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Number of Protons = Number of Electrons Number of Protons = Atomic Number

Number of Protons = Number of Electrons Number of Protons = Atomic Number Atomic Mass = Protons + Neutrons. Average Atomic Mass. Average Atomic Mass.

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Number of Protons = Number of Electrons Number of Protons = Atomic Number

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  1. Number of Protons = Number of Electrons Number of Protons = Atomic Number Atomic Mass = Protons + Neutrons

  2. Average Atomic Mass

  3. 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

  4. Weighted Avg. of Test Grades • Suppose you 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?

  5. NO! • The long way to do this problem: • (90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 0)  10 • Average = 81% • I bet you like this a lot better!

  6. The smart way to do this problem! • grades of 90: 9 out of 10 is 90% • grade 0f 0: 1 out of 10 is 10% • So 90% of the test grades are 90 • 10% of the test grades are 0 • (0.90 X 90) + (0.10 X 0) = 81 + 0 • Average = 81%

  7. Weighted average:Smarter way to calculate the average if you have numerous items to average

  8. Method of Weighted Averages Convert % to decimal format (Divide % by 100) Multiply each isotope’s abundance factor by its atomic mass Find the Sum

  9. Avg. Atomic Mass of Chlorine Chlorine has two isotopes. Percent Abundance Mass Isotope 1 75.770% 34.969 amu Isotope 2 24.230% 36.966 amu

  10. Avg. Atomic Mass of Chlorine = (.75770)(34.969 amu) + (.24230) (36.966 amu) = 26.496 amu + 8.957 amu = 35.453 amu Sum Convert % to decimals Multiply by appropriate mass

  11. Avg. Atomic Mass of Chlorine To estimate the answer: 75% of Cl is 34.969 amu & 25% of Cl is 36.966 amu The final answer has to be between 34.969 amu & 36.966 amu, but closer to 34.969 amu • 35.453 amu

  12. Avg. Atomic Mass of Si 92.21% of Si has a mass of 27.97693 amu 4.70% of Si has a mass of 28.97649 amu 3.09% of Si has a mass of 29.97376 amu

  13. Avg. Atomic Mass of Si 0.9221 X27.97693  25.7975 amu 0.0470 X 28.97649  1.3619 amu + 0.0309 X 29.97376 0.9262 amu 28.0856 amu

  14. Sum 207.24amu Avg. Atomic Mass of Pb 1.5% Pb-204  0.015 X 204amu: 3.06amu 23.6% Pb-206  0.236 X 206amu: 48.62amu 22.6% Pb-207  0.226 X 207amu: 46.78amu 52.3% Pb-208  0.523 X 208amu:108.78amu

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