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The Plum Pudding Model of the Atom

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Rafael Perez

on 17 September 2013

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Transcript of The Plum Pudding Model of the Atom

The Plum Pudding Model of the Atom
The plum pudding model is an atom model proposed by J.J. Thomson, the physicist who discovered the electron. The plum pudding model is also known as the chocolate chip cookie or blueberry muffin model. An atom made of thousands of electrons would have a very high negative electric charge, but this was not observed. Thomson suggested that atoms contained far fewer electrons and that they must have been balanced by some sort of positive charge. Thomson proposed a "Plum Pudding Model" with positive and negative charge that strive to explain why most atoms are neutral.

What phenomenon did the theory attempt to explain?
* A theory is the analysis of a set of ideas that are presented as true but they have to be tested or rejected.

* Theory can be improved or modified as more information is gathered.

* Contrary to the hypothesis the theory must be true. When a hypothesis is verified to be true it becomes a theory.
The plum pudding experiment was invalidated in 1909 by the Rutherford gold foil experiment. The basic setup of the experiment: 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 exhibit scintillations (tiny flashes) when hit by the alpha particles. Patterns formed by scintillations would provide information as to how the charges were distributed inside the atom. Since alpha particles are just helium nuclei, they are positively charged. The only logical explanation therefore was that the particles charges in the atom were not as dispersed as in the pudding.

Combining Rutherford’s description of the nucleus and Planks theory of quanta, Bohr explained what happened inside an atom and developed a picture of the atomic structure. Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.

Describe in your own words theories that are now used to explain the phenomenon that your disproven theory once explained.
* In 1913 Niels Bohr proposed his model of the atom. The Bohr model shows the atom as a small, positively charged nucleus surounding by orbiting electrons.

* In 1909 Ernest Rutherford's nuclear scattering experiments showed that atoms consist of a positively charged nucleus surrounded by negatively charged
What evidence supports the newer theories?
J.J. Thomson model of the atom was dispersed positive charge. As a whole, the plum pudding representation only strived to explain why most atoms were neutral. But that is not true, thanks to new experiments and research scientist found the atom is very dense and very small. Also, its made of a positive nucleus to be surrounded by an electron cloud.

Ernest Rutherford former student of Thomson's found out by experiments with a different sort of particle beam, that atoms had a central positively charged core with electrons circling around the core.

In your own words, explain theory.
Describe the experiments and/or observed phenomena that led to the theory's downfall.
Did any famous scientists support your team's assigned theory?
* In 1910, a physicist from New Zealand, Ernest Rutherford performed an experiment known as the Rutherford's gold foil experiment.

* This experiment was determined to find out the structure of an atom.

* Rutherford worked under the discoveries and assumption of J.J Thomson.

* Rutherford reasoned that if Thomson's model was correct then te mass of the atom was spread throughout the atom.

* He decided to test this with a thin film of gold foil.
Observations and Results
* The alpha particles were scattered as they passed through the gold foil.

*Most of the particles were undeflected.

* A few were scattered at large different angles.

* Some even back in the direction they had come.

* This meant that they had collided with an object much more massive than the alpha particles themselves.

* This massive object is so small some alpha particles encounter them.

* Most of the atom is occupied by the low mass electrons.

* The nucleus is small and massive.

* When an alpha particle encounters a nucleus, it is scattered at a large angle.
The Gold Foil Experiment Process
* It has a radioactive source rich in positively charged heavy alpha particles inside a cube shaped thick lead box with a narrow opening.

* The alpha particles passed through a lead sheet through a slit.

* A thin gold foil was bombarded with the narrow beam of fast moving alpha particles.

* The alpha particles were scattered in different directions with different angles.

* Then detected by the florescent rotatable detector which has a microscope and a screen coated with zinc sulphide.

* These particles after striking on the screen caused scintillations.

* The whole experimental setup was placed in an evacuated
chamber to prevent scattering by air molecules.

Rutherford’s Atomic Theory
* Most of the part of an atom is empty.
Approximately all the mass of the atom is concentrated at the center of atom.
* In the central region of atom, the positively charged particles are present.
* Central region (nucleus) is very small in size if compared to the size of atom.
* The electrons revolve around the central positive portion in different circular orbits.

Rutherford's Last Conclusion
The positive charge in an atom must all be packed together in one small region of the atom.
The deflections of particles were not consistent with Thompson’s theory.
The only way that alpha particles could be deflected backwards was if the mass of an atom was concentrated in a nucleus.
Due to this he discarded Thompson’s theory.


The Plum Pudding Model of the atom by J.J. Thomson, who discovered the electron in 1897. The 1904 Thomson model was disproved by the 1909 gold foil experiment of Hans Geiger and Ernest Marsden. This was interpreted by Ernest Rutherford in 1911.
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.
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