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Example · Example 2

Q.State the conclusions Rutherford drew about the structure of the atom from the results of the alpha-particle scattering experiment, and describe his nuclear model of the atom.

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Reasoning from the observations (Example 1). (1) Since almost every alpha particle passed straight through, most of the atom's volume must be essentially empty. (2) Since a small fraction deflected through large angles, there must be an intensely concentrated source of positive charge somewhere inside the atom, strong enough to repel a fast alpha particle through such an angle -- and, since this happened only rarely, that source must occupy only a tiny fraction of the atom's total volume. (3) Since a very few particles bounced back almost 180∘180^\circ, this same concentrated region must also carry almost the ENTIRE mass of the atom -- a comparatively massive alpha particle cannot be reversed by a target far lighter than itself.

The nuclear model. Rutherford proposed that: (i) almost all of an atom's mass and ALL of its positive charge is concentrated in a tiny, dense central nucleus, radius ∼10−15\sim10^{-15}-10−14 m10^{-14}\ \text{m};

(ii) electrons revolve around it in circular orbits at much larger distances, giving the atom an overall radius ∼10−10 m\sim10^{-10}\ \text{m} (so the atom is mostly empty space);

(iii) the electrostatic attraction between the nucleus and each electron supplies exactly the centripetal force for its orbit, 14πϵ0Ze2r2=mv2r\frac{1}{4\pi\epsilon_0}\frac{Ze^2}{r^2}=\frac{mv^2}{r} -- the "planetary model" of the atom.

✓Final answer

The atom has a tiny, dense, positively charged nucleus (∼10−15\sim10^{-15}-10−1410^{-14} m) holding nearly all its mass, with electrons revolving around it at ∼10−10\sim10^{-10} m under Coulomb attraction; the atom is mostly empty space.

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