22.4.1—There are discrete electron energy levels in isolated atoms (e.g. atomic

An isolated atom allows electrons only in particular energy states, not every energy between them.

An electron changes level by absorbing or emitting a photon whose energy equals the gap: ΔE=hf. Larger gaps correspond to higher-frequency, shorter-wavelength lines.

A hydrogen emission spectrum contains separate lines because only certain transitions are allowed between its levels.

The electron does not radiate continuously while remaining in a stationary level; emission occurs during a transition between allowed states.