Final answer:
An electronic transition that emits a photon typically occurs when an electron falls from a higher-energy orbital to a lower-energy orbital, such as from the LUMO to the HOMO in the hydrogen molecule, or from higher n states to lower n states in atomic hydrogen.
Step-by-step explanation:
An electronic transition that emits a photon corresponds to a transition between orbitals where an electron moves from a higher energy state to a lower energy state. In the context of molecular hydrogen (H₂), when a molecule transitions from an excited state to a ground state, a photon is emitted. This occurs when an electron moves from the higher-energy antibonding σ* MO (the Lowest Unoccupied Molecular Orbital or LUMO) to the lower-energy bonding σ MO (the Highest Occupied Molecular Orbital or HOMO). For atomic hydrogen, the emission of light in the form of the Balmer series results from electrons transitioning from orbits with n ≥ 3 to the orbit with n = 2, which corresponds to light in the visible spectrum.
According to Bohr's model of the atom, these transitions occur between quantized orbital states with specific energy levels, and the energy change associated with the transition (ΔE) is equal to the energy of the emitted photon (hf). In other words, when an electron 'falls' back to a lower energy level, the lost energy is emitted as a photon, a discrete packet of light.