Q. Ionization energy of gaseous $Na$ atoms is $495.5$ kJ mol${}^{-1}$. The lowest possible frequency of light that ionizes a sodium atom is:

1. $4.76\times {10}^{14}{s}^{-1}$
2. $3.15\times {10}^{15}{s}^{-1}$
3. $7.50\times {10}^4{s}^{-1}$
4. $1.24\times {10}^{15}{s}^{-1}$