Atom, Origin of Spectra: Bohr's Theory of Hydrogen Atom
ICSE · Class 12 · Physics
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State the discovery and main idea of J.J. Thomson's atomic model.
Answer
Electron was discovered by J.J. Thomson in 1897. Thomson presented the plum pudding atomic model. In this model, the atom is neutral and negatively charged electrons are embedded in a uniformly distri…
Why did the alpha-particle scattering experiment suggest that the atom is mostly hollow?
Answer
Most alpha-particles passed straight through the gold foil without deflection. This means most of the atom is empty space, so the positive charge and most of the mass must occupy a very small central …
A thin gold foil is used in alpha-particle scattering. Why is gold foil chosen?
Answer
Gold foil can be beaten very thin, which helps ensure single collision of alpha-particles. Its heavy nucleus produces large deflection of alpha-particles.
State the relation between foil thickness and the number of scattered alpha-particles for a fixed scattering direction.
Answer
Thickness-scattering relation: N/t = constant, i.e., N1/N2 = t1/t2, where N is the number of scattered alpha-particles and t is the foil thickness.
If the foil thickness is doubled, what happens to the number of alpha-particles scattered at the same angle?
Answer
Using N/t = constant, N is directly proportional to t. Step 1: N1/N2 = t1/t2 Step 2: If t2 = 2t1, then N2 = 2N1 Answer: The number of scattered alpha-particles doubles.
State the angular dependence of alpha-particle scattering count.
Answer
Scattering angle dependence: N ∝ 1/sin^4(theta/2), where N is the number scattered at angle theta.
At ordinary conditions, only about 1 out of 20000 alpha-particles is scattered beyond 90°. What does this show?
Answer
This shows the nucleus is extremely small compared to the atom. Only a very small fraction of alpha-particles comes very close to the nucleus and gets strongly deflected.
State the formula for distance of closest approach of an alpha-particle to the nucleus.
Answer
Distance of closest approach: r0 = (1/4πε0)(2Ze^2/K), where Z is atomic number, e is electron charge, and K is initial kinetic energy of the alpha-particle.
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