Periodicity and Ionisation Energy
Periodicity is just the repeating patterns you see when elements are arranged in the periodic table - it's like nature's own organised filing system! The most important pattern to master is ionisation energy, which is the energy needed to remove an electron from an atom.
Here's what drives these patterns: nuclear charge (number of protons) pulls electrons in tighter, whilst shielding from inner electrons pushes outer ones away. As you go across a period, nuclear charge increases faster than shielding, so ionisation energy generally increases. Down a group, extra electron shells create more shielding, making it easier to remove outer electrons.
Watch out for the sneaky exceptions! Group 3 elements have lower ionisation energy than Group 2 because you're removing from a higher-energy p orbital instead of an s orbital. Group 6 has lower ionisation energy than Group 5 because paired electrons in p orbitals repel each other, making removal easier.
Quick tip: Remember "nuclear charge wins across, shielding wins down" - this explains most periodic trends!
Group 2 elements become more reactive down the group. Their hydroxides get more soluble and more alkaline as you go from magnesium to barium. You'll see Ca(OH)₂ used in agriculture to neutralise acidic soils, and Mg(OH)₂ in antacids to neutralise stomach acid.
