Name: Per: Average Atomic Mass How are the masses on the periodic table determined?

Why? 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 ac-tivity will help answer that question.

Model 1—A strip of Magnesium Metal 1) Write in the atomic number for each Mg atom in Model 1:

2) What are the mass numbers of the naturally occurring isotopes of Mg

show in Model 1?

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

4) For the sample of 20 atoms of Mg shown in Model 1, fill in the table of the mass numbers of the three isotopes and the number of atoms of each isotope present.

5) Which isotope of Mg is the most common in Model 1?

6) Based on Model 1 and the table in Question #4, for every 10 atoms of Mg, approximately how many atoms of each isotope will be found? Fill in the new table.

24Mg 25Mg 26Mg

# of atoms

24Mg 25Mg 26Mg

# of atoms

Model 2—Natural Abundance Information for Magnesium

7) Consider the natural abundance information in Model 2. a) Calculate the expected number of atoms of each isotope that will be found in a sample of 20 atoms of Mg. Hint: # of atoms has to be a whole number!

b) Is Model 1 an accurate representation of Mg at the atomic level? Explain.

8) If you could pick up a single atom of Mg and put it on a balance, the mass of that atom would most likely be amu. Explain why.

9) Refer to your periodic table and find Magnesium. a) Write down the decimal number shown in the Mg box: . b) Does the decimal number shown on the table for Mg match any of the atomic masses listed in Model 2? Yes No

10) Why doesn’t the periodic table show the atomic mass of every isotope of an element?

11) What would be a practical way of showing the mass of Mg atoms on the periodic table given that most elements occur as a mixture of isotopes?

12) Propose a possible way to calculate the average atomic mass of 100 magnesium atoms. Your answer may include a mathematical equation, but it is not required.

Isotope Natural Abundance on Earth (%) Atomic Mass (amu)

24Mg 78.99 23.9850

25Mg 10.00 24.9858

26Mg 11.01 25.9826

Model 3—Proposed Average Atomic Mass Calculations

13) Complete the three proposed calculations for the average atomic mass in Model 3 using a calculator. Round to 2 decimal points.

14) Consider the calculations in Model 3. a) Which methods shown in Model 3 give an answer for the average atomic mass that matches the mass of Mg on the periodic table? b) Explain why the mathematical reasoning was incorrect for any method(s) in Model 3 that did not give the correct answer for the average atomic mass from the periodic table. c) For the method(s) in Model 3 that gave the correct answer for average atomic mass, show that they are mathematically equivalent methods.

15) Use one of the methods in Model 3 that gave the correct answer to calculate the average atomic mass for oxygen with the information given in the table. Show all your work and check your answer with the mass from the periodic table.

Read This! Recall that all isotopes of an element have the same physical and chemical properties, with the exception of atomic mass (and for unstable isotopes, radioactivity). Therefore, the periodic table lists a weighted average atomic mass for each element. In order to calculate this quantity, the natural abundance and atomic mass of each isotope must be provided.

16) Consider the individual atomic masses for Mg in Model 2.

a) Which isotope has an atomic mass closer to the average atomic mass from the periodic table? b) Give a mathematical reason for your answer to part a.

17) Boron has two naturally occurring isotopes: boron-10 and boron-11. Which isotope is more abundance on Earth? Explain your choice.

Answer the following (#1-5) on a separate paper. Show all work!

Isotope Abundance (%) Atomic Mass (amu)

16O 99.76 15.9949

17O 0.04 16.9991

18O 0.20 17.9992

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