JEE Main 2021 · 27 July · evening · Single correct · +4 / −1
If the Thompson model of the atom was correct, then the result of Rutherford's gold foil experiment would have been :
- (a) All of the α-particles pass through the gold foil without decrease in speed.
- (b) α-particles are deflected over a wide range of angles.
- (c) All α-particles get bounced back by 180°
- (d) α-particles pass through the gold foil deflected by small angles and with reduced speed.
Answer and solution
Answer (d) α-particles pass through the gold foil deflected by small angles and with…
Solution
Chemistry ma’am · 4 steps
In Thomson's atom the charge is spread thin, so nothing is strong enough to turn an alpha particle sharply.
- Thomson's picture. Positive charge is spread evenly through the whole atom, with electrons embedded in it like seeds in a watermelon. There is no small, dense centre of charge or mass.
- Weak, spread-out forces. An α-particle passing through feels only a weak electric force, because the positive charge is diluted over a large volume. The light electrons cannot turn a heavy α-particle much either.
- What you would see. Each α-particle would be nudged only slightly, giving small-angle deflections. It would also lose some speed as it pushes through the charged matter of the foil. No particle would bounce back.
- So. The expected result is option (d): α-particles pass through, deflected by small angles and with reduced speed.
The idea to keep. Large-angle scattering needs a tiny, dense, positive nucleus. Spread the charge out, as Thomson did, and you only get small deflections. Rutherford's rare back-scattering is what killed the Thomson model.
Answer from the NTA final answer key. Solution by PrepWiser.