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What's the Difference between Mass Number and Atomic Mass?

What's the Difference between Mass Number and Atomic Mass?
Let’s talk about the

difference

between

mass

number

an

atomic

mass

. These things sound very similar but they're actually very different. Just so you know,

atomic

mass

can go by other names. Sometimes it's called average

atomic

mass

or relative

atomic

mass

or even

atomic

weight. Maybe your teacher calls it one of these. For our purposes, in this video we're just going to call it

atomic

mass

. So we're talking about the

difference

between these two things. Let's start by
what s the difference between mass number and atomic mass
talking about

mass

number

. So up here, I've got two atoms. They've got protons and neutrons in their nucleus and I'm particularly concerned with the

number

of protons and neutrons that there are. So this atom over here has five protons. The proton are these red circles, we could count them up, 5 protons. And the neutrons are these blue circles, we can count them up, we got 5 of those. This atom over here also has 5 protons but it’s got 6 neutrons. So

mass

number

is the

number

of
protons plus the

number

of neutrons. So this atom here has a

mass

number

of 10. Five plus five. And this atom over here has a

mass

number

of 11. Five plus six.

Mass

number

tells us how many protons and neutrons are in the nucleus of an atom but it is also important because it gives us an idea about how much the atom weighs. You might remember, we've said before, that 1 proton or 1 neutron weighs about 1 amu, 1

atomic

mass

unit. So if you have a

mass

number

of 10, 5 protons and 5 neutrons,
your atom is going to weigh about 10 amu. If you have a

mass

number

of 11 that tells you that the atom here weighs about 11 amu. These atoms both have electrons but electrons are so tiny and they have such a small amount of

mass

that we don't even worry about the

mass

of electrons when we're weighing up stuff to determine how much the larger atom would weigh. So 10 amu and 11 amu. So that’s

mass

number

, protons plus neutrons, and it gives you an idea of how much the atoms weigh in amu.
Now, let's talk about

atomic

mass

. You might notice that both these atoms have the same

number

of protons in their nucleus, 5 protons here and 5 protons here. This means that they are the same element, right? Because the

number

of protons an atom has determines

what

kind of an element it is. So we can look on the periodic table to find out

what

element these atoms are and it turns out that they’re Boron because Boron, up here, has an

atomic

number

of 5, 5 protons and your Boron. So both
what s the difference between mass number and atomic mass
these atoms are different versions of Boron, they’re different isotopes and isotope is a word that just means a different version of an atom that has the same

number

of protons but a different

number

of neutrons. We can give distinct names to these two isotopes or versions of Boron. This atom over here is an example of Boron 10 because it has a

mass

number

of 1, 5 plus 5, and this atom here is an isotope of Boron called Boron 11, 5 plus 6 has a

mass

number

of 11. So there are billions and
trillions and gazillions of Boron atoms in the world. If we could pick one up at random, it could be either one of these two types, okay? It could be a Boron 10 atom with 5 protons and 5 neutrons or it might be a Boron 11 atom with 5 protons and 6 neutrons. So Boron comes in these two versions. Now the thing is there is not an equal

number

of Boron 10 atoms and Boron 11 atoms. In fact, if we pulled 50 Boron atoms at random and represented them here, we'd see that the majority of the Boron
atoms are Boron 11 which I'm indicating with little orange circles here, okay? Most of them are Boron 11 and only a small minority of them are Boron 10 atoms. We can look at this on a pie chart just to get this point home. You can see that the vast majority of them are Boron 11 with a small amount that's Boron 10. If you crank through the math here, you'll find out that about twenty percent of the Boron atoms in the world are Boron 10, where eighty percent of them are Boron 11. This
leads us to the idea of

atomic

mass

. Now,

atomic

mass

asks

what

is the average

mass

of a Boron atom. Alright, we got these two types of Boron atoms. We've got Boron 10 that was 10 amu, that's

what

its

mass

is. And we got Boron 11 which weighs 11 amu, that’s

what

its

mass

is. But here's the thing. There is only 20 percent of this guy in 80 percent of this guy so now we're asking, got these two types of Boron,

what

is the average

mass

of a Boron atom. Now you might think average
what s the difference between mass number and atomic mass
and say that's pretty easy, I know how to take an average. This guy weighs 10, this guy weighs 11 so why don’t I just add the two of them up-- 10 plus 11 and divide by two and then I'll get 10.58. That's how I take an average. Okay, no it's wrong. That is one way that you can take an average but it won't work here and the reason why is because this type of thing added up and divided by 2 assumes that we have the same amount of both of these. If we had 50 percent Boron 10
and 50 percent Boron 11 then we could just add them up and divide by 2 but we can't do that because we have different amounts of these two types of atoms, okay? And we have to come up with a mathematical equation that’s more complex than this to take into account the different amounts that we have of these two atoms. Here's how we do it. We take the fact that there are 20 percent Boron 10 atoms and that they weigh 10 amu each. So 20 percent times 10 amu and then we add that to 80
percent, which is over here, times 11 amu. And when we do that math we have to convert the percentages into decimals but when we crank through this we end up with 10.8 amu and this tells us

what

the average weight of a Boron atom is, taking into mind that only 20 percent of the Boron atoms out there are Boron 10 and 80 percent of the Boron atoms out there are Boron 11. Check this out, the

number

that we get here, 10.8 amu, is

what

's the bottom of this element name on the periodic table so
that's

what

this

number

s is. It's the

atomic

mass

that tells you

what

the average weight of one of these Boron atoms. Now, we said we had these two different types that weigh 10 amu or 11 amu. Look at the average, the average is 10.8 which is much closer to 11 than it is to 10 and that makes sense because since we have so many more of these and they weigh more, the average for these two types should end up closer to this just because we have so many more of them. So that's how we
determine

what

atomic

mass

is. Just review these two things.

Mass

number

is a

number

of protons and neutrons in a particular atom or in a particular isotope and from the

atomic

number

you can figure out how much the atom weighs in amu. But now,

atomic

mass

looks at all the different types of isotopes for a particular element, in this case Boron 10 and Boron 11. It takes into account the amount of each that you have also known as the abundance, how many of them there are and then you do this math
taking into account the abundance to find the average

mass

of all of the different types of Boron. So that is the

difference

between

mass

number

and

atomic

mass

. If you want to learn more about how to calculate

atomic

mass

, I've got videos on that.