1. Of the Isotopes of an Element
Percent Composition
Of the Isotopes of an Element

2. Percent Composition At the conclusion of our time together, you should be able to:
Determine the percent composition of 2 isotopes of an element if the average atomic mass and the mass of each isotope is given.

3. Remember How to Determine Average Atomic Mass
Determine Relative Abundance –
% abundance divided by 100
Determine mass of each isotope and multiply relative abundance by this mass
(keep all digits your calculator gives you)
Determine Average Atomic Mass by adding up all the individual masses, round to “2” decimal places

4.
(0.98 x 1 amu)
+ (0.01 x 2 amu)
+ (0.01 x 3 amu)
1.03 amu

5. But What If…
We know the average atomic mass and the mass of each isotope but not the percentage of each???

6. How to Determine Percent Abundance
Use the following equation:
Ave. Atomic Mass = (X)(Mass of 1st isotope)
+(1-X)(Mass of 2nd isotope)
Solve for “X”
Answer will be a decimal
Convert the answer to a percentage
Subtract this answer from 100 % for the other answer.
The sum of the two answers should equal 100 %

7. % Abundance Practice
2) Lithium exists as 2 isotopes, Li-6 and Li-7. Calculate the % abundance for each isotope of lithium.

8. Answer
6.941 = (x)(6) + (1-x)(7) = 6x + 7 – 7x = -1x + 7 x = .059 Li-6 is 5.9% abundant Li-7 is 94.1% abundant

9. When I Went for my Physical this Summer…
My Doc asked about my physical activity the day before…
I told him that I waded around a lake, climbed a mountain, ran from a snake, and got poison ivy.
Inspired by my story, my doc said, "You must be an awesome outdoors man!“
"No," I replied, "I'm just a terrible golfer."
When I Went for my Physical this Summer…

10. % Abundance Practice Boron, B, exists as 2 isotopes, B-10
and B-11. Calculate the % abundance for each isotope of Boron.

11. Answer
= (x)(10) + (1-x)(11) = 10 x + 11 – 11 x = -1 x + 11 x = B-10 is 18.9 % abundant B-11 is 81.1 % abundant

12. Percent Composition Let’s see if you can:
Determine the percent composition of 2 isotopes of an element if the average atomic mass and the mass of each isotope is given.

13. When the teen asked her parents what they were doing they replied…”None of your
Bismuth” - Bi

14. Copper consists of two naturally occurring isotopes with masses of 63 and 65 amu. The average atomic mass of Cu is amu. Calculate the abundance of each isotope.
63Cu = 40.3% 65Cu = 59.7%
63Cu = 69.7% 65Cu = 30.3%
63Cu = 29.7% 65Cu = 70.3%
63Cu = 31.3% 65Cu = 68.7%
63Cu = 72.7% 65Cu = 27.3% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22