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

Let's talk about

mass

number

and

atomic

weight

, these things may seem very similar but they are actually quite different, so here I have two atoms and I am particularly interested in the

number

of protons and neutrons that are in the nuclei of both atoms. so the protons are in red the neutrons are in blue we could count them the atom here has five protons and five neutrons while the atom on this side also has five protons but has six neutrons then the

mass

number if, as you remember, is the number of protons plus a number of neutrons, so the atom here has a mass number of 10 and the atom here has a mass number of 11 5 + 6.

Okay, now the mass number also tells us a little about how much the atom actually weighs. because it turns out that a proton or a neutron weighs about one AMU, an

atomic

mass unit, it's a unit that we can use to measure how much an atom weighs, so if we have 10 protons and neutrons here with a mass number of 10, it means that this atom will weigh about 10 AMU 10 atomic mass units and our atom here with a mass number of 11 will weigh about 11 AMU, so that is the mass number, the mass number is something that we can use to describe a atom.

Prot protons plus neutrons and tells us a little bit about how much that atom weighs in atomic mass units. Now let's move on to talk about atomic

weight

, so you can notice here that both atoms have the same number of protons, which means they are the same element right because the number of protons an atom has in its nucleus determines which element it is. So

what

element are these two atoms? We can look at the periodic table to find out and it turns out that Boron with an atomic number of five is

what

both atoms are because they have five protons in their nucleus, so these two atoms are different versions of Boron. , different versions of boron that have a different number of neutrons but the same number of protons, we call them isotopes, okay, isotopes are like different versions of an atom that has the same number of protons but differ in their number of neutrons, so we call this atom here Boron 10, that's its isotope name because 10 the mass number is 10 and this one here is boron 11 because it has a mass number of 11, so there are two versions of boron.

Now there are billions and billions and billions of boron atoms in the world and if you pick a boron atom, it could be either of these two versions. Well, it could be a boron 10 atom with five neutrons or it could be a boron 11 atom with six neutrons, but here's the thing: there are not the same number of boron 10 and boron 11 atoms in the world. Okay, if we pick 50 boron atoms at random anywhere, this is what they are. It seems like okay, the vast majority of them would be these orange Boron 11 atoms and a much smaller minority would be Boron 10, which I'm representing with these green circles here, turns out if we do the math, only about 20%.

Of all the boron atoms in the world are boron 10 with five neutrons and the vast majority 80% are boron 11 atoms. We can see what this looks like on a pi chart just to get a better idea of ​​how boron 11 occupies everything. this room unlike the small amount of boron 10 atoms So 20% of the boron atoms are Boron 10 80% of the boron atoms are Boron 11 this brings us directly to the question that makes the atomic weight and the weight atomic asks us what is the average mass of a boron atom, so some weigh 10 AMU, others weigh 11 AMU, what is the average mass of a boron atom?

Now you might think the average is easy, 10 AMU plus 11 AMU and I can divide it by two, but no, that's wrong. that is not the correct way to find the average mass of a boron atom, here is why it is wrong because this formula 10 + 11/2 assumes that we have the same amount of boron 10 and boron 11 if it were 50% this and 50 % So, sure enough, we could just add the two weights and divide by two, but since we only have 20% of this and 80% of this, we can't just add them and divide by two, we have to use a more complicated equation. that takes into account the amount of each that we have, this is how we do it, this is how we would determine the atomic weight, so Boron 10 we have 20% of the total Boron atoms, they are Boron 10, so we are going to do the 20% times. 10 AMU, which is what the Boron 10 atom weighs, and then we're going to go here and do the 80%, that's the amount of diameter in 11 that we have multiplied by 11 AMU, if you want, you can put them in parentheses so that you can.

To see the math a little easier, when we end up multiplying this, we will have to convert the percentages to decimals in 0, 2 and 8, we end up with 10.8 AMU, which is the average mass of a boron atom if you take into account There are only 20% of 10 and 80% of 11, so the average mass turns out to be much closer to 11 AMU than 10 and that makes sense because there are many more of these, the average should be closer to 11 AMU. to 10 and here's the last thing now you know what this number is here at the bottom of these elements in the periodic table 10.8 this is not a mass number this is an atomic weight this tells you the average mass of a boron atom Based on the fact that there are different amounts of different isotopes, so just to check the mass number is something that applies to an individual atom or an individual isotope.

Well, the mass number of this is 10 because it has five protons and five neutrons. The number of this is 11 because it has five protons and six neutrons. These are individual atoms or individual isotopes. The atomic weight takes into account these two isotopes and also takes into account the amount of each. We have 20% of this 80% of this. We have to use these percentages when we do the calculations to figure out what the average mass of all the different types of boron would be. If you want to learn more about atomic weight, there are a few videos that go into depth on how to actually do the calculations, but this video should give you a good conceptual understanding of the

difference

between mass number and atomic weight.