The ionisaton potential of hydrogen atom is 13.6 eV. The energy required to remove an electron in the n = 2 state of the hydrogen atom is

Energy of electron in ground state   E=−13.6eV
Energy of electron in second state  E₂​=−13.6​/2²=−3.4 eV
Therefore, 3.4 eV energy is required to remove an electron from second orbit.