Average Atomic Masses - .::* Ms. Roman's Chemistry Page



Name: _______________________________ Date: _____________

Aim: Can you explain why atomic mass is not an integer?

Do Now: Use the diagrams above to answer the following:

1. What element is represented by "protium"? _____________________

2. What is the difference between protium, deuterium, and tritium?

_____________________________________________________________

3. What term can be used to describe these three atoms? _______________

Isotope:

_______________________________________________

_______________________________________________

Example:

Chlorine has two isotopes, which have an average

atomic mass of 35.453 amu: ________ & _________

Average atomic mass is calculated by taking the weighted average of the naturally occurring isotopes of an element based on percent abundance.

With Your Partner…

How are the masses on the periodic table determined?

Most elements have more than one naturally-occurring isotope. As you learned previously, the atoms of those isotopes have the same atomic number (number of protons), making them belong to the same element, but they have different mass numbers (total number of protons and neutrons) giving them different atomic masses. So which mass is put on the periodic table for each element? Is it the most common isotope’s mass? The heaviest mass? This activity will help answer that question.

1. What is the atomic number for each Mg atom in Model 1. ________

2. What are the mass numbers of the naturally occurring isotopes of magnesium shown in Model 1? _______, _______, & _______

3. Do all of the atoms of magnesium in Model 1 have the same atomic mass? Explain.

_______________________________________________________________

4. For the twenty atoms of magnesium shown in Model 1, fill in the table below, indicating the mass numbers and the number of each isotope present.

|Isotope |# of Atoms Present |% Abundance |

|Mg-24 | | |

|Mg-25 | | |

|Mg-26 | | |

5. Which isotope of Magnesium is the most common in Model 1? _______

6. Based on Model 1, and the table you created in Question 4, for every 10 atoms of magnesium, approximately how many atoms of each isotope will be found?

Mg-24: __________ Mg-25: __________ Mg-26: __________

7. Complete all calculations in Model #2. Which of the following methods of calculating Average Atomic Mass is correct?

Model #2

8. Explain why the mathematical reasoning was incorrect for any method(s) in Model 2 above that did not give the correct answer for average atomic mass (the one on the periodic table).

_______________________________________________________________

_______________________________________________________________

_______________________________________________________________

Practice Regents Questions

1. There are two known isotopes for copper. Cu-63 has a percent abundance of 69.17% and Cu-65 has a percent abundance of 30.83%. Calculate the average atomic mass for copper.

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2. Element X has two isotopes. 72.0% of element has an isotopic mass of 84.9 amu and 28% has an isotopic mass of 87.0 amu. Determine the average atomic mass of element X.

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3. The atomic masses and the natural abundances of the two naturally occurring isotopes of lithium are shown in the table below.

Calculate the atomic mass of lithium in the space below. Show all work in the space below, including mathematical setup, and answer with proper units.

4. The table below gives the atomic mass and the abundance of the two naturally occurring isotopes of chlorine.

Calculate the atomic mass of chlorine in the space below. Show all work in the space below, including mathematical setup, and answer with proper units.

5. The atomic mass of titanium is 47.88 atomic mass units. This atomic mass represents the

a) total mass of all the protons and neutrons in an atom of Ti

b) total mass of all the protons, neutrons, and electrons in an atom of Ti

c) weighted average mass of the most abundant isotope of Ti

d) weighted average mass of all the naturally occurring isotopes of Ti

6. The atomic mass of element A is 63.6 atomic mass units. The only naturally occurring isotopes of element A are A-63 and A-65. The percent abundances in a naturally occurring sample of element A are closest to

a) 31% A-63 and 69% A-65

b) 50% A-63 and 50% A-65

c) 69% A-63 and 31% A-65

d) 100% A-63 and 0% A-65

7. The atomic mass of an element is the weighted average of the

a) number of protons in the isotopes of that element

b) number of neutrons in the isotopes of that element

c) atomic numbers of the naturally occurring isotopes of that element

d) atomic masses of the naturally occurring isotopes of that element

8. Hydrogen has three isotopes with mass numbers of 1, 2, and 3 and has an average atomic mass of 1.00794 amu. This information indicates that

a) equal numbers of each isotope are present

b) number of neutrons in the isotopes of that element

c) more isotopes have an atomic mass of 2 or 3 than of 1

d) isotopes have only an atomic mass of 1

9. A sample of element X contains 90. percent 35X atoms, 8.0 percent 37X atoms, and 2.0 percent38X atoms. The average isotopic mass is closest to

a) equal numbers of each isotope are present

b) number of neutrons in the isotopes of that element

c) more isotopes have an atomic mass of 2 or 3 than of 1

d) isotopes have only an atomic mass of 1

Average Atomic Mass

Steps for Calculating Average Atomic Mass

(When given percentages of each isotope and each isotopes mass)

1. Identify the correct abundance and percent for each isotope.

2. Multiply the percentage of each isotope by its mass abundance and divide by 100.

3. Add the numbers from step two together.

Try it:

|Calculate the average atomic mass for Si |

|Isotope |Atomic mass |Percent Abundance |

|Cl-35 |34.969 amu |75.53 % |

|Cl-37 |36.966 amu |24.47 % |

Step 1:

35Cl: _____________% and ______________ amu

37Cl: _____________% and ______________amu

Step 2:

[(___________) (___________)] + [(___________) (___________)]

100 100

Step 3:

_________________ amu + _________________ amu =

Practice Problems:

9. Determine the average atomic mass of the following mixtures of isotopes:

a. 80% 127I, 17% 126I, 3% 128I

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b. 15% 55Fe, 85% 56Fe

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10. There are two known isotopes for copper. Cu-63 has a percent abundance of 69.17% and Cu-65 has a percent abundance of 30.83%. Calculate the average atomic mass for copper.

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11. Element X has two isotopes. 72.0% of element has an isotopic mass of 84.9 amu and 28% has an isotopic mass of 87.0 amu. Determine the average atomic mass of element X.

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12. Which symbol represents atoms that are isotopes of each other?

a. 14C and 14N

b. 16O and 18O

c. 131I and 131I

d. 222Rn and 222Ra

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