so let's say that we have some mystery substance here and we know that it's a pure element and we need to figure out what it is well scientists have a method and we go into the details or more details in other videos called mass sometimes it's known as mass spectrometry or mass spectroscopy it's a technique where you could take a sample of a substance and think about the various atomic masses of the different isotopes in that substance and that's what we have right over here they tell us the mass spectrum for an average sample of a pure element is shown below so let's say it's this pure element so what this is telling us is this looks like maybe i don't know let's call this 82 percent of our sample has an atomic mass of 88 universal atomic mass units about this looks like about seven percent of our sample has an atomic mass of 87 universal atomic mass units it looks like 10 percent has an atomic mass of 86 universal atomic mass units and it looks like about 1 of our sample has an atomic mass of 84 universal atomic mass units and so from this information we can try to estimate what the average atomic mass of this mystery element is we could calculate it as 0.82 times 88 times 88 plus let's call this 7 so 0.07 times 87 plus 10 0.1 times 86 plus unless it should add up to 100 percent this is 89 and then this gets us to 99. so then another 1 percent 0.01 times 84. and so if we were to do this calculation this is our estimate of the average atomic mass of this element we could type this into a calculator and get some number and then look that up on a periodic table of elements or we could just try to estimate it we can see that it's going to be close to 88 because that's where the highest percentage is we have the when we're taking the weighted average we have the highest weight right over there but these other isotopes these other versions of the element that have a different number of neutrons which changes its atomic mass they're going to bring the average down so our average atomic mass is going to be a little bit less than 88 so let's look up a periodic table of elements what element here has an atomic mass a little bit less than 88 well yitrium is 88.91 but we know it can't be that because none of the isotopes have an atomic mass above 88 so we can rule out yitrium strontium is looking pretty good it's exactly what we predicted a little bit less than 88 and rubidium is a lot less than 88 so our even if we did the calculation we could feel confident we're not going to be as low as rubidium so i'm feeling very confident just eyeballing it just estimating this is going to be a little bit have an average atomic mass a little bit less than 88 which tells me that this is strontium