Isotope/Element Model

Isotope/Element Model

Description

Prior to 1982, US pennies were made of an alloy containing mostly copper with some zinc. Since 1982, US pennies have been made by electroplating zinc blanks; these coins contain very little copper. The "copper" pennies have a significantly larger mass than the "zinc" pennies. Also, the copper pennies are unreactive to acid when scratched. Under similar conditions, the cores of the "zinc" pennies dissolve away yielding very thin copper foils which bear the image of a penny. This experiment develops some relationships between pennies and isotopes.

Precautions

Use caution when handling the 6 M HCl. Wear eye protection.

Procedure

Weigh 20 pennies, one at a time, to the nearest 0.1 g. Record the data.
Determine mass differences in the pennies. Can they be separated into two piles on the basis of mass? If so, determine a mass that is greater than that of the light pennies but smaller than that of the heavy pennies. Set a triple beam balance with the middle mass.
Drop a heavy penny on a flat, hard surface and note the sound. Repeat for a light penny. Note and record and differences.
Boot a spreadsheet program, and bring up an empty document.
Take a sample of about 100 pennies. Weigh the pennies, one at a time. A penny that causes the beam to move up past the midway point is a "heavy" penny. Else, it is a light penny.
Separate the pennies into two piles on the basis of mass.
Use a row of the spreadsheet for each penny. Enter the year of the penny in column A, and the mass of the penny in column B. (Use the numerical value for the mass obtained earlier.) Repeat for all pennies in the sample. Save the data in a file.
Use the arrange (sort) feature of the spreadsheet to resort the data first using increasing year in column A, and then using decreasing mass in column B. Save the rearranged data in another file on data disc.
Count out and weigh 100 pennies. Record the mass.
Prepare a histogram of your data with the computer is possible. Otherwise, plot this on graph paper.
Share your data with another pair of students. Use the spreadsheet controls to add their data to your data. Repeat the data rearranging procedure. Plot a new histogram on the same graph paper by adding to the existing histogram with a different color or shading.
Select one heavy penny and one light penny. Notch the edge of each coin using a triangular file.
Place both coins in a 150-mL beaker containing 20 mL of 6 M hydrochloric acid. Note any evidence for reaction. Set the beakers aside for a day, swirling occasionally.
Use forceps to remove the metallic residue of the "reacted" penny. Wash this residue in a beaker of tap water.
Examine closely the residue.
Use a file to remove the entire outer edge of copper from a light penny. Place the penny in acid overnight.
Retrieve two pieces of copper that retain the impression of the coin.

Handout

Name ___________________________ Class ________

Teacher___________________________

DoChem 022 Isotope/Element Model

Examine the data. Does one year stand out? What is unique about that year?
Assume that the pennies are dateless isotopes. Compute the average penny (isotope) mass. Predict the mass of 100 pennies. Compare the predicted and observed masses.
Predict changes likely to occur in the average mass of a penny as a function of time. Explain your response.
Predict changes likely to occur in the average mass of 1981, 1982, and 1983 pennies as the result of time. Explain your response.
Supposing that the only effect involved is the burning of fossil fuels, predict and explain any change in the percentage of carbon-14 found in the carbon dioxide of the atmosphere.
There is a major flaw in the analogy between pennies and isotopes related to reactivity. Identify this flaw, and explain how one might develop a better analogy.

Teachers Guide

Purpose

To study a model analogous to atomic isotopes or closely related elements.

Materials

(for 10 students working in pairs)

5 triple beam balance
5 sheets graph paper
500 pennies
computer and spreadsheet software
10 150-mL beaker
100 ml 6 M HCl
5 triangular file
5 forceps

Lab Hints

Electronic balances may be used instead of triple beam balances. When using an electronic balance, set the sensitivity to 0.1 g and weigh each penny. Review the procedures for moving data from one file to another using the spreadsheet.

Time

Teacher preparation: 10 minutes

Class Time: 2 periods

45-50 minutes first period
5 minutes second period (Observe product)

Hazards

6 M HCl is corrosive.

Disposal

Neutralize the acid and discard at the sink. Dispose of copper foils with ordinary solid waste. Save unused pennies for reuse.

Sample Data

Closure?

Closure Questions:

Examine the data. Does one year stand out? What is unique about that year?
Assume that the pennies are dateless isotopes. Compute the average penny (isotope) mass. Predict the mass of 100 pennies. Compare the predicted and observed masses.
Predict changes likely to occur in the average mass of a penny as a function of time. Explain your response.
Predict changes likely to occur in the average mass of 1981, 1982, and 1983 pennies as the result of time. Explain your response.
Supposing that the only effect involved is the burning of fossil fuels, predict and explain any change in the percentage of carbon-14 found in the carbon dioxide of the atmosphere.
There is a major flaw in the analogy between pennies and isotopes related to reactivity. Identify this flaw, and explain how one might develop a better analogy.

Answers to Closure Questions:

1982 stands out as the only year that has both light and heavy pennies.
((Number light) x (mass light) + (number heavy) x (mass heavy))/ total

(Rest of answer depends upon model data set.)
The average mass of a penny will decline, since only light pennies are being minted for circulation.
The average masses of 1981 and 1983 pennies will not change, since all pennies of each type are identical. If anything, friction wear may reduce their mass. Barring any special reasons why the light and heavy versions of the 1982 penny are selectively removed from circulation, their average mass should remain the same too.
All of the carbon-14 in fossil fuels has had time to decay. Burning fossil fuels releases large amounts of carbon into the air containing no carbon-14. (Carbon-14 is produced in the upper atmosphere by cosmic ray processes with nitrogen.) The fraction of carbon-14 found in the atmosphere is therefore decreasing.
Because old and new pennies look identical, the first image that comes to a chemist's mind is that of isotopes. Isotopes have similar reactivities. In most situations, two isotopes of the same element behave similarly, with differences in the rate of chemical reaction usually being the most dramatic differences.

Rather than serve as a model for isotopes, the pennies better serve the model of two different but closely related elements. Two elements have similar appearances and atomic masses, but they differ in patterns of reactivity. They sound different when dropped on a hard surface. They react quite differently with acid. For example, rubidium and cesium or strontium and barium might be better examples for which the pennies serve as a model.

Applications

Stable isotopes in nature concentrate differently depending upon their environs. Thus a sample of oxygen obtained from sea water will contain a different mix of isotopes than will a sample obtained from air. When two (or more) stable isotopes comprise a large percentage of an element, the variation in the natural abundance and atomic mass from sample to sample can be quite large. This accounts for the relatively small number of significant digits in the element nickel, for example. Although oxygen has one of the largest variations in relative isotope abundance, the heavier isotopes contribute such a small fraction of the total natural abundance mass that the natural abundance atomic mass which appears on periodic tables is written with high precision.

Background

In 1982, the US penny was changed. Prior to 1982, the penny consisted of a coin minted from an alloy containing 95% copper and 5% zinc. In 1982, the coins were changed to a blank alloy of 99.2% zinc and 0.8% copper which are then barrel electroplated with copper giving a net of 97.5% zinc and 2.5% copper.
In 1984 it cost about $600 to manufacture $1,000-worth of pennies. Pre-1982 pennies weigh 3.1 g, while post-1982 pennies weigh 2.5 g.

Key Words

mass
isotope
atomic mass
element
atom
natural abundance