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Q.
Electromagnetic radiation of wavelength $242\, nm$ is just sufficient to ionise the sodium atom. The ionisation energy of sodium in$ kJ\, mol^{-1}$ is
Structure of Atom
Solution:
$ \lambda=242\, \mathrm{nm}=242 \times 10^{-9} \mathrm{~m}$
I.E. $=\frac{h c}{\lambda}=\frac{6.626 \times 10^{-34} \times 3 \times 10^{8}}{242 \times 10^{-9}}$
$=8.214 \times 10^{-19} \mathrm{~J} /$ atom
Ionisation energy of sodium in $\mathrm{kJ} \mathrm{mol}^{-1}$
$=8.214 \times 10^{-19} \times 10^{-3} \times 6.022 \times 10^{23}$
$=494.65 \mathrm{~kJ} / \mathrm{mol}$