1. Average
Atomic Mass

2. 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.

3. Isotopes
Atoms of the same element that have different # of Neutrons are said to be ISOTOPES of that element
All isotopes of an element are chemically the same, i.e., they react exactly the same

4. Isotopes All elements found in nature are MIXTURES of isotopes
Isotopes of an element are designated by mass number in either of two ways
1: U
2: U-235 or Uranium-235
Using the above how would we designate the isotope of carbon that has 8 neutrons?
146C or C-14 (Carbon-14)

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?
= Divide by 2 = 75.
Would you be happy?

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

8. The smart way to do this problem!
 out of 15 is 93.3%
 out of 15 is 6.7%
So … 93.3% of the test grades are 90, and
… 6.7% of the test grades are 60.
Weighted Average = [.933 x 90] + [.067 x 60] =
= 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
Convert % to decimal format. (Divide by 100%.)
Multiply each isotope’s abundance factor by its atomic mass.
Sum.

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

12. Avg. Atomic Mass of Chlorine =
Convert percents to decimals.
(.75770)( amu) +
(.24230)( amu)
= amu amu
= amu
Multiply by appropriate mass.
Sum

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

14. Avg. Atomic Mass of Si 92.21% of Si has a mass of 27.97693

15. Avg. Atomic Mass of Si
.9221 X 
.0470 X 
X 

16. Check your work! Answer is 28.0856.
Answer has to be between high & low.
Answer has to be closest to
Answer is

17. Avg. Atomic Mass of Pb 1.5% Pb-204.  23.6% Pb-206.  22.6% Pb-207. 

18. Avg. Atomic Mass of Pb 1.5% Pb-204.  .015 X 204 : 3.06
207.24