Plum pudding model

The plum pudding model of the atom was proposed by J. J. Thomson, who discovered the electron in 1897. The plum pudding model was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons (which Thomson still called "corpuscles," though G.J. Stoney had proposed that atoms of electricity be called electrons in 1894), surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge.

The model was disproved by the 1909 gold foil experiment, which was interpreted by Ernest Rutherford in 1911 to imply a very small nucleus of the atom containing a very high positive charge (enough to balance about 100 electrons in gold), thus leading to the Rutherford model of the atom, and finally (after Henry Moseley's work showed in 1913 that the nuclear charge was very close to the atomic number) to the Antonius Van den Broek suggestion that atomic number is nuclear charge. Eventually, by 1913, this work had culminated in the solar-system-like (but quantum-limited) Bohr model of the atom, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells.

Thomson's model was compared (though not by Thomson) to a British treat called plum pudding, hence the name. It has also been called the chocolate chip cookie model or blueberry muffin model, but only by those who think of the particles as static, not having read Thomson's original paper. This paper's title is On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure. It was published in the Philosophical Magazine (the leading British science journal of the day). For an excerpt see.

A little-known (or now forgotten) fact about the original Thomson "plum pudding" model is that it was dynamic, not static. The electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit (see Gauss's law). In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomson's model (along with a similar Saturnian ring model for atomic electrons, put forward also in 1904 by Nagaoka after the Maxwell model of Saturn's rings), were earlier harbingers of the later and more successful solar-system-like Bohr model of the atom.