Describe the emission or absorption of photons by atoms.

• Energy transfer occurs when photons are absorbed or emitted by an atom, which is modeled as a system consisting of a nucleus and an electron.
• 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.
• Because an atom is modeled as a system consisting of an electron and a nucleus, a change in the energy state of an atom corresponds to a change in the interaction energy between the electron and the nucleus.
• 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.
• 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 the substance has absorbed.
• Energy level diagrams are commonly used to visually represent the energy states of an atom.
• Binding energy is the energy required to remove an electron from an atom, causing the atom to become ionized. An atom in the lowest energy level (ground state) will require the greatest amount of energy to remove the electron from the atom.

In AP Physics 2, only energy level diagrams of single-electron atoms will be considered.

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