Final answer:
A photon is emitted from a gaseous atom when an electron moves from an excited state back to its ground state, releasing its previously absorbed energy.
Step-by-step explanation:
A photon is emitted from a gaseous atom when an electron moves to its ground state from an excited state. When an atom receives energy, electrons may transition to a higher energy level, becoming excited. This excited state is not stable, and when the electron returns to the ground state, the energy difference is emitted as a photon. Conservation of energy principles dictate that the energy released as a photon is equal to the energy initially absorbed to excite the electron.