Chapter 7 - Periodic Properties of the Elements - Integrative Exercises - Page 296: 7.111a

$E=21.2\ eV$

$E=\frac{6.262\times10^{-34}\ J-s\times2.9998\times10^8\ m/s}{58.4\times10^{-9}\ m}=3.4015\times10^{-18}\ J/photon$ $\frac{3.4015\times10^{-18}\ J}{photon}\times\frac{1\ kJ}{1000\ J}\times\frac{6.022\times10^{23}\ photons}{mol}\times\frac{1\ eV-mol}{96.485\ kJ}=21.230=21.2\ eV$