15.3: Emission and Absorption Spectra

Describe the emission or absorption of photons by atoms.

15.3.A.1 Energy transfer occurs when photons are absorbed or emitted by atoms. 

15.3.A.2 Energy can only be absorbed or emitted by an atom if the amount of energy being absorbed or emitted corresponds to the energy difference between two atomic energy states.

• • An atom in a given energy state may absorb a photon of the appropriate energy and transition to a higher energy state.

  • An atom in an excited energy state may emit a photon of the appropriate energy to spontaneously move to a lower energy state.

15.3.A.3 Transitions between two energy states of an atom correspond to the absorption or emission of a photon of a single frequency and therefore a single wavelength. Relevant equation: 

Atoms of each element have a unique set of allowed energy levels and thereby a unique set of absorption and emission frequencies. The unique set of frequencies determines the element’s spectrum.

• An emission spectrum can be used to determine the elements in a source of light.

• An absorption spectrum can be used to determine the elements composing a substance by observing what light has been absorbed.

15.3.A.4.c If an electron acquires an energy greater than its binding energy, then it will leave the atom. Electrons in the lowest energy level have the greatest possible binding energy.