Thomson's Atomic Model

The experimental background

• Thomson worked with a glass vacuum tube in which a voltage was applied between two electrodes inside an evacuated tube.
• The cathode was seen to emit rays which produced a glow where they struck the glass behind the anode.
• Studying the properties of these rays, he concluded they were made of negatively charged particles, which he called electrons.
• This demonstrated that atoms are not indestructible: they contain electrons, and the electrons can be emitted by the cathode.

The model (1903)

• An atom is a sphere having a uniform positive charge, in which the electrons are embedded.
• It is called the plum-pudding model: the electrons are the plums scattered through the pudding of positive charge.
• Diagram in words: a large circle shaded uniformly to represent the continuous positive charge, with several small dots scattered inside it representing the embedded electrons.

What the model explained

• Electrical neutrality: the total positive charge equals the total negative charge of the electrons in the atom.
• Why only negative particles come out of an atom: the whole solid sphere is uniformly positively charged, so the positive charge cannot come out; only the small negatively charged electrons can be emitted.
• The formation of ions and of ionic compounds, by the gain or loss of electrons.

Why it was abandoned

• Later experiments on the structure of atoms, above all the Geiger-Marsden alpha-scattering experiment, showed the distribution of charge inside an atom to be very different from a uniform sphere.
• If the positive charge were spread over the whole atom its volume density would be very small, and every alpha particle would be deflected only through a very small angle. Large-angle scattering and backscattering could not happen.