Hi, I'm Robin Higgins. And this is in chemistry what is the mass number of iron? Alright, so the mass number equals the number of neutrons plus the number of protons. O.k., versus the atomic number which is just the number of protons. So, we can' ask overall what is the mass number of iron because iron consists of many different isotopes. And an isotope is something that has a different number of, a different number of neutrons but the same number of protons. So there's actually four isotopes of iron. And so, we'll iron out four times. And now iron is atomic number 26 and it's always going to be atomic number 26 because this is what defines it as iron. Is that anything with 26 protons is iron. So that's set in stone, we can't change that. If we change the number of protons, we no longer have iron. However the number of neutrons is a little bit more flexible. And so, on Earth there are four different naturally occurring isotopes of iron and they're iron 54, iron 56, iron 57 and iron 58. And so, these are four different mass numbers. If you want to ask what's the mass number of iron, you'd have to know which isotope you're dealing with. And there's no way to really know that unless someone just tells you or tells you how much it weighs. So, the mass number of iron will be one of these different four numbers. However we take an average to get the atomic weight And so, we take up all the different percentages. so even though, there's four, they're not equal. So, this 54 iron is actually only five point eight percent and then, 56 is the most popular, it's 91 point seven percent. This 57 is just two point one percent. And there's not very much of 58 at all, it's zero point three percent. And if we take all these weights and average them, using the correct percentages. We get the atomic weight which is 55 point eight four five AMU. So on average, this is what iron weighs. If we know what an isotope it is, it will be one of these four. I'm Robin Higgins and this is what is the mass number of iron.