Average Atomic Mass. 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.

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Presentation transcript:

Average Atomic Mass

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

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.

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

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 60% and 14 tests of 90%. What if I said your avg. was 75?   = 150. Divide by 2 = 75.   Would you be happy?

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

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 X 60 =   Avg. = 88

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

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

Avg. Atomic Mass of Chlorine Percent Abundance Mass Isotope %35 amu Isotope %37 amu Chlorine has two isotopes.

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

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

Avg. Atomic Mass of Si 92.21% of Si has a mass of % of Si has a mass of % of Si has a mass of 30

Avg. Atomic Mass of Si.9221 X 28  X 28  X 29  X 29  X 30  X 30 

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

Avg. Atomic Mass of Pb 1.5% Pb-204. .015 X 204 : % Pb-206. .236 X 206 : % Pb-207. .226 X 207 : % Pb-208. .523 X 208 :