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Chapter 3: Problem 116

Explain why Rutherford expected all alpha particles to go through the gold foil undisturbed.

Short Answer

Expert verified
Rutherford expected all alpha particles to go through the gold foil undisturbed because of the prevailing plum pudding atomic model at the time, which depicted atoms as diffuse positively charged spheres with electrons distributed throughout. Under this model, the massive and positively charged alpha particles should not experience significant repulsion or deflection when passing through the gold foil, as the positive charge was uniformly spread out, and the electrons were too light to cause any significant deflection. However, the observed deflection of alpha particles led to the development of the modern atomic model with a concentrated, positively charged nucleus.

Step by step solution

01

Rutherford's Experiment and the Prevailing Atomic Model

During Rutherford's time, the prevailing atomic model was known as the plum pudding model, proposed by J.J. Thomson. In this model, atoms were thought to consist of a positively charged sphere with negatively charged electrons distributed uniformly throughout, resembling a "plum pudding." The positive charge was diffused throughout the atom, unlike the later discovered nucleus that holds all the positive charge in a small region.
02

Properties of Alpha Particles

Alpha particles are helium nuclei, consisting of two protons and two neutrons. They are positively charged and have relatively high mass and energy. When emitted from radioactive sources, they travel at very high speeds, up to 1/10 the speed of light, making them aggressive ionizing particles, meaning they can remove electrons from atoms.
03

Rutherford's Expectation Based on Plum Pudding Model

Given the plum pudding atomic model, Rutherford expected that alpha particles would pass through the gold foil easily. Since the positive charge was distributed uniformly throughout the atom, under this model, the positively charged alpha particles should not experience significant repulsion or deflection as they travel through the atoms in the gold foil. The electrons, being much lighter than alpha particles, would not cause any significant deflection or energy loss.
04

Conclusion

Rutherford expected all alpha particles to go through the gold foil undisturbed because, according to the plum pudding model of the atom, the positive charges were diffused throughout the atom, and the negatively charged electrons were not massive enough to cause any significant deflection to the alpha particles. However, Rutherford's observations of the alpha particles being deflected at various angles and even some being reflected back led to the realization that the plum pudding model was incorrect, giving birth to the modern atomic model with a concentrated, positively charged nucleus.

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