Question

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

• A. $494.5 \times 10^{-6}\, J/atom$
• B. $8169.5 \times 10^{-10}\, J/atom$
• C. $5.85 \times 10^{-15}\, J/atom$
• D. $8.214 \times 10^{-19}\, J/atom$

Answer 1

Answer: (D)

$\lambda=242\,nm$
$=242\times10^{-9}\,m$
Energy required to ionise one atom of $Na$, $E=\frac{hc}{\lambda}$
$=\frac{\left(6.626 \times 10^{-34}\,J\,s\right) \times \left(3 \times 10^{8}\,m\,s^{-1}\right)}{242 \times 10^{-9}\,m}$
$=8.214 \times 10^{-19}\,J /atom$