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Average Atomic Mass/ Relative Abundance
1. Boron has two naturally occurring isotopes, boron-10 and boron-11. The actual masses of these isotopes are 10.81 amu and 11.01amu respectively. The relative abundance is 19.8% for B-10 and the remaining percentage belongs to B-11. Calculate the average atomic mass for Boron.
100 – 19.8 = 80.2 %
Average atomic mass = (10.81 x 0.198) + (11.01 x 0.802)
Average atomic mass = 10.97 ( 11.0 amu
2. Calculate the percent abundances of Cu-63 and Cu-65 if the exact masses of the isotopes are 62.93 amu and 64.93 amu, respectively.
63.55 = (62.93 x X) + (64.93 x (1-X)
63.55 = 62.93X + 64.93 – 64.93X
-1.38 = -2X
X = .69
Cu-63 = 69.00%
Cu-65 = 31.00%
3. Bromine, like chlorine has two stable isotopes. For bromine the isotopic masses are 78.9183 amu, with relative abundances of 50.69% and 80.9163 amu with a relative abundance of 49.31%. Calculate the average atomic mass of bromine.
Average atomic mass = (78.9183 x 0.5069) + (80.9163 x 0.4931)
Average atomic mass = 79.90 amu
4. What are the two regions that make up all atoms?
• Nucleus and the electron cloud
5. What are isotopes?
• Elements that have the same protons but different number of neutrons.
6. What would happen if the number of protons were to change in an atom?
• It would be a different element.
7. Thallium has two stable isotopes, 203Tl and 205Tl. Knowing that the atomic mass of thallium is 204.4 amu, which isotope is the more abundant of the two?
204.4 = (203 x X) + (205 x (1-X))
204.4 = 203X + 205 -205X
-0.6 = 2X
X = .3
Tl-203 = 30.0%
Tl-205 = 70.0% - most abundant
|A |X |
|Z | |
8. Write the following isotopes using the notation:
(a) has 59 protons and 82 neutrons:
|141 |Pr |
|59 | |
(b) has a mass number of 172 and 102 neutrons:
|172 |Yb |
|70 | |
(c) has 2 protons and 2 neutrons:
|4 |He |
|2 | |
Complete the following chart:
|Element |Atomic Number |Mass |Protons |Electrons |Neutrons |
| | |Number | | | |
|9. Se |34 |79 |34 |34 |45 |
|10. Chromium |24 |53 |24 |24 |29 |
|11. Cu-65 |29 |65 |29 |29 |36 |
|12. Helium |2 |4 |2 |2 |2 |
|13. H-3 |1 |3 |1 |1 |2 |
|14. 14C |6 |14 |6 |6 |8 |
|6 | | | | | |
|15. 13C |6 |13 |6 |6 |7 |
|6 | | | | | |
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Came from periodic table
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