1. Average Atomic Mass

2. Average Atomic Mass IS on the Periodic Table – it’s the number (usually) with decimals

3. What are the units? Amu – atomic mass unit
Approximately the same as the mass of one proton (or one neutron, since they’re the same size)
A weighted average of all the isotopes found in nature

4. What’s a WEIGHTED average?
It is an average that is weighted by the percent of each isotope present in nature.

5. Huh?
Think about a middle school basketball team. There are 5 boys on the team. Their weights are below:
ALEX – 100 pounds
BILL – 100 pounds
CARL – 100 pounds
DAVID – 100 pounds
EDWIN – 150 pounds What is the average weight of the boys on the team?

6. That’s one way to solve the problem.
= pounds 5
That’s one way to solve the problem.

7. Another way to look at it…
4 boys weigh 100 pounds. 4 = 80%
1 boy weighs 150 pounds 1 = 20%

8. So… 80% = 0.80 20% = 0.20 0.80 X 100 pounds = 80 pounds + 110 pounds
80% = % = 0.20
0.80 X 100 pounds = 80 pounds +
0.20 x 150 pounds = 30 pounds
110 pounds
Same answer we got when we added the 100 and and / 5

9. Remember: the various isotopes occur in different percentages
For Example:
Rubidium has two common isotopes, Rb - 85 and Rb - 87.
If the abundance of Rb-85 is 72.2% and the abundance of Rb -87 is 27.8%, what is the average atomic mass of rubidium that would be found on the Periodic Table?

10. Solution
0.722 x 85 = x 87 = amu

11. Try this one Chlorine – 35 = 75% Chlorine – 37 = 25%
What is the average atomic mass of Chlorine that would be found on the Periodic Table?

12. Solution X 0.75 = 26.25 amu + x 0. 25 = 9.25 amu 35.50 amu
Notice: 75% is Chlorine -35 so the weighted average is closer to 35 amu than 37.

13. Inferring the average based on %
Two isotopes of an element exist:
X – 39 = 89% and
X – 40 = 11%
Which would the weighted average be closer to, 39 or 40?

14. Inferring the most abundant isotope…
It is possible to look at an average and take a good inference at which isotope is the most abundant in nature…
Argon – weighted average on PT is
Carbon – weighted average on PT is
Helium – weighted average is 4.002

15. More inferences
Which isotope of Argon is most common if average is , Argon -38 or Argon – 40?
Which isotope of Carbon is most abundant (average is ), Carbon- 12 or Carbon – 13?
Which isotope of Helium is most abundant (average is 4.002), Helium- 3 or Helium – 4?

16. Look at the Periodic Table
What is the most abundant isotope of Rhodium, atomic number 45?
What is the most abundant isotope of Yttrium, atomic number 39?
What is the most abundant isotope of Sodium, atomic number 11?

17. Quick Review What is the unit of atomic mass?
Oxygen has two numbers on the periodic table, 8 and Which one is the atomic mass? Which one is the atomic number?
If two isotopes of iodine are I-126 and I The atomic mass on the Periodic Table is Which isotope is most abundant?_____________