12 eV energy is given to electron in third orbit of H -atom, then...

Given, total energy given to electron in third orbit,

$E = 12eV$

Energy of third orbit of $H$-atom, $E ' = \dfrac{- 13. 6}{n2}$

$= \dfrac{- 13. 6}{32} = - 1. 51eV$

lonisation energy of electron of third orbit, E^{' '} = - E '

= - \left(- 1. 51eV\right) ⇒ E^{' '} = 1. 51eV

Hence, final energy of electron when it comes out of $H$-atom from third orbit

{E}_{f} = E - E^{' '}

$= 12 - 1. 51 = 10. 49eV$