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Bean BAG Isotopes Lab (5opts)
Introduction: John Dalton’s atomic theory that stated all atoms of the same element are identical
and equal in mass was simple yet revolutionary. Unfortunately, it was not quite right. More
research started to show that atoms of the same element could have different masses. These
atoms were call isotopes. Isotopes have the same # of protons and electrons but different #
of neutrons. Isotopes are chemically indistinguishable from one another; the only different is their
mass number. Isotopes do not show up equally in nature. An element’s atomic mass represents
the weighted average (based on percent abundance) of the masses of the isotopes. In
today’s lab, we are going to learn how to calculate the atomic mass of a “bean bag” element!
Purpose: The purpose of this experiment is to investigate the mass properties and relative
abundance of isotopes for the “bean bag” element and to calculate the atomic mass of this element.
A new element has been discovered and it is your responsibility to learn the following about the
element:
 the number of naturally occurring isotopes present in the element
 the percentage of each isotope present in your sample of the element
 the atomic mass of the element
Materials: balance, 3 weighing boats, bag of beans
Procedure:
1. Answer the pre-lab questions.
2. Obtain a sample of the element from your teacher and record its ID number
3. Count the total number of beans (atoms) in your sample and record in the data section.
4. Assign each different bean (isotope) a code or letter (ie, W for white).
5. Sort the atoms from the “bean bag” element into three isotope groups according to the type
of bean into a separate weighing boat. Count the number of atoms of each isotope and
record in your data table under “Number of atoms in isotope group.”
6. Mass each isotope group of atoms and record in the third column under "Mass of Isotope
Group”. (the weighing boat does not add any measurable weight)
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Pre-Lab Questions (10 pts)
1. Neutrons were discovered in 1932, more than 10 years after the existence of isotopes was
confirmed. What property of electrons and protons lead to their discovery? Suggest a
possible reason why neutrons were the last of the three classic subatomic particles to be
discovered?
2. In 1935 a scientist received the Nobel Prize in Physics for his discovery of the neutron.
What is the name of the scientist that discovered the existence of neutrons and what
scientist was he working with at that time?
3. Silicon occurs in nature in the form of three isotopes, Si-28, Si-29, and Si-30. Determine the
number of protons, neutrons, and electrons in each isotope of silicon.
# of protons
# of neutrons
# of electrons
Silicon -28
Silicon -29
Silicon -30
4. “The atomic mass of chlorine represents the mass of the most common naturally occurring
isotope of chlorine.” Decide whether this statement is true or false and why.
5. What is a quick estimate for the average atomic mass of your bean bag element? (total
weight of beans divided by the number of beans)
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Data Table (10pts): Use the data table provided to collect the following information:
Total number of beans in sample: __________
Code for Bean Bag Isotope
Number of atoms in group
Total Mass of isotope group (g)
Analysis (10pts):
 Calculate the average mass of one atom of that isotope (Mass of Isotope Group ÷ # of atoms
of that isotope). Record your answer in the analysis table below.
 Calculate the percentage abundance of each isotope present in your element (# of atoms of
isotope ÷ total # of atoms in sample). Record your answer in the analysis table below.
Code for Bean Bag Isotope
Average Mass of one atom
Total mass of isotope group ÷
# of atoms of that isotope
Percent abundance
Number of atoms of isotope ÷
total number of atoms x 100
Calculations (5pts):
 Determine the weighted atomic mass of your element to three significant figures using the
below equation. Be sure to show your work and use correct significant digits!
Weighted atomic mass = (avg. mass of isotope 1)(% abundance) + (avg. mass of isotope
2)(%abundance) + (avg. mass of isotope 3)(%abundance)
(For percent abundance, use the decimal form rather than percentage form in your calculations.)
Space for you to calculate your Weighted Atomic Mass:
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Conclusion (15 pts): Please answer in complete sentences!
1. The atomic mass of the “bean bag” element represents a weighted average of the mass of
each isotope and its relative abundance. What did you calculate as the weighted atomic
mass for your element? (Be sure to put your answer in the correct number of significant
digits)
2. How close is your quick average atomic mass from pre-lab question 4 to the weighted
atomic mass from post-lab question 1? Why was it not the same?
3. Did the most common individual beans or atoms have the same weight as your weighted
atomic mass? If yes, which one? If not, did any come close? Why not? How does this
relate to pre-lab question number 4?
4. The isotopes of magnesium (and their percent abundances) are Mg-24 (79%), Mg-25
(10.0%), and Mg-26 (11.0%). Calculate the atomic mass of magnesium. Use the mass
number for the mass of each isotope of magnesium.
5. Give your element a name and symbol and record the information here. Remember that this
is a new element so you, as a scientist, get to name this element whatever you want!
Bean Bag ID #: ______________
Element Name:______________ Symbol_____ Weighted Atomic Mass_________
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