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Global Chemistry
Lab/Activity: 2.1 Isotopes and atomic mass
Problem: How do isotopes affect average atomic mass?
Materials
8 small plastic cups for each group
Approximately 64 blackeye peas and 56 white bean peas for each group
Procedure
1. Place 8 blackeye peas and 7 small white beans in each of 8 plastic cups. This cup
with beans represents a particular isotope of an atom. Call this isotope “X-15”.
Mass each “isotope” using the analytical balance in the center lab area. What is
the average mass of this isotope? Record your data here.
2. Place 8 blackeye peas and 5 small white beans in each of 2 plastic cups. This cup
represents the same atom as in step #2, but it’s a different isotope, “X-13”. Mass
each “isotope” using the analytical balance in the center lab area. What is the
average mass of this isotope? Record your data here.
3. Calculate the % abundance of isotope X-15 and X-13. Record your data.
4. Calculate the average atomic mass of isotope X-15 and X-13, using the following
procedure: average isotopic mass = %abundance of isotope x average mass of
isotope
5. Calculate the average atomic mass of element X. To do this simply add the
average isotopic masses from step #4.
Data Table
Isotope
Average mass of
each isotope
Fractional
abundance
Average isotopic
mass. (Mass x
abundance)
X-15
X-13
Average atomic mass of element X is _____________
6. Repeat steps 1-5, but this time use:
 2 plastic cups, each with 3 blackeye peas and 4 white beans.
 7 plastic cups, each with 3 blackeye peas and 10 white beans.
 1 plastic cup, with 3 blackeye peas and 1 white bean.
All of these cups represent isotopes of a different element, atomic symbol “R”
Data Table
Isotope
Average mass of
each isotope
Fractional
abundance
Average isotopic
mass. (Mass x
abundance)
R-7
R-13
R-4
Average atomic mass of element R is _______________
Analyze
1. Which subatomic particles do the blackeye peas and small white beans represent?
2. A naturally occurring sample of hydrogen is composed of 99.985% 1H and
0.015% 2H atoms. A 1H atom has a mass of 1.00783 u and a 2H atom has a mass
of 2.01410 u. Calculate the average atomic mass of naturally occurring hydrogen.
3. An element needed for most nuclear reactors is uranium. It’s two major isotopes
are U-235 and U-238. Look at the mass of uranium on the periodic table. Which
isotope is the most common? Explain.
4. How do the masses of isotopes affect the average atomic mass of an element so
that it is often not a whole number?