'm going to give you two sentences and you can justify that false answer.

So, only protons and neutrons have a relative mass equal to one.

Protons have a relative mass that is very small compared to an electron.

Pause the video now and then press play when you've got your answer.

Well done if you said only protons and neutrons have a relative mass that is equal to one.

Remember it's electrons that have got that very, very small mass, not the protons.

Here's the periodic table.

Now the periodic table tells us a number of things.

The atomic number of an element states that that is the number of protons in the nucleus of every atom of that element.

So, this is magnesium.

It's element number 12, that means that it's got 12 protons in the nucleus of every single atom of that element.

Here's another example, this is boron.

This is element number five, and so it's got five protons in the nucleus of every atom of that element.

So if I picked up an atom, if I could, I could look at that and I could see that it's got five protons in it, I know that that is boron.

If it's got a different number of protons, it means that it's a different element.

Different periodic tables might show the atomic number at the bottom or at the top of the chemical symbol.

So if we have a look here, here's fluorine.

Here, you can see that the atomic number is at the bottom, and here is another example that you might see from the periodic table, and here it's at the top.

And the smaller of the two numbers is the atomic number.

So if you're ever confused, and you're thinking, "I don't know which one's which, which is the atomic number," you're going to look for the smallest number.

So in this case, the smallest number for fluorine is 9, and that means every single atom of that element in its nucleus has got nine protons in it.

Here's a quick check for understanding.

The atomic number tells you the number of what in the nucleus of an atom? Is it the number of protons, the number of electrons, or the number of neutrons? Pause the video now for some thinking time, press play when you've got your answer.

So well done if you said it was protons, the atomic number tells you the number of protons in the nucleus of an atom.

Well done if you got that one right.

Here's another check for understanding.

The atomic number is.

Is it always the smaller of the two numbers for an element on the periodic table? Is it always the larger of the two numbers for an element on the periodic table? Or is it the sum of both of the numbers for an element on the periodic table? Pause the video now and press play when you've got your answer.

Well done if you said it's A, it's always the smaller of the two numbers for an element on the periodic table.

Well done if you got that one right.

The atomic number shows you how many protons there are in the nucleus of that element.

It's unique to that specific element, it's the one thing that always stays constant.

So, here's boron.

We can see here the smaller number there is five, so that tells us that there's five protons in the nucleus.

Then let's have a look at magnesium.

You can see here, magnesium's atomic number is 12, and you can see there that there are 12 protons in the nucleus of magnesium.

The number of electrons in an atom is equal to the number of protons in the nucleus of the atom.

So let's have a look at an example.

So here's helium.

You can see here, helium's got an atomic number of 2, that means that there are two protons in the nucleus of helium atom.

So there's our two protons.

Now, the overall charge on any atom on the periodic table is neutral.

So if we've got two positives there, we must have two negatives, and the negatives in this case are the electrons, and that's going to balance it up.

So we've got two protons in the nucleus and we've got two electrons in the shells, because the positive charge of the protons is cancelled out by the negative charge of the electrons.

So here's a bigger atom here.

So we've got 14 protons, so the atomic number for silicon is 14.

So we've got 14 protons in the middle, and that must mean that we must have 14 negative charges to cancel that out.

So that's 14 electrons orbiting the nucleus for silicon.

A quick check for understanding now: Which element has 18 electrons in its atoms? Pause the video now for some thinking time, press play when you've got your answer.

Well done if you said it's C, it's argon.

You can see there the atomic number is 18, which must mean that there are 18 positive charges.

So 18 protons, which means that you must need to have 18 electrons to cancel those out to give it an overall neutral charge.

So you need 18 electrons to cancel out 18 protons.

Well done if you got that one right.

Here's a quick check for understanding: The atomic number tells you.

The number of atoms of the element, the number of protons and also electrons in the atom, or the number of neutrons plus protons in the atom.

Pause the video now and then press play when you've got your answer.

Well done if you said B, it tells you the number of protons and also the number of your electrons in the atom.

Now I want you to select the two correct statements.

A, the number of protons in an atom is equal to the number of electrons.

B, the number of protons in an atom is more than the number of electrons.

C, the negative protons cancel out the charge of the positive electrons in an atom.

Or D, the negative electrons cancel out the charge of the positive protons in an atom.

Remember to select two statements, so pause the video whilst you do that, and press play when you've got your answer.

So well done if you said the number of protons in an atom is equal to the number of electrons, that's correct, remember they cancel each other out, so the negative electrons cancels out the charge of the positive protons in an atom.

Well done if you got that right.

Here's part one of Task A.

State the atomic number for the following elements.

You will need a periodic table to do this.

We've got the elements, beryllium, manganese, tin, bromine, platinum, krypton.

Pause the video now, complete the task, and then press play to check your answers.

Well done for completing part one of Task A: State the atomic number for the following elements.

So beryllium's atomic number is 4, manganese, hope you didn't get that confused with magnesium, manganese is 25.

Tin is 50, bromine is 35, platinum is 78, and Krypton, finally, is 36.

Well then if you got all of those correct.

Here's part two of Task A: State the number of protons in the atoms of the elements pictured.

So what I want you to do now is, for each of the elements, tell me how many protons would you find in the nucleus of an atom of that element.

Pause the video whilst you complete this task, press play when you want to go through the answers.

So well done for completing that task.

Let's have a look, so we've got scandium, that's 21, yttrium is 39, chromium is 24, iodine 53, argon is 18, tungsten is 74, mercury is 80, lead is 82, xenon is 54, and tin is 50.

All you had to do there was write down the smallest number.

Well done if you've got all those correct.

Here's part three of Task A: Complete the table to show the number of protons and electrons for each element.

Pause the video while you complete that task and press play when you've got your answers.

Well done for completing part three of Task A completing this table.

So all we've got to do here is, if we take a look at helium, we've got the atomic number of 2, number of protons is 2, that's the same as the atomic number, and the number of electrons is also 2.

Remember, if we've got two positive protons, we must have two negative electrons to balance out the charge.

Here's fluorine, so fluorine's got an atomic number of 9, the number of protons is 9, and the number of electrons is also 9.

Now we're gonna have a look at this one.

So it's telling us we've got the number of protons, which is six.

The atomic number for that must be six, and it must have six electrons to balance out those six positive protons, and that element there is carbon.

So you can just look that up using your periodic table.

Now we've got nitrogen.

So you can see here the number of protons and the number of electrons is the same as the atomic number, so that's nitrogen there.

We've got magnesium, so if we had 12 electrons, we would need 12 protons to balance that out.

And the number of protons is also the same number as the atomic number.

So we could just look that up on a periodic table and find that it's magnesium.

And our last one, phosphorus, is 15, so we've got 15 as the atomic number 'cause we can just look that up on the periodic table.

And then that tells us we've got 15 protons and we must have 15 electrons because everything on the periodic table has got an overall charge of 0 or neutral.

Well done for completing that.

Here's part four of Task A.

What we're going to do for this one is we're going to correct the information so that Lucas can submit his homework.

He's narrowed the choices down, so he is gonna give you a choice of words.

So the atomic number tells you how many protons, electrons, or neutrons are in the nucleus of an atom.

Each element has a unique atomic/element/period number.

The atomic number is always, is it the larger or the smaller of the two numbers found for each element on a periodic table.

And finally, for an atom, the number of protons is greater, the same as less than the number of electrons.

Pause the video, complete the task, and then we can help Lucas get his homework submitted.

Well done for completing that task.

The atomic number tells you how many protons are in the nucleus of an atom.

Each element has a unique atomic number.

The atomic number is always, it's always the smaller of the two numbers given for each element on a periodic table.

And for an atom, the number of protons is the same as the number of electrons.

So well done for working really hard and completing Task A.

Well done for working really hard on the atomic number learning cycle.

Now we're going to go and learn about a different number, this is the mass number.

The mass number of an element is equal to the sum of the number of protons and neutrons in one atom of that element.

Let's have a look at what this means.

So here's lithium.

We know that there are seven subatomic particles in that atom that have got mass.

So, total number of subatomic particles in the nucleus is 7, and here's the atomic number.

We know that the atomic number tells us the number of protons in the nucleus.

Now we want to calculate how many neutrons there are.

So all we've got to do there is we've got to take away the atomic number from the mass number, so we subtract the atomic number from the mass number, and that will give us our number of neutrons.

So here we can see we've then got 3 protons and 4 neutrons, that's for lithium.

So we do 7 takeaway 3 equals 4.

Here's a quick check for understanding.

True or false, the mass number of an element is equal to the sum of the number of neutrons and protons in one atom of that element.

Pause the video while you decide if it's true or false, then press play when you've got your answer.

Well done if you said that that was true.

It is true, the mass number of an element is equal to the number of neutrons, add the number of protons, in one atom of that element.

Let's have a look while we justify the answer.

The mass of electron is very small and so does not affect the mass of the atom.

The neutrons and protons are the only particles in an atom that are important, so their masses are added together.

Pause the video now while you come up with an answer and press play, and I'll tell you whether you are right or not.

Well done if you said the mass of an electron is very small and so does not affect the mass of the atom.

Different periodic tables might show the mass number at the bottom or at the top of the chemical symbol.

So the larger of the two numbers is the mass number.

So we'll have a look at our two examples.

In this case, you can see that the mass number is on the top of the left hand one and on the bottom of the right hand one.

So the math number for fluorine is 19.

The number of neutrons is sometimes equal to the atomic number.

So sometimes there are the same number of neutrons as protons.

Sometimes however, there is a greater number of neutrons than protons.

So, sometimes the number of neutrons is greater than the atomic number.

It's never smaller than the atomic number.

The number of electrons is always equal to the atomic number for an atom.

So what that means is, on the periodic table, there's always the same number of protons as electrons for an atom of an element.

So here's two example, helium here.

So this has the same number of neutrons as the atomic number.

So it's got two neutrons and two protons.

Remember, the number of electrons is always equal, so it's also got two electrons.

Then we have a look at boron here.

Boron's got five protons, but however this time round, it's got more neutrons than the atomic number because there are six neutrons here, meaning that the mass number turns out to be 11.

However, the number of electrons is equal to the atomic number for an atom, so it's got five electrons.

Which is the correct equation to calculate the number of neutrons in an element.

Is it, A, number of neutrons equals mass number add atomic number? Is it, B, number of neutrons equals mass number take away the atomic number? Or is it C, number of neutrons equals mass number multiplied by the atomic number? Pause the video, get your answer, and then press play, and I'll tell you if you are right.

Well done if you said that the number of neutrons equals the mass number subtract the atomic number.

A quick check for understanding, which element has two neutrons in its atoms? Pause the video for some thinking time and press play when you've got your answer.

Well done if you said it's helium that has two neutrons in its atoms because the mass number take away the atomic number is equal to two.

Here's another check: Which element has got 12 neutrons in its atoms? Pause the video for some thinking time, press play when you've got your answer.

How many neutrons does the element beryllium have in each atom? So how many neutrons does the element beryllium have in each atom, 4, 5, 9, or 13? Pause the video for some thinking time, press play when you've got your answer.

Well done if you said 5, 9 - 4 = 5, well done if you got 5.

How many neutrons does the element silicon have in each atom, 14, 28, 42, or 392? Pause the video for some thinking time, press play when you've got your answer.

Here's Task B for you.

Calculate the number of neutrons for the atoms of each element.

So you've got 10 elements there, pause a video, calculate the number of neutrons for each of them, and then press play, and we'll go through the answers.

Well done for completing Task B.

Let's go through the first four answers.

So here we go.

So scandium, 45 is a mass number, subtract the atomic number, gives us 24 neutrons.

Yttrium now, 89 for the mass number, subtract the atomic number of 39, will leave you with 50.

Chromium, 52, subtract 24 equals 28, so 28 neutrons for chromium.

Iodine, 127, subtract 53, should leave you with 74, so that's 74 neutrons.

Well done if you've got all those four correct.

Next four here, we've got argon, which has got mass number of 40, and then atomic number of 18.

So 40 - 18 = 22.

Tungsten here, 184, subtract by 74 gives you 110.

Got mercury here, so 201 subtract 80 gives you 121 neutrons.

Lead, 207, which is the mass number, subtract by the atomic number, gives you 125.

So 125 neutrons for lead.

Got xenon on here, 131, subtract by 54, gives you 77 neutrons.

And tin, finally, 119 subtract by 50 gives you 69 neutrons.

So well done if you've got all of those correct, you're working really well this lesson.

Here's another task for you.

Calculate the number of neutrons for the atoms of each element.

The first one's been done for you as an example.

So we've got beryllium, it's got an atomic number of 4, 'cause that's the smallest number, a mass number of 9, that's the largest number, the number of protons that's given by the atomic number and the number of neutrons, you've got to do a little calculation, which is the mass number subtract the atomic number.

So in this case it's 9 - 4 = 5.

What I want you to do is complete the rest of the table.

So pause the video whilst you do that and then press play, and we'll go through the answers.

Well done for completing that task.

Let's go through the answers.

So beryllium was our example.

So we're going to go with scandium now, we're gonna have a look.

The atomic number is 21, the math number is 45.

So when we are looking at the number of protons, we just need to look for the atomic number and that's 21, so it's got 21 protons.

And to calculate the number of neutrons, we just need to do some simple maths.

So we're going to take 45 and we're going to subtract from that 21, and that will leave us with 24.

So 24 is the answer of the number of neutrons.

The next one down here, it's got an atomic number of 26.

So we can look that up on the periodic table and we can find out what the element's called.

So that element is iron.

It's got a mass number of 56, and all we've got to do then is look at the number of neutrons, so we're gonna calculate that.

56 - 26 is going to give us 30.

Well done if you put 30 neutrons there.

So the next one here, manganese.

So again, we've looked up using the atomic number on the periodic table to find out what element that is.

And then we can see from the periodic table if it's got a mass number of, sorry, atomic number of 25, the mass number is going to be 55, we can look that up on the periodic table.

Number of protons, well that's just the atomic number of 25.

And then we're going to do our simple calculation.

55 - 25 leaves us with 30 neutrons.

Now we're gonna have a look at the next one here.

So that's got an atomic number of 27 because it is got 27 protons.

We can then look at the name for that, which is cobalt.

And then what we can do is do our simple maths then, 59 - 27 gives us 32.

Now we're gonna have a look at this final one here.

So we've got all the sums that we need to work backwards from.

So we've got 89, so that must be the mass number, 39 must be the atomic number and also the number of protons.

So 89, that's the mass number, 39 is the number of protons, atomic number of 39, so we are going to then look up the name on the periodic table and we're going to find that it is yttrium, so well done if you got that one correct.

Well done for working hard in the first two learning cycles.

We've done about atomic number, we've done about mass number, and now we're going to look at calculating subatomic particles.

This lesson, we've learned how to find the number of subatomic particles for an element from the periodic table.

We're going to put this all together now, and what I want you to do is feel really confident at the end of this lesson that you can tell me how many protons, how many electrons, and how many neutrons are in any element of the periodic table.

Now what we're going to do is we're going to look at calculating subatomic particles.

I'm going to show you a way that I like to lay this out so that I don't go wrong when calculating the number of protons, electrons, and neutrons.

So the first thing I always do is I always draw this element in a box, just like this.

So here's fluorine, and then what I do is I write proton, electron, and neutron.

You can see that it actually spells the word "pen," P-E-N.

However, I would always like you to write proton, electron, and neutron.

Don't shorten it to PEN, please.

First thing we're going to do is we're going to look at the number of protons.

Now I know that the number of protons is the atomic number, and for this element, fluorine, the atomic number is 9.

So there we go, I'm just gonna put 9.

Now I know that there are the same number of protons as electrons because if I've got nine positives, I must have nine negatives.

So for the number of electrons, I'm going to write nine because those nine negatives cancel out the nine positives.

Now for the neutrons, what I always remember is it's quite straightforward to do.

All I need to do is take the mass number, which is 19, and subtract from that the atomic number, which in this case is 9.

So I'm going to do 19 - 9, and then I can use my calculator.

Or if it's simple enough, I could do this in my head.

What I'm gonna do then is do the calculation, and that gives me 10 neutrons.

So I've written down.

So first of all, I've written down this element.

Then I've written down proton, electron, neutron, so it should spell out "pen," and that helps you remember how to get 'em in the correct order.

And this is sulphur, so we're going to look for the number of protons.

So that's the atomic number, which is a smaller of the two numbers, so that's 16.

Then the number of electrons, well I've got 16 positives, so I must have 16 negative particles.

In that case, I've got 16 electrons.

And then all I've got to do then is do a simple calculation of the mass number, subtract the atomic number.

So I'm going to do 32 - 16 = 16.

If you do this in this way, you will always get it correct.

You won't get confused, you'll always get it correct.

So let's have a look, here's some for you to do.

We've got aluminium and we've got selenium for you to do.

So pause the video, copy the layout that I've done, calculate the three subatomic particles, the protons, the electrons, and the neutrons, and then press play, and we'll go through the answers.

Well done for completing that task.

Let's see if you've laid yours out the same way that I've laid mine out.

So the first thing I did was I wrote down proton, electron, and neutron in that order, P-E-N, spells "PEN." Then what I'm gonna do is I'm gonna look for the atomic number, for the number of protons, that's a smaller of the two numbers, so that's 13.

If I've got 13 positive protons, I must have 13 negative electrons to make sure that the charges are cancelled out.

So everything on the periodic table, all the atoms on there, they're all neutral.

Then I'm going to calculate the number of neutrons.

So I'm going to take the mass number, which is the larger of the two numbers, 27.

I'm gonna subtract from that, 13, that's going to give me 14 neutrons.

Then I'm going to have a look at selenium, and I'm gonna do exactly the same.

So first thing, write down proton, electron, neutron.

Next thing, smallest number for the protons, 34.

Electrons, well I need 34 of those to cancel out the 34 positive protons.

And then the neutrons, quick calculation, 79, which is the largest number, which is the mass number, subtract from that, 34, and that will give you 45.

Hopefully you've got all those correct.

If you have, well done.

I've got a quick check for understanding: Which one of these is correct? So pause the video, have a little think, then when you've got your answer, press play.

Well done if you said C, protons, electrons, neutrons should spell out "PEN," so P-E-N.

And then we've got 26 protons 'cause that's a smaller of the two numbers the atomic number, we've got 26 electrons 'cause they're gonna cancel out the number of protons, and the neutrons, that's the one that we're going to do a calculation for.

So we've got 56, which is the mass number, subtract the atomic number, and that will leave you with 13 neutrons.

So well done if you spotted that one.

Well done for working hard this lesson so far.

What I want you to do now is calculate the number of each subatomic particles for the atoms of the following elements.

I want you to show you're working out.

If you don't show your working out, you've not done it correctly.

So pause the video, complete the task, and press play it when you want to go through the answers.

Well done for working really hard.

So if you remember, I asked you to lay it out in a specific way.

So we want to be looking, have you done it this correct way? So, protons, we've got 40, that's the smallest one.

Electrons, 40 there.

And neutrons, we're going to do that calculation for, so there's 51 neutrons in total.

And then we've got this one here, arsenic.

We've got protons in electrons, there are 33, that's a smaller of the two numbers, the atomic number.

And then the neutrons, we need to do that calculation.

The mass number take away the atomic number leaves us with 42.

Next one, beryllium.

So we've got protein, electron, neutron, 4, 4, and then that little calculation, calculating 5 neutrons.

And this last one here, we've got protons, electrons, neutrons.

We've got 49 protons, means we should have 49 electrons.

And the neutrons are 66 'cause we've done that simple calculation of the mass number subtract the atomic number.

Well done if you've got the first four correct.

Here's the next four.

So we've got neon, we've got 10 protons, 10 electrons, 10 neutrons.

Osmium, we've got 76 protons, 76 electrons, and we've got 140 neutrons.

We've got zinc here, 30 protons, 30 electrons, 35 neutrons.

And then we've got strontium, 38 protons, 38 electrons, and 50 neutrons.

Well done if you've got these four correct.

So we've got our final two here.

We've got gold, 79 protons, 79 electrons, 180 neutrons.

And we've got tantalium here, 73 protons, 73 electrons, and 108 neutrons.

So well done if you've managed to get 10 out of 10 for this task, you've done really well.

This is part two of Task C.

What I want you to do now is complete this table with information from the previous task.

So all you've got to do now is find out how many protons, how many electrons, and how many neutrons are in these elements.

You've done all the calculations, all I want you to do now is summarise this by putting them in a table.

So pause the video whilst you do that, then press play when you've got your answers.

So we're going to go through these answers now, should be quite straightforward.

Hopefully you've managed to not get confused and put the wrong numbers in the wrong columns there.

So, we've got the first one here, number of protons, number of electrons, 33 each, number of neutrons, that's going to be different, 42.

And then we've got beryllium 4, 4, and 5.

Indium 49, 49, 66.

You should see by now that the number of protons should always be the same as the number of electrons.

We've got neon here, 10, 10, and 10.

So sometimes the number of neutrons are the same as the number of protons and the number of electrons, but you have to take care because it's not always the case.

Got 76, 76, 114 for osmium.

Then we've got zinc, 30, 30, 35.

Strontium, 38, 38, 50.

Gold, 79 protons, 79 electrons, 118 neutrons.

Tantalium, 73, 73, and 126.

So well done if you've managed to get all of those correct, you've done really well.

Here's a summary for this lesson.

The atomic number of an element states the number of protons in the nucleus of every atom of that element.

The number of electrons in an atom is equal to the number of protons in the nucleus of the atom.

The mass number of an element is equal to the number of protons and neutrons in atoms of that element.

Well done for working really hard in this lesson.

Sometimes it wasn't easy and it could be confusing.

But I hope that you've learned how to calculate the number of protons, electrons, and neutrons of atoms of any element on the periodic table.

So well done to you for working really hard.