C. C. Thompson Memphis Stote University Memphis, Tennessee 38152
Charge and Mass of the Electron An introductory experiment
In the usual introductory $hemisty course a considerable amount of lecture time is devoted to atomic structure, the discovery and properties of subatomic particles and related topics. However, few, if any, laboratory experiments dealing directly with subatomic phenomena are normally included in the curriculum. The reasons for this disparity are fairly obvious. The traditional experiments in this area reauire more time. elaborate eauinment and .. laboratory techniques than are commonly available in an introductorv chemistrv course. Unfortunatelv. " . a "lectureonly" exposure to the classical experiments of Thomson, Millikan, Rutherford, etc., all too often leaves the student with a sense of amazement a t the supposed level of sophistication and skill one must attain before he dares to explore this incredibly minute world. Moreover, there is the further risk that an honest appreciation for the chain of events which has led to our present knowledge of atomic structure mav he lackine. We have foind that ;he standard electrolysis experiment conducted with the exnressed numose of determining the charge and mass of ihe eleciron'provides the student an interesting and informative first-hand glimpse into the subatomic realm. Since the electron charge-mass adaptation of the electrolysis experiment offers many useful parallels to the actual historical development of the subject, we feel that its pedagogical advantages clearly outweigh those of the frequently-described electrolytic evaluation of Avogadro's numher.'J Some twenty years ago Nechamkin3 suggested the experiment as one of several which could he used to brine" more sienificance and meaning into the general chemistry laboratory. However, a survev of over a dozen of the more . nonular . laboratorv manuals currently in use reveals that, while nearly all include one or more exercises on electrolvsis. none of them discusses this particular application. A; added advantage is the fact that since the experiment reauires skill onlv in the use of the balance and in careful reiording of data, i t can be scheduled early in the course to coincide with the lecture material on atomic structure.
come metal atoms that acquire a specific number of electrons as they deposit on the cathode. A detailed discussion of electrochemistry is not necessary a t this point, and in fact may only serve to obscure the real objective of the experiment.
Procedure
Previously prepared electroplating baths of approximately 1M CuZ+,0.5M PbZ+ and 1M Ni2+ along with copper screen and lead and nickel strips to serve as electrodes are provided.' Students are assigned one of the metals and proceed to construct a simple electrical circuit consisting of the electrolytic cell, flashlight cells (one D cell for Cu and P b and three cells for Ni have been found satisfactory) and an inexpensive ammeter (0-1 amp range) connected in series. Working in pairs, the students determine the mass of the cathode to milligram accuracy, complete the circuit, and record the current at l-min intervals for 20-30 min. At the end of this time the cathode is disconnected, thoroughly rinsed with water and with methanol, air-dried, and again weighed. From the change in mass of the cathode, the atomic weight of the metal, and Avogadm's number the number of metal atoms deposited and the total number of electrons are found. The total charze (in coulombs) is calculated frum the average current and the time of electrolys:~Thew dnrn nliow rhe charge per electrun 10 be cmnputed, and finally, from the c, m ratio the mass of the electron r a n he calculated.
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Discussion of Results
Typical student data and computed results for three different metals are given in the table. Since copper seems to present the fewest experimental problems (little tendency for eeli inn, flaking or "treehe" of the de~osited metal) t h e disproportionatkly large number of 'values merely reflects our prejudice toward this system. Never-
Student Results for the Electrolytic Determination of the Charge and Mass of the Electron
Pre-Laboratorv Discussion
In a short pre-laboratory discussion period the students are reminded of Thomson's determination of the electronic charge-to-mass ratio and are given the e / m value of 1.76 X los C/g. They are quickly convinced of the impossihility of evaluating e and m separately from this ratio alone and of the necessity for an independent measurement of one of the quantities. The discussion leads naturally to Millikan's experiment and the recognition that our electrolysis approach, though different in appearance, is conceptually quite similar. Our "oil drops" heData for Lead (Time: 1800 see1
'Sunier, A. A., J. CHEM. EDUC., 6, 299 (1929); Johnston, J. E., J. CHEM. EDUC., 16, 333 (1939); and Ellis, R. H., and Rauch, R. B., J. CHEM. EDUC., 30,460 (1953). 2Slahaugh, W. H., J. CHEM. EDUC., 46,40 (1969). 3Nechamkin, H., J. CHEM. EDUC., 29,92 (1952). *Specific details of the preparation of electroplating solutions may be obtained from the author or from the Instructor's Guide to "College Introductory Physical Science 11," prepared by the Physical Science Group of Newton, Mass.
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Volume 50, Number 6, June 1973 /
theless, with reasonable care the other metals also give satisfactory results. As the data show, the computed e and m values are remarkably consistent with a total variation of about 10%. Our experience with the method indicates that reported deviations significantly greater than this are generally traceable to computational errors. Moreover, the class averages are in excellent agreement with the accepted values of 1.60 x 10-l9 C and 9.11 x 10-28 g for the electronic charge and mass, respectively. These results vividly re-emphasize that in a great many cases reliable data can be obtained through the use of a simple and inexpensive apparatus. During a post-laboratory discussion, student results for a given metal are pooled and attention is called to the fact that to within the expected limits of uncertainty the e and
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rn values are independent of the metal-a situation analogous to that in the early studies with discharge tubes containing different kmds of electrodes and residual gases. Although we have not tested modifications of the basic procedure, the set-up described by Slahaughz in which three or four electrolytic cells each with a different metal are operated in series should provide an interesting variation of the method used here. In addition to the advantages already mentioned, the experiment provides practice-in the use of exponential notation as well as insight into the methods of indirect measurements of very small quantities. Perhaps the most rewarding feature has been the exceedingly high student interest in the experiment itself and an increased awareness of the "realities" to be associated with individual subatomic particles.
