To remove an electron from the first orbit to infinity of hydrogen atom, the amount of energy required will be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 13.6 eV
The energy needed to free the electron is the ionisation energy: the electron goes from the first orbit (\( n = 1 \), energy \( -13.6\,\mathrm{eV} \)) to infinity (\( n = \infty \), energy zero).
\( \Delta E = E_\infty - E_1 = 0 - (-13.6\,\mathrm{eV}) = +13.6\,\mathrm{eV} \)
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27):
Energy Levels – “… The lowest state of the atom, called the ground state, is that of the lowest energy, with the electron revolving in the orbit of smallest radius, the Bohr radius, a0. The energy of this state (n = 1), E1 is –13.6 eV. Therefore, the minimum energy required to free the electron from the ground state of the hydrogen atom is 13.6 eV. It is called the ionisation energy of the hydrogen atom.”