Energy of electron in nth orbit of hydrogen atom, E=‌
−136
n2
‌eV energy of electron in ground state (n=1), E0=‌
−13.6
12
=−13.6‌eV for second excited state, n=3 ∴ energy of electron in H-atom in second excited state E′=‌
−13.6
32
=−1.51‌eV Energy required to excite the electron in hydrogen atom to second excited state =E′−E0 ‌=−1.51−(−136)=1209‌eV ‌‌≈12‌eV Energy of ionised electron of (n=∞)H - atom. E′′=‌
−13.6
∞
=0‌eV energy required to ionise the electron =E′′−E0=0−(−13.6)=13.6‌eV