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

Explanation

Energy associated with a photon of `242 nm`
`= (6.625 xx 10^(-34) xx 3.0 xx 10^(8))/(242 xx 10^(-9))`
`= 8.21 xx 10^(-19) J`
Since one atom of requires `= 8.21 xx 10^(-19) J` for ionistion
`6.023 xx 10^(23)` atoms of Na for ionisation required `8.21 xx 10^(-19) xx 6.023 xx 10^(-23) = 49.45 xx 10^(4)J = 494.5 kJ mol^(-1)`