Group 2 and Group 7 Chemistry
Group 2 — the alkaline earth metals
Group 2 metals (Mg, Ca, Sr, Ba) have 2 outer electrons and form 2+ ions. Going down the group:
- Reactivity increases — the outer electrons are further from the nucleus and more shielded, so they are lost more easily (first ionisation energy decreases).
- Reaction with water gets more vigorous, producing a metal hydroxide + hydrogen:
Ca + 2H₂O → Ca(OH)₂ + H₂. - Solubility of the hydroxides increases down the group (Mg(OH)₂ is nearly insoluble; Ba(OH)₂ is fairly soluble).
- Solubility of the sulfates decreases down the group (BaSO₄ is insoluble — used in the test for sulfate ions and as "barium meals" in medicine).
Uses: Mg extracts titanium and in flares; Ca(OH)₂ neutralises acidic soils; BaSO₄ in medical imaging.
Group 7 — the halogens
Halogens (F, Cl, Br, I) have 7 outer electrons, exist as diatomic molecules, and form −1 ions. Going down the group:
- Boiling point increases (larger molecules → stronger van der Waals forces).
- Reactivity/oxidising power decreases — larger atoms gain an electron less easily.
- Electronegativity decreases.
Displacement reactions
A more reactive halogen displaces a less reactive halide from solution — e.g. chlorine displaces bromine: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ (solution turns orange). This shows the trend in oxidising power.
Disproportionation
Chlorine reacts with water and with cold dilute NaOH in disproportionation reactions (chlorine is simultaneously oxidised and reduced), used in water treatment and to make bleach.
Testing for halide ions
Add dilute nitric acid then silver nitrate:
- Cl⁻ → white precipitate; Br⁻ → cream; I⁻ → yellow.
- Confirm with ammonia: AgCl dissolves in dilute NH₃; AgBr in concentrated NH₃; AgI is insoluble.
Worked example
Explain why the reactivity of Group 2 increases down the group but Group 7 decreases down the group.
- Group 2 atoms lose 2 electrons — further down, the outer shell is further/more shielded, so electrons are lost more easily → more reactive. Group 7 atoms gain an electron — further down, the outer shell is further from the nucleus, so an electron is attracted less easily → less reactive. ✓
Common mistakes
- Getting the two solubility trends the wrong way (hydroxides more soluble down; sulfates less soluble down).
- Reversing the reactivity trends (Group 2 up, Group 7 down).
- Forgetting to acidify with nitric acid before the silver nitrate halide test.
Exam tips
- Explain reactivity trends with ionisation (Group 2) vs electron gain (Group 7).
- Learn the halide test colours and the ammonia confirmation.
- Know disproportionation of chlorine in water treatment/bleach.
Key facts to remember
- Group 2: 2+ ions, reactivity increases down; hydroxides more soluble, sulfates less soluble (BaSO₄ test).
- Group 7: diatomic, −1 ions, oxidising power/reactivity decreases down; displacement shows this.
- Halide test: AgNO₃ (after HNO₃) → Cl⁻ white, Br⁻ cream, I⁻ yellow; confirmed with ammonia.