Factors affecting Ionisation Energy

• Greater positive nuclear charge = Greater force of attraction
• Higher atomic radius = Lower force of attraction
• Lower atomic radius = Higher force of attraction
• More shielding = Less attraction between outer electrons and nucleus
• Across a period

  1. Nuclear charge increases
  2. Atomic radius decreases

  So ionisation energy increases
• Down a group

  1. Atomic radius increases
  2. More shielding effect

  So ionisation energy decreases
• Drop at Al

  1. Removed from 3p
  2. Higher electron energy, so less energy needed to remove
• Drop at S

  1. Spin paired electrons in 3p
  2. Mutual repulsion lowers energy requirement to remove