Which elements are correctly listed in order of increasing ionization energy? 1. o < f < ne 2. o < s < se 3. n < p < as 4. c < n < o 5. n < o < f?

Ionization energy is the energy required to remove an electron from gaseous atom or ion.
Ionization energy increases from left to right in the periodic table and decreases from up to down
According to this fact we have three correct answers which are 1, 4 and 5 but
There is an exception for this trend is that Nitrogen has higher First ionization energy than Oxygen due to Oxygen has 8 electrons distributed as: 1s² 2s² 2p⁴ so by losing one electron from p it converted to 2p³ (half filled --> more stable) so that the only correct answer is number 1) O < F < Ne