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 get going with learning all about the atomic number.

There are three subatomic particles in an atom.

Here's an atom of beryllium that's labelled up for you with the protons, electrons, and neutrons.

We find the protons in the nucleus, so that's in the centre of the atom.

They have got a positive charge, so we write that as a relative charge of plus one and a relative mass of one.

Electrons are found orbiting the nucleus in shells, so going around the nucleus in those shells.

They're negative and they've got a relative charge of minus one and they've got a relative mass of 0.

0005.

We've got some neutrons and the neutrons are found in the nucleus.

They're neutral, as the name suggests, and they've got a relative mass of one.

Now a quick check for understanding.

True or false, all subatomic particles in an atom have a relative mass equal to one.

Pause a video for some thinking time and then press play when you've decided if it's true or false.

Well done if you said that that was false.

Now I've got two sentences for you to justify that answer.

Only protons and neutrons have a relative mass equal to one, only protons and electrons have a relative mass equal to 0.

0005 or 1/2,000.

Pause the video now for some thinking time, press play when you've decided what the answer is.

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

Remember, it's only electrons that have that very small mass, so that 0.

0005 or 1/2,000.

Well done if you got that correct.

Here's a periodic table.

You've probably seen these before.

And the atomic number on this tells you the number of protons in the nucleus of every atom of that element.

And what this means is, so here's an example, we've got beryllium here.

It's element number four and it's got four protons in the nucleus of every single atom of that element.

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

So here's another example.

Here's carbon.

You can see there carbon is element number six, you can see that that's the atomic number, and that's got six protons in the nucleus of every atom of that element.

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

Let's have a look at an example.

So here's fluorine.

You can see here it's at the bottom.

So fluorine's got nine protons.

Or sometimes it's written at the top.

And if you get confused and you don't know which one's which, you're going to look for the smaller of the two numbers, and that's the atomic number.

So in this case, we can see that the atomic number here is nine, because nine is the smallest number out of 9 and 19.

And that means that if you looked in the nucleus of an atom of fluorine, you would see that there's nine protons in there.

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 are in the nucleus of that element.

It's unique to that specific element.

So here's beryllium.

Beryllium's atomic number is four, and that shows you that there's four protons in the nucleus.

Then let's take a look at another example, so here's carbon.

You can see here that carbon's got six protons in the nucleus, because the atomic number for carbon is six.

If it had a different number, it would not be carbon anymore.

It needs to have six protons in the nucleus to be carbon.

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

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

We can see there the atomic number for helium is two, and that means it's got two protons in the nucleus.

So if it's got two positives, everything on the periodic table has got an overall charge of neutral, so zero, we must have two negatives, so two electrons to cancel out those charges.

So the positive charge on the protons is cancelled out by the negative charge of the electrons.

So we've got two positives in the middle there in the nucleus, and we've got two electrons orbiting the nucleus in shells.

They're cancelling that out, so overall the charge on helium is zero.

Let's have another look at another example.

This is phosphorus.

Phosphorus is element number 15, it's got an atomic number of 15, so that means it's got 15 protons in its nucleus.

And orbiting around that must be 15 electrons to cancel out the charges of the 15 protons.

Here's a quick check for understanding.

Which element has six 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 A, it's carbon.

So carbon has got an atomic number of six.

That means it's got six protons.

So it's got six positive charges, and to cancel those out to have an overall neutral atom we need to have six negatives, so six negative electrons.

So carbon has got six protons and six electrons in its atoms. 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 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 four.

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 done if you got all of those correct.

This is part two of Task A.

What I want you to do here is state the number of protons in the atoms of the elements pictured.

So you've got 10 elements pictured there.

How many protons would you find in the nucleus of each of those atoms? Pause the video while you complete the task, then press play and we'll go through the answers.

Well done for completing part two of Task A.

Let's go through the answers now.

Here's scandium.

Scandium got 21 protons in the nucleus.

Here's yttrium, it's got 39.

Chromium's got 24, iodine's got 53, argon's got 18, tungsten's got 74, mercury's got 80, lead's got 82, xenon's got 54, and tin has got 50.

All you needed to do there was find the smallest number of the two and tell me that that was the atomic number, which tells you the number of protons in the nucleus of an atom of that element.

Well done if you got all of 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 two, number of protons is two, that's the same as the atomic number, and the number of electrons is also two.

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 nine, the number of protons is nine, and the number of electrons is also nine.

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

So it is 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 zero, 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 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, learning all about the atomic number in our first learning cycle.

Now we're going to move on to learn about another number, this time the mass number, in our second learning cycle, 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 what this means.

So here's beryllium.

Here's its atomic number.

Its atomic number is four.

That tells us the total number of protons in the nucleus.

And here's the mass number.

Now, the mass number tells us the number of subatomic particles in the nucleus.

In this case it tells us that there's nine.

Now, we already know the number of protons in the nucleus, but what we want to figure out is the number of neutrons in that nucleus.

So we just must do a simple calculation.

So the number of neutrons equals the mass number subtract the atomic number.

So here's our four protons and five neutrons in there, and we can have a look at this because nine take away four equals five neutrons.

So we take the mass number, which is nine, and we subtract the four, which is the number of protons.

We must know that the rest of them are neutrons.

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

Is that true or is that false? Pause the video, make your decision, and then press play and I'll tell you the answer.

Well done if you said that that statement was true.

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

I do want you to justify your answer, so you've got two choices here.

The relative mass of an electron is 0.

0005 compared to a proton and neutron, which each have a mass of one, and so does not affect the mass of an atom.

Or B, 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 make your choice, then press play and I'll tell you the answer.

Well done if you said the relative mass of an electron is 0.

0005 compared to a proton and neutron, and so it does not affect the mass of the atom.

In other words, what we're saying is that electron is too small to count compared to the mass of a proton and neutron.

Well done if you got that right.

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

The larger of the two numbers is the mass number.

So in this case we've got an example of fluorine, and you can see here, these are the two larger numbers because 19 is larger than 9, so that must mean that 19 is the mass number.

Different periodic tables show the mass number to different decimal places.

Here's an example of magnesium, and this mass number is shown to the nearest whole number.

Compared to this periodic table, where the mass number here is shown to one decimal place.

The number of protons, however, is always a whole number.

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

So sometimes the number of neutrons is the same as the number of protons, but not always, because sometimes the number of neutrons is greater than the atomic number.

However, it's never smaller than the atomic number.

So there are never less neutrons than protons.

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

So that means that there are always the same number of electrons as protons for atoms on the periodic table.

Let's have a look at two examples.

So here's helium.

You can see here helium would have two neutrons, which is the same as the atomic number.

And beryllium here, that would have five neutrons, which is greater than the atomic number.

Helium is going to have two electrons, because it's always the same as the atomic number, and beryllium would have four electrons, because it's always the same as the atomic number.

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 were right.

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

Which element has two neutrons in its atoms? Pause the video now for some thinking time, press play when you've got your answer.

Well done if you said C.

Helium has got two neutrons in its atoms. Which element has six neutrons in its atoms? Pause the video for some thinking time, press play when you've got your answer.

Well done if you said A.

It's carbon.

Carbon has six neutrons, well done.

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

Well done if you said five.

Nine subtract four equals five, so that tells you the number of neutrons that the element beryllium has in each atom.

Well done if you got that correct.

How many neutrons does the element fluorine have in each atom? Is it 9, 10, 38, or 171? Pause the video now for some thinking time, press play when you've got your answer.

Well done if you said 10.

19 subtract 9 equals 10, so that shows you how many neutrons fluorine has.

Well done if you got 10 for your answer.

Here's Task B.

What I want you to do now is calculate the number of neutrons for the atoms of each of these elements.

Pause the video whilst you do this, then press play when you've got your answers.

Well done for working hard and completing Task B.

Let's go through the answers now.

All we've got to do here is we've got to take the mass number, which is the largest number, and subtract the smallest number, which is the atomic number, and that will give us a number of neutrons.

So in this case the number of neutrons here is 24.

We've got beryllium here.

So that's nine, subtract four equals five.

And then we've got carbon, 12, subtract 6, which is the atomic number, will leave you with six.

So that's got six neutrons.

Fluorine here, got 19, subtract 9, will leave you with 10 neutrons.

Here's the next four.

Helium, four, subtract by two.

Here's the next four, helium.

Four, subtract two, leaves you with two neutrons.

Nitrogen here, we've got 14, subtract 7, will leave you seven neutrons.

And we've got sodium here.

So here we've got 12 neutrons, 'cause 23 subtract 11 equals 12.

And then we've got this last one here, which is gold.

197 minus 79 gives you 118.

Well done if you got all those four correct.

Here's chromium, 52 subtract 24 leaves you with 28 neutrons.

And bismuth finally, 209 minus 83 leaves 126 neutrons.

So well done if you got all those correct.

You've done really well working hard 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 four 'cause that's the smallest number, a mass number of nine, 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 nine subtract four equals five.

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's 21, the mass number's 45.

So when we're 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 that 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 subtract 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 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 subtract 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 subtract 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're 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 so far this lesson.

We've looked at atomic number, we've looked at 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 we're going to have a look at calculating subatomic particles.

I'm going to show you a way to lay your work out to make sure that you always calculate the correct number of protons, electrons, and neutrons.

So the first thing I'm going to do is I'm going to write down proton, electron, and neutron.

You'll see it spells out pen, P-E-N.

That makes sure that you've got them in the right order.

However, I don't want you to shorten them to just P-E-N.

I'd like you to keep the full words there, please.

So we've got proton, electron, neutron.

Nine for protons, because that is a smaller of the two numbers.

That's the atomic number.

Then if we've got nine positive protons, we must also have nine negative electrons.

Next thing I'm going to calculate, the number of neutrons.

So simple sum at the end here, I've got 19, which is the mass number, subtract from that the atomic number, that gives me 10.

So I've got 10 neutrons.

I'm now gonna do another one to show you.

So here's nitrogen.

First thing I'm gonna do is I'm gonna write down proton, electron, and neutron.

So it spells pen, it's in the right order, I know that.

Then I'm going to write down the number of protons, so that's the smaller of the two numbers, seven.

Then I've got seven positive protons.

What I'm gonna do there is I'm gonna need seven negative electrons to cancel out the charges.

Then I've got neutrons.

I've got to do a simple calculation for that.

So that's 14 subtract 7 equals 7.

What I want you to do now is I want you to have a go at these two and then pause the video and then have a go laying it out just the way that I have, and then press play when you've completed this quick check.

Well done for completing this check for understanding.

Let's see how well you got on.

So first thing, we're going to write down proton, electron, neutron.

So this should spell out pen so you know you've got them in the right order.

Then I'm going to find the number of protons, which is the atomic number, so that's number 13.

If I've got 13 positive protons, I must have 13 negative electrons, because the charges cancel out because everything on the periodic table has got a neutral charge.

Then I want to calculate the number of neutrons, so I'm going to take the mass number, which is the largest number, and I'm gonna subtract from that the atomic number, which is the smaller of the two numbers.

So for this aluminium I'm going to have 27 subtract 13 equals 14.

Now what I'm gonna do is I'm gonna have the same process for gold.

So this is gold here.

Proton, electron, neutron, I'm gonna write those down in that order so it spells pen.

Then I'm gonna look for the smaller number, the atomic number, and I'm gonna write that next to the protons.

And I know that that's also the same number of electrons, 'cause remember, each electron cancels out one proton for the charges.

Then I'm going to calculate the neutrons, so I'm gonna take the larger of the two numbers, the mass number, and I'm gonna subtract from that the atomic number, and that will give me 118 in this case.

And I know now that I've got 118 neutrons.

So well done if you've managed to do that all correctly, you've done really well.

A quick check for understanding now.

Which one of these is correct? I want you to pause the video, have a think, and then press play when you want the answer.

So well done if you said it was C.

So proton, electron, neutron should spell out pen, P-E-N.

Then 21.

So 21 is the atomic number, that's the smaller of the two numbers.

If I've got 21 positive protons, I must need 21 negative electrons to cancel out those charges.

And then for the neutrons, I just need to do a simple calculation, which is the mass number, so that's the larger of the two numbers, subtract from that the atomic number and that will leave me with 24.

So that's 24 neutrons.

So well done if you got C, if you've got that one correct.

Here's part one of Task C.

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

I want you to show you working out like I've shown you earlier in this lesson.

So pause the video, complete the task, and then press play when you've got your answer.

Well done for completing part one of Task C.

Let's have a look at the answers.

So here's helium.

Protons, electrons, neutrons.

So we've got two protons, two electrons, we do that simple calculation, which is the mass number take away the atomic number equals two.

Got carbon here.

We've got six protons, so we've got six positive subatomic particles.

We definitely need six negative electrons to cancel out the charge of that, because everything on the periodic table has got a neutral charge.

And then we've got the neutrons.

All we've got to do there is a simple calculation, which is 12 subtract 6 equals 6.

Nitrogen there, you can see that they're all seven, seven, seven, seven.

We still need to show our calculations though for the neutron to show that we know the principle of how we can calculate the number of neutrons in an atom.

And then we've got oxygen here, eight, eight, and eight.

Let's have a look at the next four.

So we've got fluorine.

Nine protons, nine electrons to cancel out those nine protons.

And we've got the neutrons, we're going to take the largest number, which is the mass number, we're gonna subtract the atomic number, and that's going to give us 10.

So 10 neutrons there.

Beryllium here.

We've got four protons, four electrons.

Do our little calculation there, mass number subtract atomic number gives us five neutrons.

And here we've got a bit of an unusual one here because we've got one proton, one electron.

But you can see here that if you subtract the atomic number from the mass number, you get zero.

Hydrogen is the only atom to not have any neutrons.

Then we've got chromium here, 24 protons, 24 electrons, and 28 neutrons.

Well done if you got those four correct.

The last two here, so this is gold.

We've got 79 protons, 79 electrons to cancel out those 79 protons, and then we've got 118 neutrons.

So 197, which is the mass number.

Subtract from that the atomic number will give us 118.

And we've got bismuth, finally.

We've got 83 protons, 83 electrons.

Calculate those neutrons.

So we get the mass number, which is the larger of the two, 209, and we're gonna subtract from that 83, which is the atomic number.

That'll give us 126.

So well done if you got those all correct.

You've done really well.

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.

Suppose 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, four, four, and five.

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.

Tantalum, 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 today's 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 learnt 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.