The energy of an atom in its ground state is \( -3.4\,\mathrm{eV} \). What is the ionisation energy of this atom?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( 3.4\,\mathrm{eV} \)
The ionisation energy is the minimum energy needed to free the electron from the ground state. A freed electron at rest, completely removed from the nucleus, has an energy of 0 eV. So the energy supplied must raise the atom from its ground-state energy \( E_g \) to zero.
The ionisation energy of the atom is:
\( \begin{aligned} E_{\mathrm{ion}} &= 0-E_g \\ &= 0-(-3.4\,\mathrm{eV}) \\ &= 3.4\,\mathrm{eV} \end{aligned} \)
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.”