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Published byBeverly Harper Modified over 6 years ago

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

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Average Atomic Mass

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The atomic masses reported in the periodic table represent the weighted average of the masses of the naturally occurring isotopes of that element

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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?

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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!

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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%

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

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

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Avg. Atomic Mass of Chlorine 36.966 amu24.230% Isotope 2 34.969 amu75.770% Isotope 1 Mass Percent Abundance Chlorine has two isotopes.

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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

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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

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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

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Avg. Atomic Mass of Si 0.9221 X 27.97693 25.7975 amu 0.9221 X 27.97693 25.7975 amu 0.0470 X 28.97649 1.3619 amu 0.0470 X 28.97649 1.3619 amu + 0.0309 X 29.97376 0.9262 amu + 0.0309 X 29.97376 0.9262 amu 28.0856 amu 28.0856 amu

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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 Sum 207.24amu

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