The “Coinium” Lab Activity
(Part I)
Background Information:
In this activity we will use pennies to represent isotopes of the element “coinium.”
The element coinium is analogous to any element for which we find some unstable
radioactive isotopes. Coinium, like other radioactive elements, has some isotopes
which are unstable and will decay over time. During normal radioactive decay,
elements decompose into other elements, smaller particles (α and β) and energy (γ),
while coinium atoms decay from “heads” up (or “flowers” up in Singapore) to "tails"
up (or “shields” up).
Objective:
In this activity we will look at the rate of decay for the
fictitious element coinium and track the stability of one
representative atom.
Procedure:
1. Create a data table in your logbook (using the given data table guidelines). The
data table should have the following headings; time, number of decayed atoms, and
percentage of atoms remaining.
2. Place 100 pennies "flowers" up in a laboratory tray. Use a small piece of masking
tape to mark one of the pennies. Start your stop watch (this is time 0:00).
3. Place another laboratory tray over the original so that no pennies will escape when
you shake them.
4. After 30 seconds, shake the pennies a number of times. Some of the coinium atoms
should have decayed (flipped to shields). Record the number of atoms that decayed
and remove them.
5. Be sure to record when the marked atom decays by marking its decay time with an
asterisk.
6. At 30 second intervals, cover the original tray and flip the pennies. Again record
and remove the decayed "atoms".
7. Continue flipping until all of the "atoms" have decayed.
8. Construct a graph (using the given graphing guidelines) plotting time versus
percentage of atoms remaining.
9. Answer the Post-Lab questions using your graph.
- Post-Lab Questions (Part I) 1) Scientists describe the stability of an isotope by reporting its half-life. Half-life is
the amount of time required for one half of a radioactive sample to decay. What is the
"1/2 flip life" of your pennies?
2) How many pennies would you expect to remain after 3 minutes?
3) How long would it take for 33 pennies to "decay"?
4) How long would it take to have 1/8 as many pennies you started with?
*The graph below shows the radioactive decay curve for the isotope Th-234. Use the
graph to answer these questions.
5) What percent of the isotope remains after 60 days?
6) How many grams of a 250-g sample of Th-234 would remain after 40 days?
7) How many days would pass while 44 g of Th-234 decayed to 4.4 g of Th-234?
8) What is the half-life of Th-234?
The “Coinium” Lab Activity
(Part II)
Background Information:
The periodic table provides us with the “Average Mean Mass” for each of the
naturally occurring isotopes of the elements. Scientists cannot mass individual
isotope atoms in order to find out the average isotopic mass or relative abundance.
Therefore, they had to come up with a method to indirectly calculate the average
isotopic mass of a given sample element.
With pennies, you will simulate one way that scientists can determine the
relative amounts of different isotopes present in a sample of an element (without
being able to directly observe the isotopes). You may have learned earlier that pre
1982 and post 1982 pennies have a different composition and thus different masses.
In this laboratory activity, a mixture of these pennies will represent the naturally
occurring mixture of two isotopes of the imaginary element “coinium.”
The relative abundance of our coinium isotopes can be calculated using this
equation:
Mass(penny mixture) = [X * Mass(pre-1982 penny)] + [(10-X) * Mass(post-1982 penny)]
Let (X) = the number of pre-1982 pennies in the canister. Then (10-X) = number of
post-1982 pennies in the canister.
This equation can be solved for X after the three masses have been determined.
The mass of all the pre-1982 pennies is equal to the number of pre-1982 pennies (X)
multiplied by the mass of one pre-1982 penny. The mass of all post-1982 pennies is
equal to the number of post-1982 pennies (10-X) times the mass of one post-1982
penny.
To better illustrate how this calculation is done, think it
through as if you had billiard balls and ping pong balls instead of
coins. Say you have several samples of 10 balls each. Ten
billiard balls will weigh much more than 10 ping pong balls. The
less a mixture of 10 of these billiard/ping pong balls weighs, the
more ping pong balls you have present. The relationships can be
represented mathematically by the following equality:
Total mass of mixture of balls = Number of billiard balls * Mass of one
billiard ball
+ Number of ping pong balls * Mass of one ping pong ball
Objective:
You will be given a sealed canister containing a mixture of pre-1982 and post1982 pennies. Your canister could contain any combination of the two "isotopes."
Do not open the canister. Your task is to determine the isotopic composition of the
element coinium without opening the sealed canister.
Procedure:
Materials: You will be given a supply of pre-1982 pennies, post-1982 pennies, and a
sealed canister of 10 mixed pre- and post-1982 pennies (the mass of the empty sealed
canister will be written on the outside of it).
Precaution: Do not open the canister! Exposure to Coinium has been directly linked
to student score reduction.
1. Record the code number of your sealed canister.
2. Determine the average mass of each type of penny. (You determine the method).
3. Find the mass of the penny mixture sealed in the canister.
- Post-Lab Questions (Part II) –
1. Calculate the value of X (the number of pre-1982 pennies) and 10 - X (the number
of post-1982 pennies). Record them.
2. Calculate the percent composition, by MASS, of the element "coinium" from your
data, i.e., for the 10 pennies, what percent by mass is contributed by the pre-1982
pennies?
3. Why is the element “coinium” a good analogy or model for actual element isotopes?
In what ways is the analogy misleading or incorrect?
4. Name at least one other familiar item that could serve as a model for isotopes.