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

Let's talk about the

difference

between

mass

number

and

atomic

mass

. These things sound very similar but are actually very different. Just so you know,

atomic

mass can have other names. It is sometimes called average atomic mass or relative atomic mass or even atomic weight. Maybe your teacher will call it one of these. For our purposes, in this video we will simply call it atomic mass. So we're talking about the

difference

between these two things. Let's start talking about the mass

number

. I have two atoms up here. They have protons and neutrons in their nucleus and I am particularly concerned about the number of protons and neutrons there are.

So this atom here has five protons. The protons are these red circles, we could count them, 5 protons. And the neutrons are these blue circles, we can count them, we have 5 of those. This atom here also has 5 protons but it has 6 neutrons. So the 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 here has a mass number of 11. Five plus six. The mass number tells us how many protons and neutrons there are in the nucleus of an atom, but it is also important because it gives us an idea of ​​how much the atom weighs.

You may remember, we have said before, that 1 proton or 1 neutron weighs approximately 1 amu, 1 atomic mass unit. So if you have a mass number of 10, 5 protons and 5 neutrons, your atom will weigh about 10 amu. If it has a mass number of 11, that tells you that the atom here weighs about 11 amu. Both atoms have electrons, but the electrons are so small and have such a small amount of mass that we don't even worry about the mass of the electrons when we weigh things to determine how much the larger atom would weigh. So 10 amu and 11 amu. That's the 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 atoms have the same number of protons in their nuclei, 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

type of element it is. So we can look at the periodic table to find out

what

element these atoms are and it turns out that they are Boron because Boron, up here, has an atomic number of 5, 5 protons and its Boron. So both atoms are different versions of boron, they are different isotopes and isotope is a word that simply means a different version of an atom that has the same number of protons but a different number of neutrons.

We can give different names to these two isotopes or versions of Boron. This atom 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 thousands of millions. and trillions and trillions of boron atoms in the world. If we could pick one at random, it could be either of these two guys, okay? It could be a Boron 10 atom with 5 protons and 5 neutrons or it could be a Boron 11 atom with 5 protons and 6 neutrons. So boron comes in these two versions.

Now, the thing is, there are not the same number of boron 10 and boron 11 atoms. In fact, if we were to pull out 50 boron atoms at random and plot them here, we would see that the majority of boron atoms are boron 11. , which I indicate with little orange circles here, okay? The majority of them are Boron 11 atoms and only a small minority are Boron 10 atoms. We can see this on a pie chart just to clarify this point. You can see that the vast majority of them are Boron 11 with a small amount being Boron 10. If you do the math here, you will find that about twenty percent of the Boron atoms in the world are Boron 10, where eighty percent of they are Boron 11.

This brings us to the idea of ​​atomic mass. Now, atomic mass asks what is the average mass of a boron atom. Alright, we have these two types of boron atoms. We have Boro 10 that had 10 amu, that is its mass. And we have Boron 11 that weighs 11 amu, that is its mass. But here's the thing. There is only 20 percent of this type to 80 percent of this type, so now we ask, given these two types of boron, what is the average mass of a boron atom. Now you might think about an average and say it's pretty easy, I know how to get an average.

This guy weighs 10, this guy weighs 11, so why don't I add them two together: 10 plus 11 and divide them by two and then I'll get 10.58? This is how I take an average. Okay, no, that's wrong. That's one way to take an average, but it won't work here and the reason is that this kind of thing adding and dividing by 2 assumes that we have the same amount of both. If we had 50 percent Boron 10 and 50 percent Boron 11, then we could just add them together and divide them by 2, but we can't do that because we have different amounts of these two types of atoms, okay?

And we have to find a mathematical equation that is more complex than this to take into account the different amounts we have of these two atoms. This is 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 multiplied by 10 amu and then we add it to 80 percent, which is here, multiplied by 11 amu. And when we do those calculations we have to convert the percentages to decimals, but when we do this we end up with 10.8 amu and this tells us what the average weight of a boron atom is, taking into account that only 20 percent of the boron The atoms there are Boron 10 and 80 percent of the Boron atoms there are are Boron 11.

Look at this, the number we get here, 10.8 amu, is what is at the end of the name of this element in the table periodic, so that's what these numbers are. It is the atomic mass that indicates the average weight of one of these boron atoms. Now, we said we have these two different types that weigh 10 or 11 amu. Look at the average, the average is 10.8, which is much closer to 11 than 10 and that makes sense because since we have a lot more of these and they weigh more, the average of these two guys should end up closer to this. only because we have many more of them.

This is how we determine what atomic mass is. Just check these two things. The mass number is the number of protons and neutrons in a particular atom or in a particular isotope and from the atomic number one can calculate how much the atom weighs in amu. But now, atomic mass looks at all the different types of isotopes of a particular element, in this case Boron 10 and Boron 11. It takes into account the amount of each, which is also known as abundance, how many of them there are. . and then you do these calculations taking abundance into account to find the average mass of all the different types of Boron.

So that's the difference between mass number and atomic mass. If you want to learn more about how to calculate atomic mass, I have videos on that.