Electromagnetic radiation of wavelength 242 nm is just sufficient to ionise the sodium atom. What is the ionisation energy of sodium per atom?

Explanation

Ionization Energy in brief is the minimum amount of energy needed to knock off the valence electron of an atom to infinity
`E = (hc)/(lamda) = (6.626 xx 10^(-34) xx 3 xx 10^(8))/(242 xx 10^(-9)) = 8.21 xx 10^(-19)J//"atmon"`
For mole, `E = (8.21 xx 10^(-19)J//"atom") xx (6.022 xx 10^(23)"atom/mol")` = 494406.2J/mole = 494.4 kJ `"mole"^(-1)`