Alkali Metal Reactivity Increases Downward
Reactivity increases down Group 1. Greater atomic radius and shielding lead to lower IE, making it easier to lose the single valence electron.
With water: \(2\text{M(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{MOH(aq)} + \text{H}_2\text{(g)}\)
Li: steady fizz | Na: melts to silver ball | K: lilac flame
Halogen Reactivity Decreases Downward
Reactivity decreases down Group 17. Smaller atomic radius attracts incoming electrons more strongly, so oxidising power is greatest at the top.
Oxidising power: \(\text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2\)
Halogen Displacement Reactions
Halogen Single-Displacement Reactions
A more reactive halogen can displace a less reactive halide from aqueous solution:
\(\text{Cl}_2\text{(aq)} + 2\text{KBr(aq)} \rightarrow 2\text{KCl(aq)} + \text{Br}_2\text{(aq)}\)
Chlorine oxidises bromide ions to bromine (\(2\text{Br}^- \rightarrow \text{Br}_2 + 2\text{e}^-\)). The colourless solution turns orange/brown as \(\text{Br}_2\) forms.
Opposing Reactivity Trends in Groups 1 and 17
Group 1 and Group 17 have opposite reactivity trends. Group 1 reactivity depends on ease of losing an electron (easier down the group). Group 17 reactivity depends on ease of gaining an electron (harder down the group). Increased radius and shielding cause both trends to operate in opposite directions.
Study this topic on the go
Get active recall flashcards, notes, and topic quizzes in ChemEasy, or build your revision schedule with ChemPlan IB.