1. Average Atomic Mass

2.   atomic masses reported in periodic table represent: weighted average of masses of all naturally occurring isotopes of an element

3. Weighted Average:Test Grades   suppose have: 1 test of 0% and 9 tests of 90% What if I said your average was 45%? (0 + 90 = 90; divide by 2 = 45%)   would you be happy?

4. NO!   the long way to do this problem: (90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 0) then  10   average = 81% I bet you like this a lot better!

5. better way to solve this average!   grades of 90: 9 out of 10 is 90%   grades of 0: 1 out of 10 is 10% 90% test grades are 90 & 10% test grades are 0   so calc average like this… (90%)(90) + (10%)(0) = (0.90 X 90) + (0.10 X 0) = 81 + 0 average = 81%

6. weighted average: better way to calculate average when have numerous items of differing value

7. Method of Weighted Averages of Isotopes of Atoms ● ● convert % to decimal format (divide by 100) ● ● multiply each isotope’s abundance factor by its atomic mass ● ● find the sum

8. chlorine’s average atomic mass 36.966 amu24.230% Isotope 2 34.969 amu75.770% Isotope 1 Mass Percent Abundance chlorine has two isotopes

9. Avg. Atomic Mass of Chlorine = (.75770)(34.969 amu) + (.24230) (36.966 amu) (.24230) (36.966 amu) = 26.496 amu + 8.957 amu = 35.453 amu convert % to decimals multiply by appropriate mass sum

10. Avg. Atomic Mass of Si 92.21% of Si has mass of 27.97693 amu 4.70% of Si has mass of 28.97649 amu 3.09% of Si has mass of 29.97376 amu (0.9221)(27.97693) + (0.0470)(28.97649) + (0.0309)(29.97376) = 28.0856 amu