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A Reconstruction of Structure of the Atom and Its Implications for General Physics Textbooks: A History and Philosophy of Science Perspective

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A Reconstruction of Structure of the Atom and Its Implications for General Physics Textbooks: A History and Philosophy of Science Perspective
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   Journal of Science Education and Technology, Vol. 13, No. 3, September 2004 (  C   2004) A Reconstruction of Structure of the Atom and ItsImplications for General Physics Textbooks:A History and Philosophy of Science Perspective Mar´ ıa A. Rodr´ ıguez 1 and Mansoor Niaz 1 , 2 Recent research in science education has recognized the importance of history and philoso-phy of science. The objective of this study is to evaluate the presentation of the Thomson,Rutherford, and Bohr models of the atom in general physics textbooks based on criteria de-rived from history and philosophy of science. Forty-one general physics textbooks (all pub-lished in the United States) were evaluated on two criteria based on Thomson’s work, threeon Rutherford’s work, and three on Bohr’s work. Results obtained show that general physicstextbooks do not systematically include a history and philosophy of science perspective. Mosttextbooks present an inductivist perspective in which experimental details are considered tobe paramount. On the contrary, a historical reconstruction of the experimental details in-evitably includes: the context in which an experiment is conducted, the theoretical frame-work that guides the scientist, and alternative interpretations of data that lead to conflictsand controversies. Examples are provided to show how historical reconstructions of atomicmodels can provide students an opportunity to appreciate how scientists work and scienceprogresses. It is plausible to suggest that textbook presentations based on a history and phi-losophy of science perspective can perhaps arouse students’ interest in the subject and hencelead to greater conceptual understanding. KEY WORDS:  history and philosophy of science; structure of the atom; general physics textbooks. INTRODUCTION In an attempt to understand, What is Science?,The American Physical Society has drafted a pol-icy statement, which has been endorsed by theAmerican Association of Physics Teachers (AAPT):“Science is the systematic enterprise of gatheringknowledge about the world and organizing and con-densing that knowledge into testable laws and the-ories. The success and credibility of science is an-chored in the willingness of scientists to: (1) ... (2)abandon or modify accepted conclusions when con-fronted with more complete or reliable experimen- 1 Epistemology of Science Group, Department of Chemistry, Uni-versidad de Oriente, Cuman´ a, Estado Sucre, Venezuela 6101A. 2 To whom correspondence should be addressed; e-mail: niazma@cantv.net tal evidence” (AAPT, 1999, p. 659). For most prac-tical purposes this definition is acceptable. Never-theless, a closer look reveals the problematic na-ture of What is Science?, and hence the need foran epistemological framework based on history andphilosophy of science. For example, the emphasison “gathering knowledge,” i.e., experimental dataand “complete or reliable experimental evidence”with no reference to controversy, explanation of databy rival theories, and the tentative nature of scien-tific knowledge, shows the complexity of the issuesinvolved. No wonder, one critic considers that byendorsing this statement the members of the Exec-utive Board of the AAPT had, “ ...  classified them-selves as nonscientists” (Auerbach, 2000, p. 305).Furthermore, on the basis of a history and philoso-phy of science (HPS) perspective, this critic pointsout that the word “complete” could be construed by 409 1059-0145/04/0900-0409/0  C  2004 Plenum Publishing Corporation  410 Rodr´ ıguez and Niaz Kuhnians (Kuhn, 1970) as “ ...  monotheistic Pop-perianism” (Auerbach, 2000, p. 305), viz., scientificknowledge is moving toward an increasing absolutetruth. Another critic recommends that we consultphilosophers of science on such issues and that, “ ... an important missing element in the definition is thatit is perfectly rational and acceptable to ‘believe’ oraccept as provisionally true, the best or most usefultheory available. If we understand the history of sci-ence we can’t help but think that better theories willemerge” (Forinash, 2000, p. 788).Philosopher-physicist Holton (1996) has recom-mended the inclusion of history and philosophy of science in the science curriculum and raised a voiceof concern with respect to the lack of interest inscience courses. Tobias (2000) in a similar vein hascalled for making physics more attractive by stim-ulating students’ interest and efforts to understand.Philosopher-physical chemist Michael Polanyi hasexplained that although scientific discoveries are richin controversy and interesting details, textbooks onthe contrary fail to arouse students’ interest: Yet as we pursue scientific discoveries through theirconsecutive publication on their way to the text-books, which eventually assures their reception aspart of established knowledge by successive gener-ations of students, and through these by the gen-eral public, we observe that the intellectual passionsaroused by them appear gradually toned down to afaint echo of their discoverer’s first excitement at themoment of Illumination  ...  A transition takes placehere from a heuristic act to the routine teaching andlearning of its results, and eventually to the mereholding of these as known and true, in the courseof which the personal participation of the knower isaltogether transformed (Polanyi, 1964, pp. 171–172). Kuhn (1970, p. 140), on the other hand, has ar-gued that science textbooks serve an important rolein initiation of students in scientific disciplines. Hav-ing recognized this pedagogical aspect of textbooks,Kuhn also deplores the fact that, “ ...  the student re-lies mainly on textbooks until, in his third or fourthyear of graduate work,  ... ” (p. 165).More recently, philosopher-physicist StephenBrush has emphasized the importance of includinghistory and philosophy of science in science text-books: “I have not seen any systematic studies of theinfluence on science textbooks of historical research.But, judging by the result of this very modest survey[srcins of quantum theory and relativity in physicstextbooks], I think we have a long way to go in per-suading textbook authors to pay attention to such re-search” (Brush, 2000, p. 54).At a Symposium “History of the Atom” held atthe annual meeting of the American Physical Soci-ety and the AAPT, Chicago, January 1980, GeraldHolton emphasized the importance of historical re-construction in order to understand our present the-ories of atomic structure, “ ... recount the intellectualfeats and defeats over more than 2500 years, fromThales to Bohr, without which we would not havegained the current bright prospects” (Holton, 1981,p. 25). At the same symposium, John Heilbron out-lined the difference between the recollections of aphysicist and that of a historian, which clearly consti-tute an important guideline for the historical recon-struction of theories in science textbooks: The historian necessarily has a point of view differ-ent from that of the recollecting physicist. As Diracdiscovered during a week spent with historians, wetake an interest in precisely what the physicist wants(and manages) to forget, ‘the various intermediatesteps and  ...  false trails’  ...  The false trails taken to-gether lead more directly to the historian’s goal of reconstructing the past state of science than the ret-rospectively clear highway of discovery (Heilbron,1981a, p. 223). This precisely is the dilemma of science text-books, as with retrospective hindsight all discoveriesappear to be the work of geniuses who did not haveto face the criticisms, rivalries, conflicts, passionatedebates, alternative interpretations of data by fellowscientists (Niaz, 1998). Apparently, we have two op-tions: leave the textbooks as they are, or to incorpo-rate pertinent historical details (reconstructions) inorder to present a picture of scientific progress basedon human efforts and vicissitudes.At this stage it is important to briefly review lit-erature to show that the inclusion of history and phi-losophy of science (HPS) can perhaps improve stu-dents’ understanding of science. Galili and Hazan(2000) have reported an year-long experimentalstudy designed to improve students’ understandingof physics by including historical details: “This studyprobed the effectiveness of teaching high school op-tics by means of materials heavily loaded with his-torical contents, which addressed known difficultiesin understanding optics. The positive evaluations,assessed by means of diagnostics  ... , qualitativelyand quantitatively support this approach to teachingoptics. The historical approach may be valuable inother fields as well, where the match between histori-cally employed models and na ¨ ıve conceptions of stu-dents is strong” (pp. 13–14). In a recent study, Niaz et al.  (2002) have shown how history and philosophy  Reconstruction of Structure of Atom 411 of science can facilitate freshman students’ concep-tual understanding of atomic structure (based on themodels of Thomson, Rutherford, and Bohr). Controlgroup students received instruction in the traditionalmanner, whereas the experimental group studentsparticipated in six classroom sessions (about 45 mineach) that involved arguments, counterarguments,and discussions within a HPS perspective. To eval-uate the HPS-based teaching strategy both groupswere evaluated on a monthly exam (three items) anda semester exam (two items). One of the items inthe semester exam is reproduced here: “How wouldyou have interpreted, if on using different gases inthe cathode-ray tube (Thomson’s experiment), therelation ( e / m ) would have resulted different?” Re-sults obtained showed that 37% of the experimen-tal group students and 5% of the control group pro-vided conceptual responses, and the difference wasstatistically significant ( χ 2 = 22.37,  p  <  0.001). Simi-larly, on the four other evaluation items experimen-tal group performed better than the control groupand the difference was statistically significant in allcases. Solbes and Traver (2001) have studied the ef-fect of introducing a history of science course in ahigh school curriculum (15- to 17-year olds). Theyear-longcourseincludedbiographiesillustratinghu-man aspects of scientific research based on srcinaltexts that emphasized the role of controversies in thedevelopment of scientific theories. Results obtainedshowed that (a) 58% of the control group and 38% of the experimental group believed that science devel-ops through discovery and the difference was statis-tically significant ( α <  0.001); (b) 42% of the controlgroup and 33% of the experimental group believedthat Newton’s work has not been modified, andthe difference was statistically significant ( α  =  0.1);(c) 29% of the control group and 15% of the ex-perimental group believed that experiments can in-validate a theory, and the difference was statisticallysignificant ( α <  0.001); (d) 87% of the control groupand 5% of the experimental group believed that sci-ence evolved through accumulation and the differ-ence was statistically significant ( α <  0.001).At this stage it is also important to briefly reviewsome studies that have evaluated textbooks within aHPS perspective. Justi and Gilbert (2000) analyzed12 high school textbooks (9 from Brazil and 3 fromUK) with respect to the representation of atomicmodels within a HPS perspective. It was found thatmost textbooks use “hybrid” models, i.e., compositesdrawn from several distinct historical models, whichby their very nature do not allow the manifestationof the different HPS aspects. Leite (2002) has devel-opedandvalidatedachecklistforanalyzinghistoricalcontextofsciencetextbooks.Shehasreportedresultsfrom the evaluation of five Portuguese high schoolphysics textbooks, which show that an adequate im-age of science and scientists’ work is not provided tothe students. Following studies have shown that mostchemistry textbooks published in the United Stateslack HPS perspectives: atomic structure (Niaz, 1998),elementary electrical charge (Niaz, 2000a), kinetictheory (Niaz, 2000b), covalent bond (Niaz, 2001a),and stoichiometry (Niaz, 2001b). Recent literaturealso provides a critique of previous attempts andalternatives for incorporating HPS in the curricu-lum and the textbooks (Rodr´ ıguez and Niaz, 2002;Seroglou and Koumaras, 2001). Similarly, variousstudies have drawn attention to the importance of epistemological beliefs of students, viz., their under-standing of what is science and how it progresses(Abd-El-Khalick and Lederman, 2000; Elby, 2001;Hammer, 1994; Redish  et al. , 1998).A recent study by Niaz (1998) based on a recon-struction of developments that led to the formula-tion of atomic models by Thomson, Rutherford, andBohr found that most general chemistry textbookslack the conceptualization of the “heuristic princi-ples” (Schwab, 1962) that led the scientists to designand interpret their experiments. For example, in thecase of J.J. Thomson’s work, besides the experimen-tal details of cathode ray experiments (emphasizedby most chemistry textbooks), the heuristic principleinvolved the testing of rival hypotheses, viz., a de-termination of the mass-to-charge ratio would havehelped toidentifycathode rayparticlesasions oruni-versal charged particles.The objective of this study is to evaluate thepresentation of the Thomson, Rutherford, and Bohrmodels of the atom in general physics textbooksbased on criteria developed in the previous study(Niaz, 1998) and a review of the literature in thisstudy. A HISTORICAL RECONSTRUCTIONOF STRUCTURE OF THE ATOMThomson’s Model of the Atom When Thomson conducted his experiments withcathode rays he was well aware of the controversywith regards to the nature of cathode rays: Weretheyparticlesorwavesintheether(Achinstein,1991;  412 Rodr´ ıguez and Niaz Falconer, 1987). It was the discovery of X-rays thattriggered in 1895 Thomson and other physicists’ in-terest in cathode rays. The controversy actually be-gan in 1879 with Crookes support for a particle the-ory of cathode rays. Deflection of cathode rays bya magnetic field was considered to provide strongsupport for a particle theory. Hertz (1892), on theother hand, showed that cathode rays were not de-flected by an electrostatic field, contrary to the pre-dictions of the particle theory (Falconer, 1987). Thisfinding provided support for the ether theory of cath-ode rays, according to which they were some sortof ethereal disturbance similar to light. Thomson’s(1897) article in the  Philosophical Magazine  can in-deed be considered a masterpiece of scientific andpedagogical reasoning and one can easily see howdifficult it is to interpret experimental data. Thom-son (1897) expressed the dilemma he faced in lucidterms: As the cathode rays carry a charge of negative elec-tricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on bya magnetic force in just the way in which this forcewould act on a negatively electrified body movingalong the path of these rays, I can see no escapefromtheconclusionthattheyarechargesofnegativeelectricity carried by particles of matter. The ques-tion next arises, What are these particles? Are theyatoms, or molecules, or matter in a still finer stateof subdivision? To throw some light on this point, Ihave made a series of measurements of the ratio of the mass of these particles to the charge carried byit. To determine this quantity, I have used two inde-pendent methods (Thomson, 1897, p. 302). This was the crucial aspect of Thomson’s article andhe clearly visualized that the determination of themass ( m ) to charge ( e ) ratio ( m / e ) of the cathoderays would help him to identify them as ions or auniversal charged particle. Let us now look at howsome of Thomson’s colleagues received his findingsat that time. FitzGerald (1897) proposed an alterna-tive explanation for cathode rays based on “free elec-trons” in the same issue of   The Electrician  (May 21,1897) in which an abstract of Thomson’s article hadappeared prior to publication in the  Philosophical Magazine  (October 1897). Apparently, FitzGeraldaccepted Thomson’s hypothesis that cathode rayswere corpuscles/primordial atoms/free electrons, buthe questioned (precisely the “hard core”) that thesecorpuscles were constituent parts of all atoms. Thisshows the importance of peer interaction and therole of alternative interpretations in scientific theorybuilding.Interestingly, two German physicists, Kaufmann(1897)andWiechert(1897)alsodetermined( m / e )ra-tios of cathode rays in the same year as Thomsonand their measurements agreed with each other.Falconer (1987) shows cogently that although exper-imental determinations of ( m / e ) ratios were impor-tant, their interpretation was even more important:“Kaufmann, an ether theorist, was unable to makeanything of his results. Wiechert, while realizing thatcathode ray particles were extremely small and uni-versal, lacked Thomson’s tendency to speculation.He could not make the bold, unsubstantiated leap,to the idea that particles were constituents of atoms.Thus, while his work might have resolved the cath-ode ray controversy, he did not ‘discover the elec-tron”’ (p. 251). It appears that Thomson’s ability tospeculate,elaboratealternativehypotheses andmod-els, and perhaps most importantly formulate a theo-retical framework for his experimental findings, ledhim to foresee and conceptualize what his contem-poraries ignored.Thomson, although no philosopher of science,perhaps tried to respond to his critics by outlining hismethodology in the following terms: From the point of view of the physicist, a theory of matter is a policy rather than a creed; its object is toconnect or coordinate apparently diverse phenom-ena, and above all to suggest, stimulate and directexperiment. It ought to furnish a compass which, if followed, will lead the observer further and furtherinto previously unexplored regions (Thomson, 1907,p. 1). Rutherford’s Model of the Atom In the very first paragraph of his famous arti-cle in the  Philosophical Magazine  Rutherford (1911)starts on a controversial note: It has generally been supposed that the scattering of a pencil of alpha or beta rays in passing through athin plate of matter is the result of a multitude of small scatterings by the atoms of matter traversed(p. 669). This of course referred to the experimental workof Crowther (1910), a colleague of Thomson.Rutherford (1911) explicitly points out thatCrowther’s experimental results provided supportfor Thomson’s hypothesis of compound scattering: The theory of Sir J.J. Thomson is based on the as-sumption that the scattering due to a single atomicencounter is small, and the particular structure  Reconstruction of Structure of Atom 413 assumed for the atom does not admit of a very largedeflexion of an alpha particle in traversing a sin-gle atom, unless it be supposed that the diameterof the sphere of positive electricity is minute com-pared with the diameter of the influence of the atom(p. 670). This served as a preamble for Rutherford (1911) topresent his side of the story in the following terms: The observations, however, of Geiger and Marsden(1909) on the scattering of alpha rays indicate thatsome of the alpha particles must suffer a deflexion of more than a right angle at a single encounter. Theyfound, for example, that a small fraction of the inci-dent alpha particles, about 1 in 20,000 were turnedthrough an average angle of 90 ◦ in passing througha layer of gold-foil  ...  A simple calculation based onthe theory of probability shows that the chance of an alpha particle being deflected through 90 ◦ is van-ishingly small ... A simple calculation shows that theatommustbeaseatofanintenseelectricfieldinorder toproducesuchalargedeflexionatasingleencounter  (p. 669, emphasis added). It is interesting to note that Rutherford had the ex-perimental data as early as June 1909 (Geiger andMarsden, 1909), to postulate his model of the nu-clear atom, and yet he did not do so until March1911. Darwin (1962) has traced the srcin of the “nu-clear atom” to a dinner conversation at Rutherford’sresidence just before Christmas in 1910. What hap-pened between June 1909 and March 1911 is im-portant not only for historians and philosophers of science, but also for physics teachers. Soon afterGeiger and Marsden (1909) published their results,Thomson and colleagues started working on the scat-tering of alpha particles in their own laboratory. Al-though results from both laboratories were similar,interpretations of Thomson and Rutherford were en-tirely different. Thomson propounded the hypoth-esis of   compound scattering , according to which alarge angle deflection of an alpha particle resultedfrom successive collisions between the alpha parti-cle and the positive charges distributed throughoutthe atom. Rutherford, in contrast, propounded thehypothesis of   single scattering , according to whicha large angle deflection resulted from a single col-lision between the alpha particle and the massivepositive charge in the nucleus. The rivalry betweenRutherford’s hypothesis of single scattering basedon a single encounter and Thomson’s hypothesis of compound scattering, led to a bitter dispute betweenthe proponents of the two hypotheses. At one stage,Rutherford even charged Crowther (1910), a col-league of Thomson, to have “fudged” the data inorder to provide support for Thomson’s model of theatom (Wilson, 1983). Heilbron (1981b) has provideda succinct account of the rivalry between Thomsonand Rutherford: Now in 1910 Thomson was the undisputed worldmaster in the design of atoms, and neither Braggnor Rutherford had yet tried their hand at thetype of quantitative model making at which heexcelled: they were challenging the acknowledgedleader of English physics, their own former teacher,in what had been his most exclusive preserve. AndRutherford was coming into open conflict withThomson’s ideas for the first time. The situation thusdid not lack a competitive aspect, and this, I thinkhelps explain the harshness of the attacks againstCrowther (Heilbron, 1981b, p. 134). On the one hand, Rutherford was entirely convincedand optimistic that his model of the atom explainedexperimentalfindingsbetter,andyetitseemsthattheprestige,authority,andevenperhapssomereverencefor his teacher made him waver. However, in a letterto Schuster (Secretary of the Royal Society), writtenabout 3 years later (February 2, 1914), Rutherford ismuch more forceful: ... I have promulgated views on which J.J.[Thomson] is, or pretends to be, skeptical. At thesame time I think that if he had not put forwarda theoretical atom himself, he would have comeround long ago, for the evidence is very stronglyagainst him. If he has a proper scientific spirit Ido not see why he should hold aloof and the merefact that he was in opposition would liven up themeeting (Reproduced in Wilson, 1983, p. 338). Bohr’s Model of the Atom On the very first page of his now famous tril-ogy published in the  Philosophical Magazine , Bohr(1913) starts on a controversial note by pointing outthe difficulties associated with Rutherford’s model of the atom: In an attempt to explain some of the proper-ties of matter on the basis of this atom-model[Rutherford’s] we meet, however, with difficulties of a serious nature arising from the apparent instabil-ity of the system of electrons: difficulties purposelyavoided in atom-models previously considered, forinstance, in the one proposed by Sir J.J. Thomson(p. 2). In the fourth paragraph Bohr (1913) formulates hisepoch-making postulate:
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