The diagram shows energy levels of an atom. An electron transitions from level n = 3 to level n = 1. What happens?
AThe atom absorbs a photon of energy hf
BThe atom emits a photon with energy equal to E₃ − E₁
CThe atom loses one proton
DNothing happens
Explanation
📌 When an electron drops from a higher energy level to a lower one, the atom emits a photon whose energy = E_higher − E_lower. Frequency of the emitted photon is set by hf = ΔE. This is the origin of emission spectra. Distractor A: absorption is the reverse process; C: protons are in the nucleus, not affected by electron transitions; D: emission happens.
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