One of the concepts that crop up over and over, particularly at AS, is shielding.
If you've got this far then it seems unlikely that you haven't picked up that shielding makes it possible to remove electrons using less energy.
In other words, the more the shielding the lower the Ionisation Energy.
All electrons are negative - so they repel each other.
This will always go hand in hand with increasing distance from the nucleus to the outer electron - which will be the bigger factor in reducing ionisation energy.
- You're unlikely to asked to explain what causes shielding.
- So, just make sure that you understand its effect.
- And remember that elements with outer electrons in the same shell have the same amount of shielding.