Why is the first ionization energy in oxygen slightly more than nitrogen?
- In nitrogen: $\ce{[He] 2s^2 2p^3}$
- In oxygen: $\ce{[He] 2s^2 2p^4}$
This tells me that it should be easier to remove an electron from oxygen than it is for nitrogen as the electron in oxygen is slightly further away from the nucleus meaning lesser nuclear charge.
But why is it harder to remove an electron from oxygen, i.e. why is the first ionization energy of oxygen higher?