Ionisation energy is the amount of energy required to remove most loosely bound electron from the isolated gaseous atom.

For the sodium atom, it is equal to the electromagnetic radiation of wavelength $242 \text{nm}$.

As per the Planck's quantum theory.

The energy required for removing the one-mole electron from the one-mole sodium atoms.

$\text{E}=\frac{\text{hc}}{\text{λ}}\times {\text{N}}_{\text{A}} {\text{J mol}}^{-1}$

$\text{E}=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{242\times {10}^{-9}}\times 6.022\times {10}^{23}\text{J} {\text{mol}}^{-1}$

$\text{E}=0.4946\times {10}^6{\text{J mol}}^{-1}\text{}\text{E}= 4.946\times {10}^2{\text{kJ mol}}^{-1}$