hi this is Mrs Hollick and this is your podcast on average atomic mass the atomic mass on the periodic table is actually a weighted average of all of the Isotopes for that element so if we take a look at Carbon for example carbon's atomic mass on the periodic table says 12.011 what it really is is a weighted average of carbon's main Isotopes carbon has actually two main isotopes that contribute to the mass but we all know of carbon 14 from Carbon 14 dating so I also wanted to note that but if you look the carbon-12 isotope has a mass of 12 98.93 percent of it has a mass 12. the rest of carbon is carbon 13. that mass is 13.00335 only one percent of the actual average atomic mass of carbon is contributed by carbon 13. and then you can see there are Trace Amounts of carbon 14 contributing to carbons and average atomic mass but it is again just Trace Amounts because you can see this would add up to 100 percent so on the periodic table you'll see carbon's atomic mass its average atomic mass as 12.011 and that's due to the fact that we have weighted averages from these amounts and you'll see that the average is closest to the most abundant isotope most abundant isotope was 12 and you can see that that average atomic mass is close to 12. the symbol we use for the average atomic mass when you take it off of the periodic table is either Amu U or grams per mole for this podcast and for this practice we're going to be using either AMU or you just because it's easier you want to write this down in your notes in practice the first example it says magnesium has three isotopes 78.7 percent is magnesium 24 11.3 percent is magnesium 26 and 10.0 percent is magnesium 25. what's the average atomic mass well we have to figure out how much each of these Isotopes contributes to the total average atomic mass and to do that it's very simple you want to set up a chart and in that chart we're going to put first the percentages so 78 .7 percent that's the first percentage that we see has a mass of 24. so that's what we're going to draw our line down and the next thing we'll write is that Mass 78.7 percent is has a mass of 24 is magnesium 24. then the next one says 11.3 percent is magnesium 26. so 11 .3 percent is magnesium 26 and then the last it says 10. 0 percent is magnesium 25. so now in order to determine how much of that 24 goes into the total average atomic mass we have to figure out what 78.7 percent of that is so the first thing you want to do is change your percent to a decimal and the way that we do that remember from math class is we divide by a hundred so if 78.7 is our percent when we divide by 100 you're really just moving your decimal point to the left two places so 78.7 percent is really 0.787 and then to figure out 78.7 percent of 24 we're going to multiply that so you're going to change a percent to a decimal and multiply it by the contributing mass and when you do that you're going to get 18.8 8 and we'll do the same for the rest so change your percent to a decimal we move the decimal point back two places so that decimal would be one point one one three times the mass of 26 and that contribution would be two point nine three eight so that's what 11.3 percent of 26 is it's 2.938 that's what you're figuring out here we've done this in math class before change 10 into a decimal we've moved the decimal point back two places so that would be point one zero zero times 25 and that would equal 2.500 once you get that now this is the mass that each isotope contributes you're going to add these up when you add these up you're going to get 24 point three two six and then the symbol that's the average atomic mass we'll use U I use U it's just shorter than AMU it's just a shortcut so this is the average atomic mass for magnesium and if you actually look on your periodic table you will see that that is very close to what's on there okay let's look at the next example it says element X has three isotopes 94.2 percent is X 114 4.8 percent is x113 and one per percent is X 112. what's the average atomic mass so again set up a chart for yourself and we're going to get the information right from the problem it says 94.2 percent is X 114 so the mass is 114. so there's our chart and then it says 4.8 percent is x113 so that's our mass is 113 and 1.0 percent is 112. so now we can do our calculations you want to change a percent to a decimal so move it back two places we divide by a hundred so that would be 0.942 times 114 so 94.2 percent of 114 is 107.388 the next one changing 4.8 percent to a decimal we have to go back two places so just be careful notice it's going to be point zero four eight and then we multiply that by the mass of 113. and that will be five point four two four and then the last one change one percent to a decimal it'll be point zero one zero because we move the decimal two places back one two we have to put that zero in there times the mass of 112. and so then that comes out to 1.12 now again this is how much each isotope contributes to the total mass it's a weighted average so we add those up and you will get 113.932 and then our symbol or our unit is a u so this is the easiest way to calculate average atomic mass there are shortcuts you don't necessarily have to write this chart if you don't want to I can just quickly show you a shortcut here what you could do is you would just read your problem and you go okay point not if you can do this in your head change the percent to a decimal multiply it by the mass plus then the next one is 4.8 percent so it'll be .048 times 113 plus the last one is one percent so it's .010 times that mass which is 112 and then equals and then it would just you would do that in your calculator and it would equal the 113.932 whichever method is easier for you to do that you understand you can certainly use I would like to see some sort of work in your practice so you must either set up the chart or you can use this method like this when you're actually doing your practice you need to show your work in your practice if you have any questions please write them down and you can ask your teacher when you get to class tomorrow thanks for your attention and have a great day