Why Do Atoms Select Particular Energies?

Why can an atom absorb or emit some photon energies but not every energy in between?

Use an allowed-energy staircase, photon gaps, and reference data to connect quantum states with spectral lines.

Four discrete energy rails show a photon lifting an electron state across one exact gap, paired with one matching red spectral line. — Vastward teaching schematic. The rails represent allowed energies, not literal electron paths; the arrow links one energy gap to its matching photon and spectral line.

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