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

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

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

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?

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!

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%

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

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

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

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

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

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

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

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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