Why does the first ionisation energy generally increase across a period? Explain why there are dips in energy between groups 2 and 3 and groups 5 and 6?
1st ionisation energy increases across a period as nucleus gets more positive, no further increase in shielding due to electrons being added to same shell, atomic radius decreases, attraction between nucleus and highest energy electrons increases, so more energy is required to move the highest energy electron. Dip in energy between groups 2 and 3 due to highest energy election occupying a p orbital, which is slightly higher in energy than an s orbital.Dip between groups 5 and 6 due to extra electron being added to an already occupied p orbital, making one of the valence p orbitals full. This is a destabilising interaction due to increased repulsion between the electrons, meaning the highest energy electron in group 6 is more easily lost.
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