Ionization energy decreases down a group due to the shielding effect, which weakens the attractive force on outer electrons, making them easier to remove. Conversely, ionization energy increases across a period as the effective nuclear charge increases. This trend highlights how electron removal becomes less energetically demanding down a group and more challenging across a period.

1. Lower ionization energy
• As a result the energy required to remove an outer electron
is lower due to shielding effect
• In summery weakiong the attractive force making easier to
remove electron from outer shell
• Ionization energy decrease down a group
• Increase across the period