Atom, Origin of Spectra: Bohr's Theory of Hydrogen Atom — Flashcards

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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…

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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 …

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Gold foil can be beaten very thin, which helps ensure single collision of alpha-particles. Its heavy nucleus produces large deflection of alpha-particles.

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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.

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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.

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Scattering angle dependence: N ∝ 1/sin^4(theta/2), where N is the number scattered at angle theta.

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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.

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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.