The value of telling it like it was. The best honours project ever?

Introduction

There are differing views as to whether the historical context of the chemistry we teach should be included in chemistry instruction, or in a separate ‘science history’ course. However, wherever it is taught, there can be tremendous opportunity in summarising chemical discoveries in our chemistry teaching. Equally, valuable opportunities can be lost in the telling. For example, many current textbooks correctly ascribe the ‘discovery of the nucleus’ to Rutherford, Geiger and Marsden, specifically in the context of the detection of back-scattered ⍺-particles, reported between 1909¹ and 1911.^2,3 However, they jump from that statement to a description of the nucleus being positively charged and containing protons and neutrons. None of this is ‘wrong’ – it just didn’t happen quite like that. Such a description suggests, incorrectly, that the concept of the nucleus, the ‘Rutherford nucleus’ emerged fully fledged contemporaneously with the Rutherford, Geiger and Marsden experiments. We demonstrate, using selected quotations from the contemporary literature, that the ‘incorrect’ model of the nucleus persisted for almost one-quarter of a century. This illustrates several important pedagogical principles. Firstly, showing students that the evolution of chemical concepts is not a series of lightbulb moments. Secondly, the importance of careful observation and of not ignoring the seemingly inexplicable, thirdly, that there is great benefit in testing one’s understanding, and finally, and perhaps most importantly, that the ‘wrong’ ideas can have real value. Significantly then, students can understand that the chemistry we are teaching is not static and ‘immutable’, but that it is dynamic and evolving.

Results and discussion

Regarding the first point, Rutherford, himself, explained:

it is not in the nature of things for any one man to make a sudden violent discovery; science goes step by step, and every man depends on the work of his predecessors. When you hear of a sudden unexpected discovery – a bolt from the blue as it were you can always be sure that it has grown up by the influence of one man on another, and it is this mutual influence which makes the enormous possibility of scientific advance. Scientists are not dependent on the ideas of a single man, but on the combined wisdom of thousands of men, all thinking of the same problem, and each doing his little bit to add to the great structure of knowledge which is gradually being erected. [Rutherford, pp. 73–74⁴].

We should not teach chemistry as if it were otherwise.

Rutherford’s story starts in New Zealand. It’s been told many times,⁵ so we’ll start with a young Mancunian, Ernest Marsden, the son of a Rishton shopkeeper and scion of an ancient family of Darwen in Lancashire. Marsden completed his secondary studies at the Queen Elizabeth Grammar School in Blackburn and commenced his undergraduate degree at the University of Manchester in October 1906, as a Lancashire County Council Scholarship holder. He also supported himself by teaching at Manchester Grammar School, until, at the end of his third year, Marsden won a Hatfield Scholarship, and began his Physics Honours under the supervision of Hans Geiger, a John Harling Fellow and research assistant of the newly arrived professor of physics, the 1908 Chemistry Nobel Laureate, Ernest Rutherford.

At the time, the received wisdom, as enunciated by W. H. Bragg, was that ⍺-particles should pass through matter without deviation.⁶ However, Rutherford reported (in part in response to a query from Bragg⁷) in 1906 the scattering of ⍺-particles as they passed through matter,⁸ and soon confirmed the effect with Geiger in 1908.⁹ Geiger remarked to Rutherford ‘Don’t you think that young Marsden, whom I am training in radioactive methods, ought to begin a small research?’, to which Rutherford suggested ‘Why not let him see if any ⍺-particles can be scattered through a large angle?’,⁴ then subsequently, to Marsden, ‘See if you can get some effect from ⍺-particles directly reflected from a metal surface’.¹⁰ Marsden was tasked with counting ⍺-particles passing from an intense source through a long glass tube. Much to his surprise, Marsden observed a small number of ⍺-particles reflected through unexpectedly large angles.¹ Rutherford admitted later ‘I did not have any good reason to expect a positive result’, ‘I did not believe they [⍺-particles] would be [back-scattered]’,⁴ and Marsden supposed the experiments were designed as a test of his experimental competence.¹¹ So it was that, as Marsden later recounted:

One day Rutherford came into the room where we were counting the ⍺-particles at the end of a 4 ½ metre firing tube. We had been having trouble to obtain a constant figure and this was obviated by putting in the firing tube, at right angles to the axis a series of washer-like openings, to stop particles scattered from the sides of the wall by what we thought might be molecular-size protuberances. It almost appeared, however, as though they were reflected from the walls. We know now that such numbers would be small. Nevertheless, Rutherford had been thinking over the matter and he turned to me and said: ‘See if you can get some effect of ⍺-particles directly reflected from a metal surface’. I do not think he expected any such result, but it was one of those ‘hunches’ that perhaps some effect might be observed and that in any case that neighbouring territory of this Tom Tiddler’s ground might be explored by reconnaissance … Rutherford was ever ready to meet the unexpected and exploit it where favourable, but he also knew when to stop on such excursions. To my surprise, I was able to observe the effect looked for and I collected reflectors of metals from aluminium to platinum and made comparative measurements. I remember well reporting the results to Rutherford a week after, when I met him on the steps leading to his private room. [Rutherford, p. 68⁴; see also Pais, p. 189¹²; and Marsden, p. 8¹³]^12–14

Geiger seems to have already reported the results to Rutherford ‘I remember two or three days later Geiger coming to me in great excitement and saying, “We have been able to get some of the ⍺-particles coming backwards”.’ Geiger reminisced:

One day (in 1911) Rutherford, obviously in the best of spirits, came into my room and told me that he knew now what the atom looked like and how to explain the large deflections of the ⍺-particle. On the very same day I began an experiment to test the relation expected by Rutherford between the number of scattered particles and the angle of scattering [Geiger, in Eve et al., p. 405⁵; and Andrade, p. 111¹⁵].

Rutherford was amazed, and commented later with the benefit of 20:20 hindsight, ‘It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you’.⁴ By May 1909, Geiger and Marsden had submitted their paper describing the ‘diffuse reflection of the ⍺-particles’.¹

Of all the honours^A,16 (final year undergraduate) research projects, one would be hard pressed to find one more consequential than that given by Rutherford and Geiger to Ernest Marsden.

In his 1911 papers, Rutherford proposed several predictions of his model, the cross section of scattered particles as a function of angle, the dependence of scattering on foil thickness and nuclear charge, and the dependence of scattering on particle velocity.⁴ He also noted, ‘the main deductions from the theory are independent of whether the central charge is supposed to be positive or negative. For convenience, the sign will be assumed to be positive’.²

Geiger and Marsden began experiments^17,18 successfully testing each of Rutherford’s predictions ‘in a series of beautiful experiments’.^4,19 Fleming, in his tribute to Marsden, was to comment:

This is one of the most beautiful series of experiments which has ever been performed, as well as one of the most significant [Fleming, p. 464²⁰].

Rutherford cautiously tried out his nuclear theory in public on Tuesday 7 March 1911,³ at a meeting of the Manchester Literary and Philosophical Society to which he invited his research group, most of whom knew only that ‘he’d got some interesting things to say and he thought we’d like to hear them. We didn’t know what it was about at that time’.²¹ He described a model of the atom ‘which consists of a central electric charge concentrated at a point and surrounded by a uniform spherical distribution of opposite electricity equal in amount’ and that ‘The main results of large scattering are independent of whether the central charge is positive or negative’. Rutherford submitted his full paper to The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science the following month,² in which he reiterated, ‘The deductions from the theory so far considered are independent of the sign of the central charge, and it has not so far been found possible to obtain definite evidence to determine whether it be positive or negative’.

Rutherford in his 1911 full paper provided an explanation for the results of Geiger and Marsden as ‘it seems simplest to suppose that the atom contains a central charge distributed through a very small volume [and] that the value of this central charge for different atoms is approximately proportional to their atomic weights’,² Marsden later referred to this paper as describing ‘the nucleus theory of the atom’.²²

Rutherford was cautious. He appears not to have discussed publicly his model while attending the famous Solvay Conference in Brussels later that year.²³ However, by 1913, Rutherford had settled on a positively charged²⁴ central electric charge that he had christened the nucleus in his 1913 textbook:

To account for the experimental results, it is necessary to suppose that the atom is the seat of such intense electrical forces that occasionally the ⍺-particle can be deflected from its path through more than a right angle in an encounter with a single atom. This indicates that the atom must contain a highly concentrated charged nucleus, and that the a particle in passing through the atom close to the nucleus suffers a wide deflection of its orbit [Rutherford, p. 184²⁵].²⁴

So, although the ‘Rutherford model’ eventually incorporated a positively charged nucleus,²⁴ this was not Rutherford’s initial interpretation of the experiments of Geiger and Marsden, and was a concept that evolved with (not inconsiderable) time.

Rutherford’s nuclear model at that time was of an extremely small, heavy (relative to an electron) charged nucleus (‘the atom must contain a highly concentrated charged nucleus … [and] this central charge is concentrated at a point’²⁵), about which circulated the extranuclear electrons. He was more expansive in a paper delivered during a discussion on the structure of the atom, held by the Royal Society on 19 March 1914.^26,27 He stated that the nucleus consists ‘of positive particles and electrons’. Rutherford described the constitution of the nucleus in the following terms ‘The exceedingly small dimensions found for the hydrogen nucleus add weight to the suggestion that the hydrogen nucleus is the positive electron’,²⁷ … ‘The mass of the atom is, however, dependent on the number and arrangement of the positive and negative electrons constituting the atom’.² Notably, the term ‘positive electron’ was still in use.

Rutherford did caution ‘Until, however, the nucleus theory has been more definitely tested, it would appear premature to discuss the possible structure of the nucleus itself’²⁷ and commented, ‘An important question arises whether the atomic nuclei, which all carry a positive charge, contain negative electrons’.²⁷

As the British spring turned to summer, Rutherford set sail for Australia, to attend the 84th Meeting of the British Association for the Advancement of Science, held in Adelaide, Melbourne and Sydney. In Sydney on Tuesday 18 August 1914, he chaired a discussion of the structure of the atom and told his audience:

I have found it necessary to believe that there is a concentrated nucleus in the atom (having a certain number of units of charge), in which the main part of the mass resides; outside this there are a corresponding number of electrons … it is probable that the hydrogen nucleus is simply the positive electron with a large electrical mass due to the great concentration of the positive charge [Rutherford, p. 293²⁸].

and obtained mixed responses:

I am not inclined to agree with Professor Rutherford that the nucleus of a hydrogen atom is necessarily the positive electron. It seems to be more complicated. [Prof. Nicholson, in the British Association for the Advancement of Science report, p. 300²⁸].

Bohr recollected ‘the Rutherford work was not taken seriously. We cannot understand today, but it was not taken seriously at all. There was no mention of it in any place. The great change came from Moseley’.²⁹

The Rutherford model of the nucleus was, then, at that time, one of a very small volume, containing positive and negative electrons of sufficient relative number that the sum of their charges would balance the sum of the charges of the electrons circulating around the nucleus with the positive electron possibly being the hydrogen atom.

In his 1914 Discussion on the structure of the atom,²⁷ Rutherford stated, ‘I have suggested that … the hydrogen nucleus is the positive electron’, and the nucleus consists of ‘positive particles and electrons’,¹² and ‘the suggestion that the hydrogen nucleus is the positive electron … it is to be anticipated that the helium atom contains four positive electrons and two negative… The mass of the atom is, however, dependent on the number and arrangement of the positive and negative electrons constituting the atom’.²⁷ ‘[Geiger and Marsden] concluded that the nucleus charge was equal to about half the atomic weight multiplied by the electronic charge’.²⁷

In order to account for this large angle scattering of α particles, I supposed that the atom consisted of a positively charged nucleus of small dimensions in which practically all the mass of the atom was concentrated. The nucleus was supposed to be surrounded by a distribution of electrons to make the atom electrically neutral, and extending to distances from the nucleus comparable with the ordinary accepted radius of the atom… Since the experimental evidence indicates that the nucleus has very small dimensions, the constituent positive and negative electrons must be very closely packed together [Rutherford, p. 493²⁷].

Rutherford’s model of the atom – a positively charged nucleus containing positive and negative charges and occupying an extremely small fraction of the atomic volume and about which circulated a number of electrons sufficient to balance the charge of the nucleus, was relatively slow in gaining traction.^7,12 Not only was the model of the nucleus ‘wrong’ by today’s standards, but the model of the atom was at least improbable, if not impossible. ‘Planetary’ models had been considered previously, and it was well appreciated that the circulating electrons, particularly those of the lighter elements, would be unstable.³⁰ Carl Runge is reported to have asked Helmholtz whether a planetary model might explain Runge and Kayser’s elemental spectroscopic observations, whereupon Helmholtz had:

sass eine Weile im Gedanken und dann hörte ich ihn sagen: Hm, ja die Planeten, wie ist das doch? Die Planeten – auch nein, dass geht wohl nicht [sat in thought for a while and then I heard him say: Hmm, yes the planets, what is that like? The planets – no, that’s probably not possible]. [Runge, in Pais, p. 182¹²]

However, at that time, Niels Bohr was visiting Manchester, via Cambridge from Copenhagen, to spend a few months with Rutherford during the summer of 1912. On returning to Denmark, Bohr turned the power of quantum theory upon the ‘Rutherford atom’, publishing his results in support of the ‘Rutherford atom’ from July 1913,^31–33 in papers with immediate impact.¹²

Additionally, Henry Moseley had arrived in Manchester in 1910 as a John Harling Fellow and soon began working on the X-ray spectra of the elements.^30,34 In December 1913,³⁵ and April 1914,³⁶ Moseley published his two articles establishing that the K_α X-ray frequencies of individual elements increased uniformly between nearest neighbours of the periodic table, commenting that ‘such data … strongly support the views of Rutherford and of Bohr’, giving a measure of the charge on the nucleus.³⁵

Although Rutherford commented in Melbourne that ‘what [the difficulty in explaining atomic stability] points to is that there is something wrong with the theory of electromagnetic radiation – not of the atom’,²⁸ two decades would pass before Chadwick detected the existence of the neutron.^37,38

Then came the war and Rutherford’s research group was dispersed. Rutherford was engaged in defence work, Moseley was killed at Gallipoli, Marsden returned from New Zealand and served with the Royal Engineers in France (winning the Military Cross), Chadwick, who had been studying with Geiger in Berlin, languished in the Ruhleben internment camp near Berlin, de Hevesy had moved to Vienna and was drafted into the Austrian–Hungarian army, Bohr was in Denmark, and the mathematical physicist, Darwin,³⁹ was in France with the Royal Engineers, then with the RAF.

In 1919, Rutherford published the detection of the positively charged ‘swift hydrogen atoms’,^40–43 evidence for which had been presented by Marsden²² before the intervention of WWI and Marsden’s assumption of a professorship at the now Victoria University of Wellington. The ‘swift hydrogen atom’ would be christened the proton on a summer’s evening in Cardiff the following year. This particle was detected in a process Rutherford didn’t pretend to understand, but which was shown by mature-age student and future Nobel Laureate, Patrick Blackett, in 1925⁴⁴ to be the transmutation of nitrogen to oxygen.

In 1920, Rutherford commented in his 3 June Bakerian Lecture, ‘We also have strong reason for believing that the nuclei of atoms contain electrons as well as positively charged bodies, and that the positive charge on the nucleus represents the excess positive charge’ (p. 377⁴⁵) and ‘In considering the possible constitution of the elements, it is natural to suppose that they are built up ultimately of hydrogen nuclei and electrons’ (p. 395⁴⁵).

On 25 August 1920, in a contribution entitled The Building up of Atoms, Rutherford reprised his Bakerian Lecture at the Cardiff meeting of the British Association for the Advancement of Science. He described a model in which ‘the central nuclei of various elements are built up of aggregations of elementary positive charges and negative electrons’ and that he regarded ‘the fundamental positive charge … as the nucleus of the hydrogen atom, i.e. a hydrogen atom with the outer electron removed’.⁴⁶ At the time, Rutherford seemed reticent to name this particle and it fell to Sir Oliver Lodge to observe ‘that an entity of such importance should have a name and suggested “proton”’.^46,47 At that Cardiff meeting, Eddington in his presidential address was clear that there was:

no room for doubt that all the elements are constituted out of hydrogen atoms bound together with negative electrons [with the example that] The nucleus of the helium atom … consists of 4 hydrogen atoms bound with 2 electrons [Eddington, p. 354⁴⁸].

In response to Lodge’s invitation, Rutherford had suggested ‘prouton’ as an alternative to ‘proton’, referencing Prout’s hypothesis of 1815.^49,50

After comparing the densities of gases relative to hydrogen,⁵¹ Prout had hypothesised, initially anonymously, that ‘we may almost consider the πρώτη ΰλη [protí ÿli, raw material] of the ancients to be realised in hydrogen’.^52,53 Well intentioned, as was this homage to Prout, ‘proton’ was not universally welcomed,⁵⁴ and the name took a little time to land.

Meanwhile, half a world away, David Orme Masson, professor of chemistry at The University of Melbourne, was putting the finishing touches to a paper, ‘The constitution of atoms’, in which he observed ‘since it must now be conceded that all material atoms are compounded of positive and negative electrical atoms, it is surely time that each of these fundamental and universal constituents were known by some distinctive name’ and suggested that since ‘the outstanding characteristic of the positive particles is that they mainly determine the mass of the atom … they should be called barons (βάροσ, weight)’. He sent this to Rutherford on 8 October 1920, with the request that Rutherford submit the paper for publication. Rutherford obliged and the paper appeared in The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, with a generous footnote from Rutherford:

At the time of writing this paper in Australia, Professor Orme Masson was not aware that the name ‘proton’ had already been suggested as a suitable name for the unit of mass nearly 1, in terms of oxygen 16, that appears to enter into the nuclear structure of atoms. The question of a suitable name for this unit was discussed at an informal meeting of a number of members of Section A of the British Association at Cardiff this year. The name ‘baron’ suggested by Professor Masson was mentioned, but was considered unsuitable on account of the existing variety of meanings. Finally the name ‘proton’ met with general approval, particularly as it suggests the original term ‘protyle’ given by Prout in his well-known hypothesis that all atoms are built up of hydrogen. The need of a special name for the nuclear unit of mass 1 was drawn attention to by Sir Oliver Lodge at the Sectional meeting, and the writer then suggested the name ‘proton’.

Professor Orme Masson sent the present paper for publication through the writer, and in order to avoid the long delay involved in correspondence, his paper is printed in its original form. If the name ‘proton’ is generally approved, it is merely necessary to change the symbol ‘ b’ into ‘p’ in the chemical equations given in the paper [Rutherford, footnote in Masson, p. 282⁵⁵].

Masson, who was knighted the following year, was equally gracious:

…nobody suggested any specific name for it [the positive particle] till about four months ago. In a paper which I sent home [England] early in October, I ventured to suggest the name ‘baron’, selecting this as suggestive of its most outstanding character; for it is some 1800 times as massive as the electron and the weight of any material atom is appreciably determined by the number of positive particles in its nucleus, the electron contributing a practically negligible mass, or weight. I was not – and could not then be aware, but I heard from Sir Ernest Rutherford last week, that the question on naming the positive particle was raised by Sir Oliver Lodge at the British Association meeting in September and was discussed among the members of Section A. Rutherford himself then suggested proton and this met with general acceptance. A distinct point in its favour is that it recalls the protype of Prout’s hypothesis, which – after a century of discredit – had now been finally verified. It is, however, open to one rather serious objection – that it claims for the positive particle a priority over the electron, which is in no sense warranted and perhaps also to this other – [a comment of remarkable prescience] that the day may come when by the advance of knowledge the duty is cast upon us or our successor of christening the parents of both sorts of electrical atoms. Will it then be convenient to talk of protoproton?

However that may be, Rutherford of all men, has an incontestable right to act as god-father in this case, and I have no doubt that proton will be generally accepted [Masson, in Weickhardt, p. 115⁵⁶].

Masson with his characteristic whimsy, but sadly reflecting the mores of a thankfully all but forgotten past, continued:

We may now speak of the proton and the electron as the mother and father of all our elements, and of the whole natural universe. As to which is the mother and which the father, it is perhaps unnecessary to push the metaphor, but we may remember that the more massive and slow moving proton has its special field of influence in the nucleus of the atoms and stays at home there, whereas the more mobile electron often wanders far afield, circulating round the home or even travelling independently and producing disturbing effects on its environment. [Masson, in Weickhardt, p. 115⁵⁶]

The Nobel Laureate, Sir MacFarlane Burnet, recalled, attending, as a 19 year old, an undergraduate lecture in which Masson discussed the term ‘baron’, ‘But why use a Greek derivative’, said Masson, ‘it means exactly the same as Masson’.⁵⁶ Masson, Rutherford and the participants at the 1920 Cardiff Meeting of the British Association for the Advancement of Science were presumably unaware of an earlier, slightly different, use of the term ‘proton’.^57,58

With the publication of Rutherford’s concept of the nucleus, the scales fell from Frederick Soddy’s eyes, explaining the nature of isotopes in 1913:

The same algebraic sum [net charge] of the positive and negative charges in the nucleus, when the arithmetic sum [total number of charged particles] is different, gives what I call ‘isotopes’ or ‘isotopic elements’, because they occupy the same place in the periodic table. They are chemically identical, and save only as regards the relatively few physical properties which depend on atomic mass directly, physically identical also. Unit charges of this nuclear charge, so reckoned algebraically, give the successive places in the periodic table. For any one ‘place’, or any one nuclear charge, more than one number of electrons in the outer-ring system [the core and valence electrons] may exist, and in such a case the element exhibits variable valency. But such changes of number, or of valency, concern only the ring and its external environment. There is no in- and out-going of electrons between ring and nucleus [Soddy, p. 400⁵⁹].

The term isotopes (extra proton–electron pairs in the nucleus) had been suggested to Soddy by Dr Margaret Todd during a discussion in the drawing room of Soddy’s father-in-law.⁶⁰

In his 1922 Nobel Lecture,⁶¹ Frederick Soddy, marking his 1921 chemistry award ‘for his contributions to our knowledge of the chemistry of radioactive substances, and his investigations into the origin and nature of isotopes’ described the then current understanding of the atom. Soddy said, ‘In 1911 Rutherford put forward, in a somewhat tentative form, his now well-known “nuclear theory” of atomic structure’ (p. 391⁶¹) and summarised that Rutherford:

supposed that, at the centre of the atom, there existed a nucleus of very minute dimensions relatively to the atomic volume, upon which was concentrated a large charge of one sign, the rest of the atom being occupied by a number of single charges of the opposite sign which neutralized the central or nuclear charge… It followed therefore that the place [of an element] in the Periodic Table is an expression of the nett nuclear charge, i.e. of the difference between the numbers of positive and negative charges in the nucleus. Thus, the chemically identical elements – or isotopes, as I called them for the first time in this letter to Nature, because they occupy the same place in the Periodic Table – are elements with the same algebraic or nett nuclear charge, but with different numbers of + and − charges in the nucleus. On the view that the concentrated positive charge is the massive particle in the atomic structure, since positive electricity has never been observed free possessing less than the mass of an atom, the atomic weight of the isotope is a function of the total number of positive charges in the nucleus and the chemical character a function of the nett number. Though the nucleus possesses electrons there can be no in- or out-going of electrons between the nucleus and the external electronic system… Changes of the number of electrons in the external system are chemical in character and produce changes in the valency of the element. These are reversible and have no effect at all on the central nucleus. Whereas changes of the nucleus are transmutational and irreversible, and they instantly impress changes upon the external electronic system to make it conform to the new nucleus [Rutherford, in Soddy, p. 392⁶¹].

Soddy, then, initially envisaged isotopes as elements with the same number of extranuclear electrons, but having extra pairs of protons and electrons in the nucleus.

Conclusions

Afterword