How are ionization and electron-electron repulsion related?

[NaomiAbramowicz1H]

Identify the reasons why oxygen has a lower first ionization energy than both nitrogen and fluorine:
- Oxygen has a smaller atomic radius than fluorine.
- The 2p electrons in fluorine are more highly shielded from the nuclear charge than the 2p electrons in oxygen.
- The 2p electrons in fluorine experience a higher effective nuclear charge than the 2p electrons in oxygen.
- Upon ionization, oxygen is relieved of electron‑electron repulsion.

I know the last answer is one of the correct ones, but how does electron-electron repulsion impact ionization energy? And what kind of trend exists for electron-electron repulsion?

[Sophie Vikram 1L]

The general rule of thumb is that greater ionization energy is caused by closer proximity to the nucleus. When electrons are closer to the nucleus, it is harder to remove/ionize them because the effective nuclear charge (positive) pulls them towards the nucleus. Electron-electron repulsions occur because the negative charge on one electron repels/pushes away other electrons (think magnets with the same poles facing each other). Because the electrons repel each other, electron-electron repulsion creates decreases electron proximity to the nucleus. What we can infer from this is that more electrons in an electron cloud lead to greater electron-electron repulsion which thus can counteract/minimize the effect of nuclear charge. Minimizing nuclear charge decreases ionization energy.

[Neelaj Das 3I]

Electron-electron repulsion refers to the fact that electrons try to distance themselves as much as possible from each other because like charges repel (negative and negative). Electron-electron repulsion makes an atom less stable, so upon removing an electron from oxygen, oxygen experiences a decrease in electron-electron repulsion making it more stable. This is why is has a lower first ionization energy.

[Andrea_Perez_2F]

The way that electron repulsion and ionization energy relate is that as ionization decreases, the distance between the electron and the nucleus increases. So when two negatively charged electrons are close to one another then this decreases the ionization energy. The trends that exist for electron-electron repulsion would be that as you go down a group, and the atomic radius becomes larger, this means that there is an increase in repulsion, also it gets larger because the valence electrons are farther away from the nucleus.