Question

As we know, Rutherford had directed the 1909 Geiger-Marsden experiment to test if Thomson's plum-pudding model was correct or not. What would be the plum-pudding model's predictions about the gold foil experiment? (Alpha particles are doubly ionized Helium atoms, denoted as $\alpha$ or ${\alpha }^{2+}$.)

• A. The $\alpha$ particles would get embedded into the atoms.
• B. The $\alpha$ particles would pass through the gold foil with no or very less deflection.
• C. The $\alpha$ particles would bounce back from the gold foil due to electrical repulsion.
• D. Some of the $\alpha$ particles would go right through, while a small number would get deflected at large angles.

Answer 1

The correct option is B The $\alpha$ particles would pass through the gold foil with no or very less deflection.

Thomson's model hypothesized that the positive charge is evenly distributed throughout the atom, in which the electrons are embedded, that too more or less uniformly. The mass of the atom is, thus, homogenous and not localized.
The gold foil used in the Geiger-Marsden experiment had such a small thickness, that effectively, one layer or just one sheet of gold atoms could be assumed to had faced the bombardment of the high-momentum alpha particles $\left(\alpha \right)$. In such a case, the particles should just go right through the thin sheet of gold atoms - in Rutherford's own words, "…as if you fired a 15-inch shell at a piece of tissue paper", as the absence of any localized mass or charge meant the $\alpha$ particles couldn't get deflected at large angles.

Why even small angle deflection you ask?




I hope you remember, if the alpha particle enters the positive charged cloud slightly above the diameter then it will face a greater repulsion upwards due to more positive charges present below its line of approach than above it.

Thus, the expectation was that the $\alpha$ particles would pass through the gold foil with no or very less deflection.