What is the ionisation energy of the hydrogen atom?

The correct answer is Option (3) → \( 13.6\,\mathrm{eV} \)

The ionisation energy is the minimum energy needed to free the electron from the ground state. In the Bohr model, the ground state (n = 1) of hydrogen has \( E_1=-13.6\,\mathrm{eV} \). A freed electron at rest has 0 eV. So 13.6 eV must be supplied.

NCERT (2026-27):
Energy Levels – “… 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.”