The Discovery of the Electron, Proton, and Neutron

B a d M. Peake University of Otago. Box 56. Dunedin, New Zealand

Introductory courses in chemistry invariably include an account of the historical development of our ideas on the structure of the atom. The three fundamental particles mak- ing up an atom are introduced, naturally, but the history of their discovery and the origins of their names, particularly for the proton, does not appear to be well documented.

The existence of a fundamental unit of electricity was fvst suggested by Michael Faraday in 1834 (1) to account for his results involving the electrolysis of solutions of aqueous ac- ids and salts: " . . . if we adopt the atomic theory or phraseol- oev. then the atoms of bodies which are equivalent to each o&r in their ordinary chemical action, hive equal quanti- ties of electricity associated with them." This idea was suh- sequently extended by other scientists including George J. Stones, who first proposed in 1874 (2) the name "electrine" for thkunit of char& on a hydrogen ion; he subsequently changed this in 1891 (3) to the name "electron".

The ohservation of a small number of laree angle deflec- - - ~ ~ ~ ~~~ ~ - - tions of alpha particles (He2+) incident upon a gold metal foil led Ernest Rutherford in 1911 (4) to suggest that the ~ ~

nucleus of an atom is very small and p&itivel;iharged. Two vears later he concluded from the results of experiments k d v i n g the scattering of alpha particles from simple gases (5) that "the hvdrogen atom has the simplest possible struc- t&e of a nucieus k t h one unit char&." It appears that Rutherford was also the first tentatively to suggest the name "proton" for this fundamental particle. This occurred at an informal meeting around 1920 of the Physics Section of the British Association (6): "the name proton met with general approval, particularly as it suggests the . . . term 'protyle' eiven bv Prout in his well-known hwothesis that all atoms are huik upof hydrogen." The term;&elf is derived from the Greek orotos (first). and Samuel Classtone (7, has noted . .. that it'had heen used as far hack as 1908 or earlier as a general term for a unit from which all elements were built.

From observations of the nature of the particles formed from alpha particle scattering from Nz, Rutherford was also the first to suggest in 1920 (8) that". . . it may be possible for an electron to combine much more closely with the H uucle-

ns, forming a kind of nuclear doublet. Such an atom would have very novel properties." In discussing the classification scheme for isotopes, William Draper Harkins first intro- duced in 1921 (9) the term "neutron": " . . . a term represent- ing one negative electron and one hydrogen nucleus". It is interesting to note, however, that the same term had also been used by Walther Nernst (10) at least 10 years earlier in the context of ". . . a compound of positive and negative electrons . . . an electrically neutral massless molecule".

The actual ohservation of such afundamental particle had to wait until some time later when, in 1930, Bothe and Becker (11) reported that exposure of light elements, in particular beryllium, to alpha rays leads to a highly pene- trating radiation. In 1931-1932 the Curidoliot and Joliot (12) reported that exposure of hydrogen-containing materi- al, particularly paraffin, to this new radiation lead to the ejection of high-velocity protons. At the same time Chad- wick (13) interpreted both these sets of results in terms of radiation consisting "of particles of mass nearly equal to that of the proton and with no net charge", which he identified with the neutron species first postulated by Rutherford.

These details should enable teachers to complete the his- torical account of the development of our present knowledge of atomic structure, which appears to he otherwise well pre- sented in many introductory chemistry texts.

1. Faraday,M.Phil. l b m 8 . R ~ ~ . So.. (London) LsS1,IPI, (Paragraph8691. RepMtcdin Erpe"rncnfolReseorchesinElsct"rity:Dover: N w York, 1965.

2. Staney, G. J.Phil. Max. 1881.11,381. 3. Stoney, 0. J. Sei. TT.N_RDx Dublin So.. 1891,4,563-607. 4. Rutherford.E.Phi1. Mog. 1911.21.669-688. 5. Rutherford. E.Phil. Mag. 1913,26,702-712. 6. Footnofo (P 2821 toM-n. 0 Phil. Mog. 1921.91,~l-285. 7 . GI-tone,S.SoumbookanAfa~nieEnergy; Mecmilliao: London. 19%f&notep42. 8. Rutherford,E.Roc.Ray.Soc. 1920,97,37MW. 9. Harkin., W.D. J.Am. Cham.Soc. 1921.43,103L%1060.

10. Nernst, W . Theoretical Chernisfly, 3rd ed.; M a r m h London, 1911: p 396. 11. Bothe, W.; Brker, H. 2. Phya. 1930,67.28%306. 12. CurieJoliot, I.; Jaliot, F. Compl. R e d . 1931.143, 1412-1414,l4lb1417: 1932,144,

273-275, 708-711,876817. 13. Chadeai.k.J.Notve 1932 ,129 .312;h .Ro~. SSS. 1932,136.692708: 1935,142.1-25.

738 Journal of Chemical Education