Electromagnetic radiation of wavelength 242 nm is just sufficient to ionise the sodium atom. Calculate the ionisation energy of sodium in kJ ${{mol}^{-1}}$.
Energy E = hv = hc / $\lambda $
= 6.626 x ${{10}^{-34 }}$ x 3.0 x ${{10}^{8}}$ / 242 x ${{10}^{-9}}$
= 0.0821 x ${{10}^{-17}}$ J/atom
This energy is sufficient for ionisation of one Na atom, so it is the ionisation energy of Na.
= 6.02 x ${{10}^{23}}$ x 0.0821 x ${{10}^{-17}}$J/mol
=4.945 x ${{10}^{5}}$ = 4.945 x ${{10}^{2}}$ kJ/mol