Table 2.
Anifact
Innructor's Guide
Age ol Anifact Halflife Years
Red Cube Green Cube
Yellow Cube Blue Pyramid Purple Pyramid Green Pyramid Orange Pyramid Red Cylinder Blue Cylinder Yellow Cylinder
Prelab Assignment
Following a historical development of atomic models, including a focus on the Rutherford gold foil nuclear model experiment, each lab group researches and makes a classroom presentation on a subatomic particle (excluding protons, neutrons, and electrons). In-class reading and worksheet assienments on the tooics such as the discoverv of radioactivity, transmutation,'half-life, reasons for nuciear instability, artificial radioactivity, nuclear accelerators, and the Grand Unification Theory are completed, using sources includins the Mav 1985 issue of National Geoeraohic - . and various recent textbooks (1-3). Prior to the laboratory activity, students analyze three sample decay curves to determine the amount of isotope remaining at a given time, the half-life of the sample, and which of the three substances is the most radioactive. The Lab Activlty Purpose
of their dating with the instructor before proceeding to date another artifact. The instructor may wish to give helpful hints to groups having trouble dating their firstartifact' The author has observed three different methods of solving the radioactive dating problem in this activity. The most common method can be called "visual grouping," which can best be described bv usine an examnle. Assume that an artifact contains 2 rariioart~eisotopesiblack marbles) and 3 daughter isotopes (red marbles). At the time the object was formed it is known that 20°: of the isotopes mixture was that of the daughter. In other words, there is one red marhle for every four-black marbles. Next, group the marbles in the artifact according to color by moving two of the red marbles to the black marble pile. This will simulate what the artifact looked like a t the time of its formation. After allowing one half-life to elaose. two of the black marbles would have become red marblks, so we move two red marbles from the black pile to the redoile. Now the samolelooks the wavit did when b e found the-artifact. heref fire, the artifaccis one half-life old. A second solution method used by secondary students is graphing, in which they plot the fraction of the radioactive isotope remaining versus the number of half-lives elapsed. The third method usually involves the use of self-derived mathematical eouations. The grouping bf students is an important facet since group interaction facilitates the solving of the problem. Heterogeneous grouping works best and allows most, if not all, students tocomplete this activity successfully. Themost important outcome of this activity, however, is that students acquire a concrete image of the radioactive dating process.
The purpose of this activity is to identify six elements from cdeulations of their half-lives. Materials
For a lab group of two or three students paper or poster board, 40 cm X 60 cm meter stick or metric ruler 36 pennies (Each lab group should supply its own pennies.) reference half-life table from teat (4,5) fine line marker or fine-tipped pen unknown samples (These are constructed from heavy paper or plastic transparency material, using a scale of 1 cm per marked line. See Figure 1 and the table. Elongated lines should coincide with the boldface markings on the students' papers at the lo-, 20-, 30-, and 40-cm markings, so that when students align them with the X-axis, they can calculate the half-lifeand identify the isotope.) Half-Llves ol Various Isotopes
Samole
Half-Life
lsolo~e
i
1602 y
2
57.5 min
Zi:~r :$c
3
12 h
183
4
138 days
210
5
4.5 X lo9y
6
0.0033 s
760s
.,PO 238
,,'J
?%~m
Acknowledgment
The author is indebted to Charles Mueldener for his support in the development of this activity and to mv other colleagues whose names also appear in the element names.
An Exercise to Teach Concepts of Half-life without Using Radioactive Isotopes Mary C. Christian North Prov~denceH!gh School North Prov~dence.RI 02904 The objective of this activity, which can be used either during the study of basic atomic structure or to reinforce the conceots of nuclear chemistrv. is to eive students a workine defini'tion, rather than just aiextboolk definition of half-life: The leneth of time for a eiven amount of radioactive suhstance to undergo nuclear decay or transmutation is coucretely modeled in this laboratory exercise.
48
Journal of Chemical Education
Figure 1. Unknown "sqmples" for studem me. Each llne represems 1 om.
Procedure (1) Position the large paper or poster board so that the longest side is the X-axis. Draw an abscissa (X-axis) and an ordinate (Y-axis) approximately 10 cm from the bottom and 10 cm from the left side of the paper, respectively. (2) Divide the X-axis in 10-cm segments, placing boldface markings at the lo-, 20-, 30-, and40-em positions for clarity. Label the X-axis "Time". Label the Y-axis "Amount of Radioactive Isotope". (3) Place the pennies on the paper at the following positions (see Fig. 2): Time Location
Number of Pennies
0
8 (original sample) 7
9
6 5
4 (first half-life) 24
2 (second half-life)
36
1 (third half-life)
(4) Study the positions of the pennies. Note that they form a radioactive decay curve. Determine where half the original sample remains, where one-fourth remains; these represent the first and second half-lives, respectively. You may pencil in the "missing pennies", up to eight on each column, to indicate that they have not disappeared but have changed inlo other matter. (5) Obtain a paper strip, representing an unknown isotope, from your instructor, Align it with the X-axis on the paper and calculate the half-life; then identify the isotope. (6) Construct your own unknown strip and put information about an isotope not used in the experiment at the correct
Figure 2. A student decay curve constructed with paper and pennies (not all pennies are shown). time position. Exchange unknowns with your classmates for identification, T h i s activitv is a n effective method of giving secondary chemistry s t u d e n t s t h e opportunity t o appi; ~ h ~ c o n c e pnf ts radioactive decay t o "real isotopes" without actuallv using expensive equipment and radioactive samples
Note to Teachers Teachers a r e advised t o construct their own samples based o n t h e half-life table available i n t h e t e x t t h e v a r e currentlv using. Literature Cited
A Writing Assignment for Senior Chemistry Majors Our university, like others, has recently adopted a Writing-Acn~sa-the-Curriculum program and nox gives a junior writingeramination wall students. In the 1)epartment ofChemisrry we hare attempted 11, provide nurrtudenrs with more writing erp~riencesin the sophwnore organic lnhuratory, rne pnaw physical chemistry laboratory, and the senror-level inorganic course, as well as in some of the upper-level elective courses. In the inorganic course for the past several years I have been concerned with our student's lack of knowledge about individual chemists, for example, complete unawareness of recent Nobel Prize winners. Therefore, my writing assignments have focused on an individual chemist, and students have written abstracts of a particular chemist's articles, usually three; each student was assigned a different chemist. Most recently, a part of the assignment has involved finding biographical material about the author prior to doing the abstracts. I t came as a surprise (and as a bit of a shock) that two of my students decided to call their assigned chemist for information on their careers. Both chemists, according to the students, were most pleasant and seemed flattered. The assignment sheet that is given to the students, with a card containing their chemist's name, now indicates that the scientist is not to be phoned-once in a career i~ probably enough. l.yped,doul,le spaced, one-page ah-tracts of the rerearch arricles have alwayi been the hasic assignment. Often, oneoi' the chemist's earliest articles, a mid-carter article, and a recent article have hem assigned for abstrncting. On ~rccasion,an abstract of an article by a different author, referenced in one of the chemist's articles, has also been required. This past semester, rather than a bibliography, a resume for the assigned chemist was required. Same of my students again showed excellent resourcefulness since many of this year's chemists were not Americans. One student, who had recently done his own resume, created ambitions for a chemist in the Netherlands. He was supposedly tired of wooden shoes and tulips and wished to relocate to a midwestern university in the States (as a replacement for the present instructor). Some of the supposed hobbies and career aspirations made grading this set of papers more enjoyable than the abstracts. One student became so interested in the work of the chemist that he presented a seminar based on the chemist's work. This student is considering attending the chemist's university for PhD studies. Students often are forced to abstract articles containing chemical concepts that are unfamiliar, and the assignment provides another opportunity far them to use the library. Also, this assignment provides another check on the student's writing ability in their chosen field. Douglas X. West Illinois Slate University Normal. IL 61761
Volume 65
Number 1
January 1988
49
