The Thomson Model of the Atom

The Thomson Model of the Atom

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In 1897, J.J. Thomson discovered the electron, the first subatomic particle. He also was the first to attempt to incorporate the electron into a structure for the atom. The internal structure of the atom had been a source of speculation for thousands of years. The Greeks taught that the atom was solid, as did Dalton. Although Dalton did allow for the fact that there might be a sub-atomic structure of which he was unaware.

Thomson faced two major problems: (1) how to account for the mass of the atom when the electron was only about 1/1000 the mass of the hydrogen atom (the more modern figure is 1/1836) and (2) how to create a neutral atom when the only particle available was negatively charged.

His solution was to rule the scientific world for about a decade and Thomson himself would make a major contribution to undermining his own model.

I. Leadup to Thomson's 1904 Model of the Atom

Thomson had been in the business of proposing atomic models since at least 1881, which is when he proposed his "vortex" model of the atom. We will not go into details about it.

The first seed of the model we are discussing appear in his famous 1897 announcement of the discovery of the electron. He wrote:

"The explanation which seems to me to account in the most simple and straightforward manner for the facts is founded on a view of the constitution of the chemical elements which has been favourably entertained by many chemists: this view is that the atoms of the different chemical elements are different aggregations of atoms of the same kind. In the form in which this hypothesis was enunciated by Prout, the atoms of the different elements were hydrogen atoms; in this precise form the hypothesis is not tenable, but if we substitute for hydrogen some unknown primordial substance X, there is nothing known which is inconsistent with this hypothesis, which is one that has been recently supported by Sir Norman Lockyer for reasons derived from the study of the stellar spectra.

If, in the very intense electric field in the neighbourhood of the cathode, the molecules of the gas are dissociated and are split up, not into the ordinary chemical atoms, but into these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles are charged with electricity and projected from the cathode by the electric field, they would behave exactly like the cathode rays. "

And a few paragraphs later:

"If we regard the chemical atom as an aggregation of a number of primordial atoms, . . . ."

However, he does not go into the presence of a positive force, although he must have been aware of its necessisity.

Here is what he then said in 1899:

"I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles.... The detached corpuscles behave like negative ions, each carrying a constant negative charge which we shall call for brevity the unit charge; while the part of the atom left behind behaves like a positive ion with the unit positive charge and a mass large compared with that of the negative ion."

This last portion is interesting in that it proposes the correct mechanism for ionization; a negative electron is removed leaving behind a positive atom.

II. Thomson's Mature Model

His next statement on the structure of the atom comes in a 1904 article. The first half of the article is filled with detailed calculations about the stability of corpuscles moving about in a positive environment. In fact, Thomson is only able to make calculations where all the corpuscles are limited to roatating in a ring. Moving from ring to sphere proves too difficult a challenge.

Here is a quote from the 1904 article:

We suppose that the atom consists of a number of corpuscles moving about in a sphere of uniform positive electrification . . . .

That seems pretty straighforward, but the problem will soon become the electrons and their mass.

By the way, this is often referred to as Thomson's "plum pudding model," where the pudding represents the sphere of positive electricity and the bits of plum scattered in the pudding are the electrons. The ChemTeam likes to call it the "blueberry muffin" model. All those round little blueberries surrounded by the bread of the muffin. Ummmm, good. Some butter on top of a muffin hot from the oven and some nice, COLD milk. Oh my.

You can read more of Thomson's 1904 article in the classic papers section.

However, not everyone is convinced this is the right answer. Savante Arrhenius (the 1903 Nobel Prize winner in Chemistry) had this to say about Thomson's model in 1907:

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

Arrhenius goes on to several criticisms of the Thomson Model.

Before leaving this topic, I want to make a point about how the Thomson Model is presented today. Sometimes teachers, and even textbooks, will represent the Thomson Model as a mixture of protons and electrons, like on the right-hand side of this image:

Make sure you have the correct idea firmly in mind. The Thomson Model has negative partices (electrons) and a sphere of positive charge. There are NO protons in the Thomson Model of the atoms. Be careful, a teacher might try to trip you up on a test question. (Those teachers sure are evil, aren't they??)

The Thomson Model will hold sway for a few years, until Ernest Rutherford announces the nuclear model of the atom in 1911. This tutorial: A Brief History of Rutherford's Experiment starts the story.

Interest in the Thomson Model fell off rapidly after 1911, although in 1914 and 1915 attempts were made to resurrect it. These efforts came to nothing and the Thomson Model assumed its place in history as the first modern attempt to construct a theory of atomic structure.