we have listed here we know that carbon 12 is the most common isotope of carbon on Earth 98.89% of the carbon on Earth is carbon 12 and we know that by definition its mass is exactly 12 atomic mass units now that's not the only isotope of carbon on Earth there are other Isotopes the next most frequent One Is frequent one is carbon 13 1.11% of the carbon on Earth is carbon 133 and we can experimentally find that its mass is 13.0 034 atomic mass units so these numbers that we have here just as a review these are atomic mass these are atomic mass and so we're going to think about in this video is well how do they come up with the atomic weight number that they'll give you on a periodic table like that so Atomic uh atomic weight where does where does that come from well in the video on atomic weight and on atomic mass we see that the atomic weight is the weighted average of the atomic masses of the various isotopes of that element so to find this roughly 12.01 we take the weighted average of these two things and what are we weighted by well we weit about we weighted by how common that that isotope actually is so what we want to do is we we could take 98.89% and multiply it by 12 and I'll rewrite this percentage as a decimal so it'll be 0.988 9 time 12 and to that we are going to add we are going to add 1.11% time 13. z34 so as a decimal that's going to be 0.011 per or that's 1.11% is 0.011 oh 111 and I'm going to multiply that times 13. z34 atomic mass units so what does that give us let's get our calculator out here so we are going to have 0.988 n time 12 is equal to 11.86 68 and to that we are going to add we are going to add 0.0111 times 1334 and I know it's going to do this multiplication first because the calculator knows about order of operations and so that's all going to be as you can see 12.011 13774 which if you were to round to the hundreds place is how this this atomic weight was gotten so that's there you go that's how we calculate atomic weight so I could write this as approximately 12.01 it's the weighted average of the atomic masses now another thing that you might want to note is what's the difference between carbon 12 and carbon 13 carbon 12 this right over here is six protons the six protons are what make it carbon so both of these will have six protons and the difference is in the neutrons this right over here has six neutrons six neutrons and this right over here is going to have one more Neutron seven neutrons so when you look at the difference in atomic mass notice the change is looks like it's plus 1.34 atomic mass units so from this you can see hey look if I add a neutron plus one Neutron plus one Neutron it's roughly equal to an atomic mass unit it's not exactly an atom mass unit but roughly speaking a lot of kind of very broad High Lev terms you can kind of view it as being very close to one atomic mass unit and the same thing is true of protons but anyway hopefully you now have appreciation for the difference between atomic mass which is the mass and atomic weight which is the weighted average of the various isotopes of that element on Earth how to calculate it and roughly what the mass of a neutron is