Plum-pudding model

The plum pudding model is one of several scientific models of the atom. First proposed by J. J. Thomson in 1904 soon after the discovery of the electron, but before the discovery of the atomic nucleus, the model represented an attempt to consolidate the known properties of atoms at the time: 1) electrons are negatively-charged particles and 2) atoms are neutrally-charged.

In this model, atoms were known to consist of negatively charged electrons. Though Thomson called them "corpuscles", they were more commonly called "electrons" as G. J. Stoney proposed in 1894. At the time, atoms were known to be neutrally charged. To account for this, Thomson knew atoms must also have a source of positive charge to balance the negative charge of the electrons. He considered three plausible models that would satisfy the known properties of atoms at the time:

Thomson chose the third possibility as the most likely structure of atoms. Thomson published his proposed model in the March 1904 edition of the Philosophical Magazine, the leading British science journal of the day. In Thomson's view:

... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ...

With this model, Thomson abandoned his earlier "nebular atom" hypothesis in which atoms were composed of immaterial vortices. Being an astute and practical scientist, Thomson based his atomic model on known experimental evidence of the day. His proposal of a positive volume charge reflects the nature of his scientific approach to discovery which was to propose ideas to guide future experiments.

The orbits of electrons within the model were stabilized by the fact that when an electron moved away from the centre of the positively-charged sphere, it was subjected to a greater net positive inward force, because there was more positive charge inside its orbit (see Gauss's law). Electrons were free to rotate in rings which were further stabilized by interactions among the electrons, and spectroscopic measurements were meant to account for energy differences associated with different electron rings. Thomson attempted unsuccessfully to reshape his model to account for some of the major spectral lines experimentally known for several elements.

The plum pudding model usefully guided his student, Ernest Rutherford, to devise experiments to further explore the composition of atoms. As well, Thomson's model (along with a similar Saturnian ring model for atomic electrons put forward in 1904 by Nagaoka after James Clerk Maxwell's model of Saturn's rings), were useful predecessors of the more correct solar-system-like Bohr model of the atom.

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