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Physics Question 43 – JEE-MAIN 2026

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In Rutherford's alpha-particle scattering experiment, only a few alpha particles rebound back because A. The size of gold nucleus is very small as compared to the size of gold atom. B. Alpha particle and gold nucleus have equal charge. C. The impact parameter is minimum for a few alpha particles. D. A few alpha particles have very high kinetic energy. E. Only a few alpha particles undergo head-on collision with the nuclei. Choose the correct answer from the options given below:

Recall the key conclusions of Rutherford's alpha-particle scattering experiment regarding the structure of the atom.

Step 1: Analyze Rutherford's Model and Atomic Structure✦ Active

Rutherford's alpha-particle scattering experiment revealed that most alpha particles passed through the gold foil undeflected, indicating that the atom is mostly empty space. A very small fraction of alpha particles were deflected at large angles, some even rebounding, which led to the conclusion that the atom has a tiny, dense, positively charged nucleus at its center. This directly supports statement A: "The size of gold nucleus is very small as compared to the size of gold atom." This small size of the nucleus makes head-on collisions rare.

Step 2: Evaluate Collision Types and Impact Parameter○ Expand

The scattering angle of an alpha particle is inversely related to its impact parameter (b). For an alpha particle to rebound (scatter at nearly 180∘), it must have a very small impact parameter, meaning it is directed almost head-on towards the nucleus. Since the nucleus is extremely small, the probability of such a direct hit (head-on collision) is very low. This confirms statement C: "The impact parameter is minimum for a few alpha particles," and statement E: "Only a few alpha particles undergo head-on collision with the nuclei."

Step 3: Discard Incorrect Statements○ Expand

Statement B is incorrect because an alpha particle has a charge of +2e, while a gold nucleus has an atomic number Z=79, meaning its charge is +79e. They do not have equal charges. Statement D is also incorrect as a reason for *only a few* rebounding; while alpha particles do have high kinetic energy, this property applies to all incident particles and doesn't explain the rarity of large deflections. The rarity is due to the small size of the nucleus and the low probability of a head-on collision. Therefore, the correct reasons are A, C, and E.

💡 Teacher's Secret Hint

Remember that the kinetic energy of the alpha particles is a given property of the source, not a variable that explains the 'few' aspect of scattering.

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