Plum Pudding Model

by John Carl Villanueva on August 27, 2009

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plum pudding model of the atom

plum pudding model of the atom

The Plum Pudding Model is an atom model proposed by JJ Thomson, the physicist who discovered the electron. It is also known as the Chocolate Chip Cookie or Blueberry Muffin Model. You can easily picture it by imagining the said goodies. For example, you can imagine a plum pudding wherein the pudding itself is positively charged and the plums, dotting the dough, are the negatively charged electrons.

Thus, in contrast to today’s atom that has a very dense and very small (compared to the whole atom) positively charged nucleus, Thomson’s had a more dispersed positive charge. As a whole, the plum pudding representation only strived to explain why most atoms were neutral.

It’s interesting to note that this model was sometimes visualized as having a cloud of positive charge, a striking contrast to the most recent atomic model which describes the positive nucleus to be surrounded by an electron cloud.

Introduced by Thomson in the March 1904 edition of the UK’s Philosophical Magazine, this model was invalidated 5 years after during what is now known as the Rutherford gold foil experiment, an experiment designed to probe the atom.

The basic setup of the experiment was as follows: alpha particles coming from a radioactive source were directed to a gold foil. The foil was then surrounded by a wall of zinc sulfide that exhibited scintillations (tiny flashes) when hit by the alpha particles. Patterns formed by the scintillations would provide information as to how the charges were distributed inside the atom.

With the plum pudding model in mind, Rutherford expected very minimal alpha particle deflections as they were bombarded on the foil. Hence, the scintillations were only supposed to be observed right behind the foil, exactly opposite to the source of alpha particles. Alas, although majority of the flashes were indeed observed behind the foil, a few were also seen near the source.

According to Rutherford, it was like firing a cannonball on a tissue paper and seeing it bounce against the paper right back at him on certain occasions.

Since alpha particles are just helium nuclei, they are positively charged. The only logical explanation therefore was that the positive charge in the atom were not as dispersed as you would imagine in a pudding’s soft crust. Instead, Rutherford guessed that they should be concentrated in a very small volume in the center. This specific portion of Rutherford’s model still holds even up to this day.

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Sources:
Cambridge Physics
Wikipedia

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