Chemistry: The Central Science (13th Edition)

The energy gap between the ground state and the excited state for sodium ion is $$\Delta E=3.373\times10^{-19}J$$
The question asks for the energy gap (the difference in energy) between the ground state and the excited state for the sodium ion ($\Delta E$). It is said that the energy of a photon emitted must equal the difference in energy between the two states, or in this case, the ground state and the excited state for the sodium ion.. In other words $$E_p=\Delta E=3.373\times10^{-19}J$$