A Simple Laboratory Experiment Illustrating
the Relative Nature of Atomic Weights Randolph 6. Huff' and David W. Evans Presbyterian College, Clinton, SC 29325 The relative nature of atomic weights was deduced by Dalton during formulation of his atomic theory. The necessity for use of a standard to determine an atomic weight scale is obvious to anyone who has considered the difficulty one would encounter trying to obtain a precise weight of something as small as an atom. However, simple as this concept is, it remains a constant source of confusion to the typical firstvear chemistrv student. In order to ittempt something a little out of the ordinary in the hones of establishine" this basic conceDt. . . a laboratow experiment was devised that serves two main purposes, alone with some secondarv achievements. The first goal of this experiment was to pr&ide a direct means of illustrating the relative nature of the atomic w e i ~ h t sused commonly today. Students are provided with a set of Styrofoam balls whose weights have been adjusted to represent various isotopes of the elements. Among this group is one that is indicated to be the current standard of carbon-12. Theothers are unknowns. By recording the weights of the individual balls, the student can then calculate the relative weights, with the carbon-12 isotope representing 12 atomic mass units. The second main goal was to provide the student with experience in weighing, since a substantial number of weighings are required for this experiment. Some additional goals also achieved during this experiment involve work with the concept of isotopes and the A~~
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AUthor to whom correspondenceshould be addressed.
meaning of average atomic weights (since they are the actual reported results.) This experiment is inexpensive to perform and provides considerable experience in several areas that tend to be troublesome to the typical first-year student, be they in high school or college chemistry. When run as the first experiment of the term, i t can easily replace the lab we believe is commonly run a t this point, namely the use of balances. Our experiment emphasizes weighing technique, and in addition, achieves the other goals outlined above. Experlmental Design A set of nine Styrofoamballs, preadjusted to appropriate masses, is provided for students at each balance station. We chwe this set size on the basis of our anticipation of turnaround times at each balance, consideringour typical general chemistry laboratory size of around 30. The students are provided with a brief explanation of the theory behind the experiment along with a datasheet (see Table 1). The only information initially contained in the data sheet are the relative isotopic abundanees. The students are instructed to weigh the halls individually and then perform the requested calculations, after which they are required to identify the appropriate elements corresponding to their unknowns and write their correct symbols, including mass number. For classes on the order of the abovementioned size, a typical lab time of 1% h can be expected. Our initial set included the hydrogen, carbon, chlorine, and thallium isotopes along with arsenic, which is isotopically pure. There are several other sets that we have devised and that will be implemented later (some representative examples are provided in Table 2). The
Table 1. Data Sheet Used for the Reporting of Experlmental Resuns wlth Representative Student DataC Number of isotopic Model
weight (g) of lsotopic Atomic Model
8 9
94.53 95.44
Calculated Relative isotopic Weight 01 Atomic Model
203 205
% Natural ibtopic Abundance'
Symbol of Element including Mass Number
29.50 70.50
TI-203 TI-205
Calculated Atomic Weight 07 Natural Isotopic Mixture
205
Actual Atomic Weight'
204.383
ChemicalRubber Compeny Handbo& 01 Wlembf~'andPhysiC9.471h ed.: 1968-1967. Pure Appl Chem 1984.56.653. =,he ~igniflcantfigure ruies used in me above cslcu~stionswere from those published in ~bbing,D. D. ~eosralchembtw, 3rd ed.; noughton ~ i f f l i n1990. :
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Number 6
August 1991
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chemistry laboratories). The smallest halls required sand for their
Table 2. Sample Sets ot lsotoplc Models
tntal ndiuatment. since these were used for the least massive ...-.weieht ~. ~~~~~~
Styrofoam Ball Size (in.) 1 1.5 2
2.5 3
Set I Hydropn-1.2 Carbon-12.13 Chlorine-35.37 Arsenic-75 Thallium-203.205
Set 2
Set 3
Neon-20,21.22 Fluorine-19 Carb01~12,13 Carbon-12.13 Phosphorus-31 Bromine-79.81 Copper-63.65 Iridium-191.193 Gold-197 Aluminum-27
Table 3. Student ReruRscfrom Atomlc Welahi Exoerlmsnt
Element
Atomic Weight. ~ct&
Atomic Number of Percent of Weight, Standarc Relected Data Within: calcuiated Deviation datab lo 2 0 3a
Hydrogen 1.00796 Carbon 12.011 Chlorine 35.453 Arsenic 74.9216 Thalliulm 204.363
1.00 12.01 35.4 74.9 204
0.0147 0.0219 0.1022 0.1340 0.4736
0
74
2 1 1
96 62 70 72
0
95 100 98 98 95 98 94 100 96 99
Chemical Rubbsr Company Handboo* of Chemisfry and Physics. 47th ed.: 1986-
goal is to develop a large enough co~leetionof isotopes to allow mixing in later years. One of the advantages of utilizing Styrofoam halls as the isotopic models is the variety of sizes readily available. In order to enhance the aesthetics of this experiment, we used different-sized halls to indicate the difference in relative atomic radii. Materials
The materials required for constructi~nof the atomic models were chwen for their ready availability in usage. They consist oE Styrofoam halls (1 in., 1.5 in., 2 in., 2.5 in. and 3 in.), lead BB's, ballhead pins, sand, and rubber cement. Construction of Model Sets ~ styrofoam halls were chosen to represent the various isotopes for ~~~
several practical reasons. First, it was important that the weight he able to be adjusted precisely, easily, and permanently. The Styrofoam halls can be cut in half and their core removed easily with a knife. The weight can then be adjusted by addinglead BB's until the actual weight is within 0.2 g of the desired (calcdated) weight. The halvesare then elued toeether with rubher cement. which works well on the highly porous ~iyrofoam.After gluing, the isotopic models were allowed to dry for a week. (Shorter drying times led to significantweight lossesforthelargerhalls.) For all but the smallest (1in.) halls,pins were then inserted into the hall and used to make the final adjustments in the weight.~h~ pins worked well since it was nient to snip off appropriate lengths of the pin in order to obtain final weights to +0.01 g (the precision of the balances in our general
676
Journal of Chemical Education
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isotopes and, therefore, were consistently very light. Second, Styrofoam balls were chosen due to their ready availability and cost. They can he found in virtually any arts and crafts or discount store at a reasonable price (approximately $3 per nineisotope set). Third, Styrofoam, when treated properly, is sufficiently durable for use in this fashion. After handling by over 200 freshmen, no significant loss of weight was noticed due to abuse. The lead BB's, pins, and sand were all chosen as weight adjusters because of their densities, as well as their availability and low cost. All together, the cost of assembling a set of nine isotopes is about $5, which should be well within a normal high school or college chemistry budget. Results
A typicalstudent datasheet isshown inTable 1.Astatistical analysis of the data submitted hy 85 students is presented in Tahle3. Within the recision of the balances (i0.01 r). .--excellent results were obiained. A few data were exclud;?d due to very obvious calculation errors (see Table 3). Since the goal of this experiment was to emphasize the ~ o i n t outlined s above. each student was reauested to complete an evaluation fo;m and provide written comments. Of the 85 students who initially performed the experiment, 80 completed the evaluation form. The results of these evaluations clearlv show that the students believed that they had gained knokledge by performing this experiment. hep pooreSt average rating (2.99 on a 5-point scale) was due to the student's perception that the calculations required were difficult. I n large part, this was due to our intention of not providing too much detail during the laboratory lecture given prior to the lab. It was believed that more would be gained from the hv the students if thev had to think ~ - exneriment ~ their way tb&ugh the exercisc Another reason for this low rating probably stems from the fact that most students, in our experience, do not enjoy calculations in any case. However, both questions relating to the main goals, namely an of the relative atomic mass system increased and the o ~ e r a t i o nof a balance. received relatively high marks from the students (3.56 i d 4.46, respectiveli). he question concerning our suhsidi-Y goal of understanding the nature of isotopes also indicated attainment of the desired result through this experiment (3.59). Overall, the students rated this experiment as one which should he continued to be offered (4.19). ~
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COnCluSlOn
This experiment details a simple, inexpensive method of
reinforcing the principles of the relative atomic weight scale, the concept of isotopes, and, finally, proper weighing technique. The simplicity of this lab should allow for easy incorporation into a typical high school or freshman chemistry laboratory experience.
