Isotopes of “Pennium”
Your Prelab assignment includes:
Title, Purpose, Storyboard of the Procedure, Background information (be sure the
answers to the prelab questions below can be found in this paragraph), Blank data table (see
below).
Introduction:
Unless you’re a coin collector, you probable think all United States pennies are pretty much
the same. To the casual observer, all the pennies in circulation do seem to be identical in size,
thickness, and composition. But just as elements have one or more isotopes, with different
masses, the pennies in circulation have different masses. In 1982, the coin's composition changed
from 95% copper and 5% zinc to copper-plated zinc. These coins, which are still being produced
today, contain 97.6 % zinc and 2.4 % copper. This coin is identical in size and appearance to the
predominantly copper penny issued before 1982, but this modification saves the Government an
estimated $25 million in metal costs every year. In this lab, you are going to use pennies with
different masses to represent different “isotopes” of an imaginary element called pennium, or Pe.
Remember that chemical isotopes are atoms that have the same number of protons, but different
numbers of neutrons. Thus, chemical isotopes have nearly identical chemical properties, but some
different physical properties.
After completing this lab, you will determine the relative abundance of the isotopes of
pennium and the average mass of each isotope. You will then use this information to determine
the atomic mass of pennium. Recall that the atomic mass of an element is the weighted average
of the masses of the isotopes of the element. This average is based on both the mass and the
relative abundance of each isotope as it occurs in nature. In other words, the isotope that is most
abundant contributes most to the average and the less common isostopes contribute less to the
average.
Objective:
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

To determine the masses of the different isotopes of pennium
To determine the relative abundances of the isotopes of pennium
To determine the atomic mass of pennium
Materials:
Safety glasses
Triple Beam/electronic Balance
20 pennies
Procedure:
1. Put on your safety goggles. Remove the pennies form the container and count them to
make sure there are 20 pennies. Determine and record the mass of the 20 pennies
together. Enter this in your data table as the combined mass of the pennies.
2. Find the mass of each penny separately. In the data table, record the year the penny was
minted and its mass to the nearest 0.01 g.
3. Place the 20 pennies back in the container and return the balance to the cabinet. Wash
your hands and begin working on the calculations with your lab partner.
Pre-lab Questions: These questions are to be answered in the Background information
paragraph part of your pre-lab assignment for this lab.
1. What is an isotope?
2. What do the 20 pennies in this investigation represent?
3. What do the different masses of the pennies represent?
4. What information do you need to calculate the average atomic mass an element?
Data:
Combined mass of
20 pennies
Penny Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
g
Year Minted
Mass (g)
Isotopes of “Pennium” – Post Lab: You will add the following to your lab book to complete
your assignment: Calculations, analysis and conclusions.
Calculations: For this part you will be looking at the data you collected in the lab and
analyzing it. Your ultimate goal is to draw some conclusions about the element Pe (pennium)
and then do some calculations of its atomic mass. For all of the calculations you are required
to show all of your work neatly. This should include a word equation, number equation
and your answer neatly circled. DON’T FORGET UNITS AND SIG DIGS!
1. Look at the masses of the different pennies and group similar masses as one type of
isotope. How many isotopes do there appear to be for the element Pe? Explain your
rationale.
2. Calculate the relative abundance (also called the fractional abundance) of each of
the isotopes in your sample. Express your answer as a decimal value.
3. Calculate the average atomic mass of each of the isotopes in your sample
4. Using the fractional abundance and the average atomic mass of each of the isotopes
in your sample determine the average atomic mass of the element Pe.
Analysis and Conclusions:
1. Was the mass of the 20 pennies equal to 20 times the mass of one penny? Why or
why not?
2. In what year(s) did the mass of the Pe change? Explain how your data shows this?
3. How can you explain the fact that there are different “isotopes” of pennium?