Arrhenius' Criticism's of the Thomson Model

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I. Svante Arrhenius

I want to make it clear that this man is no lightweight, either physically (he was rather portly) or intellectually (1903 Nobelist in Chemistry). His major contribution to chemistry (and it was a big one) was the theory of electrolytic dissociation.

This is the idea that electrolytes (substances that complete an electrical current when dissolved) split into ions when dissolved in solution. It seems like a real obvious idea, but when it was proposed (in the early 1880's), it was meet by stiff resistance.

It must have been a satisfying moment when he won the 1903 Nobel, since it was given in recognition of the electrolytic dissociation theory. The Nobel Institute used this wording:

"in recognition of the extraordinary services he has rendered to the advancement of chemistry by his electrolytic theory of dissociation"

II. Three Criticisms of Thomson's Model

1) Arrhenius said about the Thomson Model:

"This conception has hitherto remained only a formal one, and has led to no new results."

By this he means there is no prior evidence which leads one to develop the Thomson Model of the Atom. Also, there is not any evidence that can only be explained by the Thomson Model. And, finally, he is explicit that it has not led to any new predictions or results. This is a sine qua non of a scientific theory, that the theory can be used to predict new results previously unknown or explain results previously unexplained.

2) Thomson's Model attributed ALL of the mass of an atom to its electrons, so atoms such as sodium or magnesium must have many thousands of electrons. Consequently, Arrhenius points out:

". . . the atomic weight ought to increase with the positive valency in every series."

Here, Arrhenius had been discussing how Thomson's model fit into the periodic table. However, there is a problem, as Arrhenius points out in his next sentences:

"This seems not to be the case. There are two very prominent exceptions to this rule. The one is argon, which has a higher atomic weight than potassium (39.8 against 39.1). The figure 38 for argon given by Mendeleeff is only a "theoretical" value, computed in order to retain the simplicity of his scheme; the experiments of Ramsay1 give 39.8 for argon, and potassium has been very accurately determined to be 39.1. The other exception is met with in the case of the two elements tellurium and iodine, with the atomic weights 127.6 and 126.97 respectively."

In other words, if electrons keep adding onto each element in turn, you should never have a case where the mass goes down when you go to the next element. It should always go up.

Arrhenius continues:

"This difficulty is only a specially striking instance of a more general one; the systems in Thomson's series differ from each other by one electron, so that the difference between two consecutive atomic weights is constant. This does not agree with the much more complicated behaviour of the natural elements. In the series 2 this difference varies between 1.05 and 3.39, i.e. in the proportion 1 : 3.2, in a somewhat irregular manner. In other series this variation is of the same order."

In other words, let's take the series of sodium, magnesium, and aluminum. Na+ loses one electron, Mg2+ loses two, and Al3+ loses three. If all the mass of an atom is in the elecrons, then the mass change from Na+ to Mg2+ to Al3+ should be constant. It is not.

3) This third difficulty has to do with the known low mass of the electron. Arrhenius knew that the electron was only about one two-thousandth the mass of a hydrogen atom. Consequently, he writes:

Another difficulty, and perhaps the greatest one, is that experiment shows that the mass of the electrons is only about the two-thousandth part of that of the hydrogen atom. According to this idea, it would be natural to suppose that the neon atom consists of 39,800 electrons, the sodium atom of 46,100. It would, therefore, not be one step from neon to sodium, but 6300. Hence it will be necessary to add to Thomson's scheme the supposition that not one electron, but a great number of electrons, together with the corresponding quantity of positive electricity, makes the difference between two consecutive elements. And as the differences between the atomic weights of consecutive elements are not at all constant, it will be necessary to assume that the complex of electrons is very different in different cases. By this amendment the Thomson scheme loses its simplicity, and at the same time much of its theoretical value.

In other words, to keep consistent with the known masses of the atoms, there have to be differences of thousands of electrons between atoms on the periodic table. However, the differences in ions is in only one-electron steps.

These two problems cannot be reconciled and Arrhenius knows this, hence his final sentence in the quote above.

There is more, but I will let you read Arrhenius' full commentary. The comments were made in the summer of 1904, but not published until 1907. Hence, Arrhenius was able to edit in words (see p. 102) to include discussion of Thomson's 1906 paper, which destroyed the idea of thousands of electrons in a given atom.

This 1906 paper has been called (by John Heilbron of UC Berkeley) one of the most important papers in the history of atomic structure.

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