I. Title:
Isotopes in a Bag
Background Information: Isotopes are atoms of the same element that are nearly identical. Isotopes
have different numbers of neutrons and thus, different mass numbers. When chemists refer to the
atomic mass (or atomic weight) of one of these elements, they are really referring to the average of
the mass numbers of the isotopes of that elements. In this lab you will use small zip-top bags to
represent isotopes of atoms of a single element. The red and black beads represent the protons and
neutrons in the nucleus of the atom. Each lab group will get 15 “isotopes” (bags with beads) of an
element and each group’s element will be different.
II.
Purpose: To investigate isotopes and discover how they contribute to the average atomic mass of an
element.
III.
Pre-Lab Questions:
1.
Define isotopes.
2.
How are isotopes distinguished from each other?
3.
Analyze your bags of isotopes. Sort them by number of red and black beads. What do the red
beads represent? What do the black beads represent?
4.
If your “isotopes” represented an element on the periodic table, what element would they
represent? Explain how you know.
5.
How would the isotope Carbon with a mass of 13 be named?
6.
What would the isotopic notation for Chromium-53 be?
7.
There are three isotopes of Potassium: K-39, K-40, and K-41. Explain why the average mass given
on the periodic table is 39.10, which is much closer to 39 than 40 or 41.
IV.
Safety: Don’t open the bags and/or lose any of the beads in the bags!
Procedure
1. Get a basket of isotopes.
2. Sort the isotopes by number of neutrons. You should have 3 separate groups of isotopes of your
element. Record the number of neutrons in each group in the column 1 of the data table.
3. Count and record the number of isotopes in each group. Add all isotopes together to get the total
number of isotopes. Record in column 2 of the data table.
4. Find the total mass of all of the isotopes with the same number of neutrons. Complete this for all
three groups of your isotopes. Record the mass of each group to the nearest .01 g. Record the total
masses in column 3 of the data table.
5. Divide the total mass of each group by the # of isotopes in that group to get the average mass for one
isotope in that group. Record in column 4.
6. To find the % abundance for each group of your isotopes, divide the # of isotopes in each group by
the TOTAL number of isotopes in your basket THEN X 100 to put in percentage form. Record your
% abundance for each group in column 5. Add together your % abundance for all three groups. Your
total % abundance should be very close to 100%.
7. To find the Atomic Mass of your element, you must first find the mass contribution for each group
of isotopes. For each group, multiply the % abundance by the average mass. *** Convert %
abundance to a decimal before multiplying!!! Record in column 6.
8. Add together the mass contributions of each group of isotopes to arrive at the Atomic Mass of your
element. Record in the last box in column 6.
V.
Data and Observations:
# of
isotopes
Total mass of
isotopes with
same # of
neutrons
Average
mass of one
isotope from
each group
%
abundance
Mass
contribution of
each isotope
Isotopes with
____neutrons
Isotopes with
____neutrons
Isotopes with
____neutrons
Atomic Mass of
element
Total
VI.
Calculations: Complete the data table. All work must be shown when doing calculations.
Neatly show your calculations below your data table in your lab report!
VII.
Graph:
none
VIII. Post-lab Questions:
1. Explain the difference between mass number and atomic mass.
2. What two things is Atomic Mass dependent on?
3. Would the atomic mass be different if you received another basket of isotopes containing bags with
the same number of red beads but different numbers of black beads? Explain why or why not?
4. In step 5, instead of measuring and using the mass of a single isotope in each group, the average
mass of an isotope from each group was determined. Explain why.
5. Analyze the Class Data Table. Describe the trend you observe in the groups’ atomic masses. Explain
why this trend occurs.
IX.
Conclusion Paragraph:
Write a conclusion paragraph which addresses the following 7 points.
 Restate the purpose of the lab.
 Provide a brief background of the topic related to the lab.
 Briefly describe the lab procedure (summarize – do not list step-by-step).
 State the results of the lab – be sure to refer to your data!
 Analyze the results of the lab. Did the results meet the purpose of the lab? Explain how. Refer to
your data.
 Discuss any possible errors that could have occurred – even if you don’t think you made any
errors.
 Real world application.