Group 1 – Alkali Metals
Reactivity increases down the group. Lower IE → easier to lose the single valence electron → more vigorous reaction.
With water: 2M + 2H₂O →
2MOH + H₂
Li: steady fizz | Na: molten ball | K: lilac flame
Group 17 – Halogens
Reactivity decreases down the group. Smaller atoms attract electrons more effectively → stronger oxidising agents at the top.
F₂ > Cl₂ > Br₂ > I₂ (oxidising power)
Halogen Displacement Reactions
A more reactive halogen can displace a less reactive halide from solution:
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Chlorine oxidises bromide ions. The solution turns orange/brown as Br₂ forms.
⚠️ Examiner Trap
Group 1 and Group 17 have opposite reactivity trends. Group 1 = easier to lose electrons down the group (metals). Group 17 = harder to gain electrons down the group (non-metals). The same factor (atomic radius / shielding) drives both trends in opposite directions.