There are four main factors that affect the ionization energy:
- Distance from nucleus
- Shielding effect
- Effective nuclear charge
- Electron-electron repulsion
the first two factors are the most effective factors, however, if we are comparing the removal of two electrons that exist on the same distance from nucleus and therefore, they are subjected to the same shielding effect (or same number of core electrons), hence we compare the last two factors.
For the electron-electron repulsion, if one electron is paired (