H2 Chemistry 1 - Atomic Structure: Why Period 3 IE1 Has Two Dips

What you will learn

• Period 3 first ionisation energy mostly rises from Na to Ar, with two dips at Mg to Al and at P to S.
• Main trend explained by effective nuclear charge: proton number rises while shielding in the third shell stays roughly constant.
• Mg to Al dip: Al loses a 3p electron, which sits higher in energy and is shielded by the 3s pair, so it is easier to remove than Mg's 3s electron.