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In this video, we’re going to talk about atomic
mass. Now sometimes people call atomic mass by other names; your teacher might call it
one of these things instead. Atomic mass is a really important characteristic for elements.
Each element like Copper, Oxygen, Sulfur, and so forth has its own atomic mass and atomic
mass is this number that’s written underneath the element’s sign on the periodic table.
Now atomic mass is an average, it’s an average of the masses of a number of different atoms
but it’s a special kind of average called a weighted average and this is different than
the kind of average that you probably already learned in math. So in order to talk about
and really understand atomic mass, we first have to understand weighted averages, what
they are, and how to calculate them. So we’re going to start out by talking about weighted
average using an analogy to cars. So let’s imagine that there’s a type of car called
the Lemona and the Lemona is called a Lemona because it looks like a lemon, it has this
very distinct shape. For the purposes in this video, we’ll imagine that the Lemona comes
in two models, the Lemona GX and the Lemona GXL and these two models have different features
that are unique to each one of them. The GX is blue, the GXL is red and this one’s kind
of a luxury model. Its got massaging seats and platinum spinner wheels whereas this one
only has cloth seats and cheap aluminum wheels. Either way, even though these are different
models that have different features they are both Lemonas because they have this distinct
lemon like shape. In this way, the models of the Lemonas are very much like isotopes
of an element. Copper for example comes in two models, Copper 63 and Copper 65. Both
of these isotopes of Copper have the same number of protons, twenty-nine, because the
atomic number of Copper is 29. But they have different numbers of neutrons. So just as
long as we have 29 number of protons, it makes you copper. It doesn’t matter how many neutrons
you have, just the way that if you have the shape of a lemon, the car is a Lemona and
it doesn’t matter what other features come in that car. This is all we’re going to talk
about with isotopes right now but just keep this in the back of your mind that the models
of a car are very similar to the isotopes of an element. Anyway, we said that atomic
mass is going to have a lot to do with the idea of averages. So let’s think about averages
for these two cars. The Lemona GX weighs 4,000 pounds whereas the Lemona GXL weighs 5,000
pounds. It’s probably the platinum spinner wheels that really add to that heft. So let’s
say you have this question. What is the average weight of the two cars? Knowing what you probably
already know about averages, you could do this math. You could take 4000 pounds for
the GX, add it to the 5,000 pounds for the GXL and then divide by two because we have
two things here. That would give you an average of 4,500 pounds which gives us a number that
is right between the weights of the two models. So I’m going to refer to this as a regular
average, it’s a kind of average you probably already learned how to do. Now what if I made
this problem a little bit more complicated by giving you some extra information? Let’s
say that there aren’t the same number of GX’s and GXL’s out there. Maybe because the GXL
is a little bit more expensive there are a lot fewer of them. If we look at all the Lemonas
that have been sold everywhere, only five percent of them are GXL’s whereas the vast
majority, ninety-five percent them, are GX’s. We could show these graphically. If we were
to pull 100 random Lemonas off the street, all the Blues would be the GX’s whereas the
ones in red show the GXL’s. Obviously there are many more but this is a hundred taken
at random and we can see the same thing on a pie chart with just five percent GXL’s and
the vast majority ninety five percent are GX’s. So there’s that. Now let’s take this
information into account when we’re asked this question. What is the average weight
of Lemonas taking into account the amount of each model? Now we have to calculate an
average that is different than the regular average that we did up here because in this
case we just found a number that was right between 4,000 and 5,000 but if we’re taking
into account the amount of each of these, is it really fair to say the average weight
is 4,500, right in the middle of these two weights? Because there are so much more of
the Lemona GX’s and they weigh less, we need to come up with an average that takes this
into account and gives us a number that’s not just right in the middle but would be
closer to this because they’re so many more of them. Here’s how we do it. This is where
we get to the idea of weighted average. So to calculate the weighted average, I’m going
to take the amount that the Lemona G weighs which is 4,000 pounds and then I’m going to
multiply it by the percent abundance. Abundance is just a really fancy word that means how
much of something you have. So here, we have ninety-five percent of the total Lemonas are
GX’s so I’m going to multiply it by the abundance of the GX. I have to turn this percentage
into a decimal. So the decimal point would be here, I move it two spaces to the left
so I’m going to get 0.95. Now what this expression is here is this is the contribution from the
GX that I have 95 percent of. Now I’m going to take that and I’m going to add it to the
amount that I have of the GXL. So I’m going to take its weight which is 5,000 pounds and
multiply by its abundance also expressed as a decimal. So again the decimal places is
here, I’m going to move it two spaces to the left so I’ll have 0.05 and this right here
is the GXL which accounts for 5 percent of my total. I multiply these two things together
and then I do the addition and I’m going to end up with a weighted average of 4,050 pounds.
Now as you can see, here’s an average that takes into account the weights of both of
these models but it also takes into account the amount the amount we have of each and
so because there are so many more of the GX’s, the average isn’t right in the middle, that
average is much closer to the weight of the GX’s. And because there is few of the GXL’s,
their weight doesn’t have a whole lot of impact on this final average. I mean it’s higher
than 4,000 but it’s not right in between and so this calculation is what we refer to as
a weighted average where we take into account the amount or the abundance of how much we
have of each thing. So now that we learned how to do weighted averages with different
types of cars, let’s talk about how to do weighted averages with different isotopes
of an element. So the atomic mass is a weighted average of the masses for all the isotopes
of a certain element. Copper as we said earlier has two versions or models, Copper 63 and
Copper 65. Just like the Lemona, these two versions of Copper or these two isotopes of
Copper have different masses. So, the mass of Copper 63 is about 63 amu and the mass
of Copper 65 is about 65 amu but also just like the Lemona we don’t have the same number
of Copper 63 and Copper 65 atoms. If we randomly pulled a 100 Copper atoms out of the world,
we find that 69 percent of them are Copper 63. Here my 100 copper atoms and the 63 ones
are represented by blue dots. And we’d find that 31 percent of those are Copper 65 atoms.
So the point is you pull a Copper atom at random from somewhere in the world and it
can be either 63 or it can be 65. You have a 69 percent chance of getting Copper 63 and
31 percent chance of getting Copper 65. So to find the atomic mass, we need to do a weighted
average calculation that takes into account the mass of each of these isotopes but also
their percent abundance and here’s how we’re going to do it. Remember how we did it with
the Lemona? What we do is we start with the mass. So Copper 63, I’ll do 63 amu. Now I
multiply that by its abundance expressed as a decimal. Sixty-nine percent move the decimal
place two spots to the left and I have 0.69 and this expression right here is for Copper
63. Now I’m going to add that to Copper 65. I’m going to do 65 amu times its abundance
0.31 expressed as a decimal and just to keep track of this I’ll put Copper 65 (Cu-65) here.
Now the math really isn’t that hard, it’s just setting it up that’s tricky. Multiply
this, multiply this, and add them together, I’m going to get 63.62 amu. Now look at this,
63 and 65, if we did a regular average we would come up with a number that was right
in the middle, 64 ,okay? But there are a lot more of the 63’s so that’s going to mean that
the weighted average isn’t going to be right in the middle, it’s going to be closer to
63 and that’s exactly what we see. We see a weighted average that is not 64 but is down
closer to 63 because we have more of these and the heavier Copper 65’s are not contributing
as much to this weighted average. Now I told you that this number here on the periodic
table represents the atomic mass. You might be wondering why the atomic mass I calculated
here came out to 63.62 and not to the .55 that I see here. Well the reason is because
I took some shortcuts here. I used cleaner numbers so that it didn’t confuse you as much
when we were doing the calculations for the first time. It turns out that Copper 63 doesn’t
really weigh exactly 63 amu but it’s actually 62.93. It’s also not 69 percent abundant but
it’s 69.17 abundant. So they’re just some extra decimals on the end that I chose to
leave off for these calculations because they’re kind of a pain. The same is true of 65 where
the numbers aren’t the perfectly nice even ones that I used for this problem but the
point is when you do take these numbers into account and you do the weighted average calculation,
you end up with an atomic mass in amu that is exactly the same as what you find on the
periodic table. So now that you understand what a weighted average is, how to calculate
it, and how to work through atomic mass, you’re ready to check out the practice problems on
this topic in other videos.

100 thoughts on “Atomic Mass: Introduction

  1. I have my mock GCSE triple science exam tomorrow and this has helped me a ton, I was wondering if you’re going to ever do videos about moles, titrations, oxerdation and reduction and atom economy and all that??

  2. I really loved your video. fabulous. what a nice way of teaching . explained in a very simple way. if I could express my gratitude toward you for such a nice explanation. thanks a lot.

  3. who else trying to cop one of them gxl lemonas. Been one of my dream cars for as long as I can remember, they even went as far to replace the platinum wheels with fidget spinners and that to me is perfection

  4. These videos are really interesting so thanks for creating them. I came here just by googling what are isotopes after reading about heavy water and not really understanding it.

    One question I have is what is the point of knowing the weighted average mass of an element? That number completely depends on how much of it is out there, which I presume is done by some sort of sampling?

    Whatever the reason, how can we sure this is correct in the full universe and if our sampling of the element is wrong, doesn't that impact the above uses quite significantly?

  5. I have a video on my channel of relative atomic mass calculations and past papers! It'll be really helpful for GCSE and A Level chemistry revision!

  6. Very nice teaching sir !!! U are really a best teacher ….books doesn't make these things so much interesting …& because of u ..I m really interested to study chemistry …thanx sir …& pls upload more for physics &chemistry ….

  7. woooooooooow!!!, best pedagogic explanation!!!! Greetings from Spain! Nos I know how to teach in a fun way this

  8. THANK U SOO MUCH , u were really helpful , and my all confusion has cleared by watching this video I wish if u would our chem professor then we would have no need to cram chem

  9. Stupid question, but where do you get the percentages youre using to calculate the copper 63 and 65?
    I understood the calculations and stuff, but say for example youre supposed to find the atomic mass for oxygen, but youre not allowed to use the periodic system, nor do you know the specific answer. Heelp

  10. I m presently in ninth class and I was so confused about this topic as I do not take any tuition.So it helped me a lot.Once again thanks 😇

  11. u are my world's best teacher I have seen…I really love the way u present …each topics…I'm building up my confidence with ur videos…I lv u…sir….and I'm thank full to u…so much….💙💙💖💖

  12. Normally don't bother to comment, however your videos your explanation is very simplified + easy to comprehend. Thanks a lot!

  13. So I pay thousands to the University, yet my time is better spent watching YouTube videos like this one, where the explanations are so much easier and clearer. Perhaps University lecturers need to learn how to teach! You'd think by now, they would have discovered this phenomenon! Thanks so much for your videos!

  14. Great video, I just would like to know how you came up with those percentages of copper 63 and 65? 69% and 31%?

  15. Hi Tyler, I home school my son and he has been very successful with his studies because of your videos. So THANK YOU!! I have a request though. Would you be willing or interested in making a video on physics specifically Newtons second law? He is trying to learn force and acceleration and having difficult time comprehending it.

  16. Bullshit . I have 2 different types of bowling balls .Both are exactly the same size and the same weight . So which one has more mass ? It's scientific bullshit .

  17. WOW, this is too good. Explaining Mass which is basically weight and showing how it's weighed to get the numbers I don't care about. Excellent analogy. Yay internet. I'm subscribing to Tyler DeWitts videos right now.

  18. Anyone else notice that there are 6 red dots for GXL when there are supposed to be 5? No hate at all. This is so helpful. I just found that kind of funny.

  19. Sir, I’m in shock! So I NEVER write reviews but I will have to one this one. Thank you for this explanatory video of how to calculate Weighted Average. Thank you!

  20. i just love how humble you are ind your videos. and these really helped me understand and change my perspective in chemistry. i am starting to like it with your teaching. thank you very much.

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