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Q12.1(a):
Choose the correct alternative from the clues given at the end of the each statement: The size of the atom in Thomson’s model is .......... the atomic size in Rutherford’s model.

Solution :

Step 1: Theoretical Analysis of Thomson’s Atomic Model

According to J.J. Thomson’s "Plum Pudding" model, the atom is envisioned as a sphere of uniform positive charge density with electrons embedded within it. In this model, the positive charge occupies the entire volume of the atom, which is approximately $10^{-10} \, \text{m}$ in radius. [Per Thomson's historical model]

Step 2: Theoretical Analysis of Rutherford’s Atomic Model

Ernest Rutherford’s alpha-particle scattering experiment led to the discovery of the atomic nucleus. Rutherford concluded that the positive charge and the bulk of the mass of the atom are concentrated in a tiny central region called the nucleus, with a radius of the order of $10^{-15} \, \text{m}$. The electrons revolve around this nucleus in a vast empty space. Thus, the actual "atomic size" remains the same order of magnitude ($10^{-10} \, \text{m}$), but the distribution of matter differs significantly.

Step 3: Comparative Assessment of Atomic Dimensions

The question asks to compare the "size of the atom" in both models. Based on the experimental outcomes of the gold foil experiment, both models conceptually define the total radius of an atom as approximately $10^{-10} \, \text{m}$.

Thomson: Positive Sphere Rutherford: Nucleus at center R ≈ 10⁻¹⁰ m

Step 4: Logical Deduction of the Alternative

Since both models attribute the outer boundary of the atom to the extent of the electron cloud, the spatial extent (radius) of the atom in both models is identical. Mathematically, $R_{\text{Thomson}} = R_{\text{Rutherford}} \approx 10^{-10} \, \text{m}$.

Feature Thomson Model Rutherford Model
Atomic Radius $\approx 10^{-10} \, \text{m}$ $\approx 10^{-10} \, \text{m}$
Charge Distribution Uniform Centralized (Nucleus)

Final Solution: The size of the atom in Thomson’s model is the same as the atomic size in Rutherford’s model.


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