# 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 bytalking 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

ofprotons 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 boththese 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 andtrillions 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 Boronatoms 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 averageand 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 sothat'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 wedetermine

## 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 mathtaking into account the abundance to find the average