Atomic Energy Absorption and Emission

1. Atomic Energy Absorption and Emission • Franck and Hertz applied high voltage across a tube filled with mercury • The lowest value (called the 1st excited state) was 4.9 eV, etc. • There are two other excited states shown here: 9.8 eV and 14.7 eV.

2. These (excitation) energy levels are different for each type of atom (each element). As the electrons move through the tube, • If the kinetic energy of the electron is < 4.9 eV when it strikes a mercury atom, there is just an elastic collision and the atom absorbs NO energy. • If 4.9 eV < Ek < 9.8 eV when the collision occurs, the atom absorbs 4.9 eV and the electron moves off with Ek' = Ek – 4.9 eV. What happens if the electron has 10.0 eV of kinetic energy on impact?

3. The atom absorbs 9.8 eV (putting it into its 2nd excited state) and the electron retains 0.2 eV of Ek. • Once an atom reaches an excited state it must give up all that excess energy and return to ground state (its lowest energy state). • The energy is emitted as photons (sometimes visible - as with fluorescent lights and sometime invisible). • This showed that atoms could only absorb energy in CERTAIN amounts.

4. This diagram shows that the photon(s) emitted have an energy equal to the excitation of the atom. • But even though the TOTAL energy emitted equals the TOTAL absorbed, there can be multiple photons emitted. • Example: photon 1 is from 3rd excited state to 2nd excited state and photon 2 is from 2nd excited state to ground state. How many possible transitions are there for an atom in the 3rd excited state?

5. To calculate the energy absorbed or emitted for an atom: 1. Calculate all possible wavelengths of photons emitted when a mercury atom falls from the 2nd excited state to the ground state.