Bohr model and quantized energy levels (HL)
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Question
Bohr energy levels of hydrogen?
Answer
$E_n = -\dfrac{13.6}{n^2}$ eV, with n = 1, 2, 3, … The levels are **discrete** (quantized).
Question
Why are the Bohr energy levels negative?
Answer
The zero is set at the just-free electron (n → ∞), so any **bound** electron has **less** energy — hence negative.
Question
What is the ground state of hydrogen?
Answer
n = 1, with $E_1 = -13.6$ eV — the **lowest** (most tightly bound) level.
Question
Energy of the n = 2 level of hydrogen?
Answer
$E_2 = -13.6/4 = -3.40$ eV.
Question
Photon energy when an electron changes level?
Answer
$hf = E_i - E_f$ — the photon energy equals the size of the energy drop.
Question
Emission vs absorption of a photon?
Answer
**Emission**: electron jumps **down**, atom gives out a photon. **Absorption**: electron jumps **up**, atom takes a photon in.
Question
Why does hydrogen give a line spectrum?
Answer
Its levels are **discrete**, so only certain energy drops — and hence certain photon energies — are possible.
Question
Ionisation energy of hydrogen?
Answer
**13.6 eV** — the energy to lift the electron from the ground state (−13.6 eV) up to 0 eV (free).
Question
Photon energy for n = 3 → 2 in hydrogen?
Answer
$(-1.51) - (-3.40) = 1.89$ eV $= 3.0\times10^{-19}$ J.
Question
Convert an energy from eV to joules?
Answer
Multiply by $1.60\times10^{-19}$ (1 eV = 1.60×10⁻¹⁹ J).
Question
Which transition gives the highest-frequency photon?
Answer
The one with the **largest** energy drop — falling down to n = 1.
Question
How do the levels change as n rises?
Answer
The gaps **shrink** (1/n²), so the levels **crowd together** approaching 0 eV.
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