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Module 8: From the Universe to the Atom

Quick questions on Bohr model and the Balmer-Rydberg formula: HSC Physics Module 8

9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is bohr's postulates (1913)?
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Bohr postulated three rules to fix both problems.
What is named spectral series?
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The Balmer series is the visible band first discovered, which is why it has its own name. The full pattern lets astronomers identify hydrogen even from the most distant galaxies.
What is limitations of the Bohr model?
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The Bohr model works astonishingly well for hydrogen (and for hydrogen-like ions such as He+^+ and Li2+^{2+}, with Z2Z^2 corrections), but it has clear limitations:
What are postulate 1: stationary orbits?
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The electron in a hydrogen atom occupies certain discrete circular orbits in which it does not radiate. Each such orbit is a stationary state with a well-defined energy.
What is postulate 2: quantisation of angular momentum?
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The allowed orbits are those for which the orbital angular momentum is an integer multiple of ℏ=h/(2π)\hbar = h/(2\pi):
What is postulate 3: photon emission?
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Radiation occurs only when the electron makes a transition between two stationary states. The photon energy equals the energy difference:
What is q1?
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State Bohr's two key postulates for the hydrogen atom. [2 marks]
What is q2?
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Calculate the wavelength of the photon emitted when a hydrogen electron drops from n=4n = 4 to n=2n = 2. [3 marks]
What is q3?
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An astronomer at Siding Spring Observatory observes hydrogen Balmer-series lines from a star. (a) Identify the energy of the n=5n = 5 level in eV. (b) Calculate the wavelength of the transition n=5→n=2n = 5 \to n = 2 (H-gamma).

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