Given, total energy given to electron in third orbit, E=12eV Energy of third orbit of H -atom, E′=
−13.6
n2
=
−13.6
32
=−1.51eV lonisation energy of electron of third orbit, E"=−E′ =−(−151eV)⇒E"=151eV Hence, final energy of electron when it comes out of H-atom from third orbit EF=E−E" =12−1.51=10.49eV