In the Bohr model, what happens when an electron transitions from a higher to a lower energy level?

• A. The electron gains energy by absorbing a photon.
• B. A photon with energy equal to the energy difference is emitted.
• C. The electron's energy remains unchanged.
• D. The atom collapses and releases no photon.

Answer: (B)

When an electron in the Bohr model moves from a higher energy level to a lower one, energy must be conserved. The atom loses the excess energy by emitting a photon. The photon’s energy exactly equals the difference between the two energy levels, ΔE = E_high − E_low. This emitted photon has a frequency f = ΔE/h and a wavelength λ = hc/ΔE, which also explains the spectral lines observed for atoms. The other ideas—absorbing energy to move up, no energy change, or the atom collapsing without emitting light—don’t fit energy conservation or the discrete energy-level structure in Bohr’s picture.