hi everybody and thank you for watching this unit to video where I'll show you how to calculate the average atomic mass of an element from percent abundance data now before we do this for real and take a look at the calculation let's just remember some things that we've looked at recently in class first of all the idea that not all atoms of a given element are the same let's take carbon for example carbon comes in three naturally occurring types carbon with a mass of 12 carbon with a mass of 13 and carbon with a mass of 14 every single one of these carbon atoms has 6 protons protons are what make these atoms the element carbon if they didn't have six protons they wouldn't be carbon what gives them the different masses is the fact that this carbon has different as six neutrons this one has 7 and this one has eight we of course call these isotopes now when you look at carbon on the periodic table you'll see two numbers one of them is six that's of course the atomic number it's a nice whole number six because carbon is always six that never changes the confusing part on the periodic table is that the mass is given as a decimal 12.01 one now it's not possible to have 0 point 0 1 1 of a proton or of a neutron so sometimes it's confusing where this decimal comes from the reason that decimals there is because periodic tables give you the average mass of all carbon atoms so like we just saw carbon can have a mass of 12 13 or 14 so on the periodic table the decimal you see is the average mass of all those different masses combined and that's what I'm going to show you how to calculate in this video now before I show you how to do the actual calculation part let's take a look at one more piece of information you need to be familiar with and that's this idea of percent abundances a percent abundance simply shows how calm or abundant each isotope is carbon-12 for example on the left has a percent abundance of 98.9 percent that means of all the naturally occurring carbon atoms 98.9 percent of those carbons are carbon with a mass of 12 carbon 13 s is only one point one percent in carbon 14 s is extremely small less than point zero zero one percent which you can say from these numbers is that carbon 14 is the least abundant isotope out of the three because very little of all the carbon atoms exist as this type whereas carbon 12 is the most common or most abundant because almost one hundred percent of all the carbon atoms you'll find our carbon 12 now we can actually take a look at how to do the actual calculation of an average atomic mass one more quick thing though before we do that you're sitting there saying I already know how to take an average you just add up all three masses 12 plus 13 plus 14 and divide by 3 you'd be wrong because we have to do a special type of average here since each isotope has its own unique percent abundance we have to use a special type of average called a weighted average that takes into account the fact that most of the carbon atoms are carbon-12 and very few of them are carbon 13 and 14 so here's how you do a weighted average i'm going to show you two examples the first one I'll do nice and slow so you can follow along the second one we'll do a little bit faster the problem is going to give you information that pretty much mirrors this every time this is about copper copper has two common isotopes copper 63 and copper 65 copper 63 has a percent abundance of 69.0 nine percent while copper 65 has a percent abundance of 30.9 one percent and the question is very simple calculate the average atomic mass so the first thing you do is take all the different masses of all the different isotopes you have and I just like the right amount in a vertical line in this case it's nice and easy we only have two isotopes 63 and 65 so you just list them like that you're going to take each of those masses multiply them by the percentage the corresponding percentage before you do that you're going to move the decimal place in the percent two places to the left so since copper 63 has a percent abundance of sixty-nine percent I'm going to multiply 63 by that percentage above the decimal to the left so point six 90 and 9 and you'll do the same thing with a 65 multiply it by 30.9 one percent just with that decimal place move two to the left so point 30 9 and 1 when you're done with that you multiply through and get your answers so when you multiply these as the numbers you get forty three point five to seven and twenty point 092 now there's one final step that's the benefit of writing these in the lines the way I've shown you you take the answers from the multiplication draw a big line beneath them and get the final average by adding all of those numbers together and in this example after you add those two numbers you get 63.6 to that is the final answer it is the average atomic mass of copper now i'm going to show you just one more example it's a little bit more complicated because in this example there's three different isotopes magnesium 24 25 and 26 each with their own percent abundance a good idea is to pause the video now and try this on your own and then check your answer as we go over it so you can see how you did after just looking at how to do this one time now that you've had a chance to try this let's check your answers with the proper answers first thing you should do is line up the masses 24 25 and 26 multiply each mass by the percentage but with a decimal place moved two places to the left get those answers those are the three numbers you should get final step is add those three numbers together for a final average atomic mass of 20 4.46 that actually concludes this video on calculating average of tom masses please enjoy the recap to follow