1. Average Atomic Mass

2. How much does an atom weigh?  Protons & Neutrons:  1.67 X 10 -24 gram  Electrons:  9.10 X 10 -28 gram  To avoid working with these really small numbers, scientists have developed the atomic mass scale.

3. What is an a.m.u.?  atomic mass unit  amu  1/12 the mass of the C-12 atom.  C-12 is used as the reference for atomic masses. This is the STANDARD.

4. Atomic Weight or Mass  Atomic mass is relative.  C-12 is the standard. 1 atom of C-12 has a mass of exactly 12.000… atomic mass units.  So all the atoms are compared to C-12.

5. Average Atomic Mass   The atomic masses reported in the periodic table represent the weighted average of the masses of the naturally occurring isotopes of that element.

6. Weighted Avg. of Test Grades   Suppose you have 1 test of 60% and 14 tests of 90%. What if I said your avg. was 75?   60 + 90 = 150. Divide by 2 = 75.   Would you be happy?

7. NO!   The long way to do this problem:   (90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 60)  15   Avg. = 88. You like this a lot better!

8. The smart way to do this problem!   14 out of 15 is 93.3%   1 out of 15 is 6.7%   So 93.3% of the test grades are 90.   6.7% of the test grades are 60.  .933 X 90 +.067 X 60 = 83.97 + 4.02   Avg. = 88

9. This is an even smarter way to calculate the average if you have lots of items to average.

10. Method of Weighted Averages 1. 1. Convert % to decimal format. (Divide by 100%.) 2. 2. Multiply each isotope’s abundance factor by its atomic mass. 3. 3. Sum.

11. Avg. Atomic Mass of Chlorine Percent Abundance Mass Isotope 1 75.0%35 amu Isotope 2 25.0%37 amu Chlorine has two isotopes.

12. Avg. Atomic Mass of Chlorine = (.75) (35 amu) + (.25) (37 amu) (.25) (37 amu)  = 26.25 amu + 9.25 amu  = 35.5 amu Convert percents to decimals. Multiply by appropriate mass. Sum

13. Avg. Atomic Mass of Chlorine   To estimate the answer:   75% Cl is 35 and 25% Cl is 37.   The final answer has to be between 35 and 37, but closer to 35.  35.5 amu

14. Avg. Atomic Mass of Si 92.21% of Si has a mass of 28 4.70% of Si has a mass of 29 3.09% of Si has a mass of 30

15. Avg. Atomic Mass of Si.9221 X 28  25.8188.9221 X 28  25.8188.0470 X 29  1.363.0470 X 29  1.363 +.0309 X 30  0.927 +.0309 X 30  0.927 28.1088 28.1088

16. Check your work!  Answer has to be between high & low.  Answer has to be closest to 28.  Answer is 28.11.

17. Avg. Atomic Mass of Pb 1.5% Pb-204. .015 X 204 : 3.06 23.6% Pb-206. .236 X 206 : 48.62 22.6% Pb-207. .226 X 207 : 46.78 52.3% Pb-208. .523 X 208 : 108.78 207.24