Group 2

Group 2 – Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)

1) Position, Valence & Electronic Configuration

·       s‑Block, Group 2; all except Be historically termed ‘alkaline earth’ metals.

·       Valence shell: ns² → predominant oxidation state +2; compounds largely ionic (higher covalency for Be > Mg).

·       Electronic configurations: Be 1s²2s²; Mg [Ne]3s²; Ca [Ar]4s²; Sr [Kr]5s²; Ba [Xe]6s²; Ra [Rn]7s² (radioactive).

2) Physical Properties & Periodic Trends

·       Good electrical/thermal conductors; harder and denser than Group 1.

·       Radii ↑ down group; smaller than Group 1 analogues in the same period.

·       Ionization energies (1st & 2nd) ↓ down group → electropositivity ↑ down group.

·       Hydration of M²⁺: Be²⁺ >> Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺ (decreases down group).

·       Electronegativity: very low; decreases Be → Ba.

·       Flame colours: Ca brick‑red; Sr crimson; Ba apple‑green; Be/Mg show no colour (electrons tightly bound).

3) Reactivity & Characteristic Reactions

Air & Water

·       Be, Mg: surface oxide film → kinetically inert to air/water (powdered Be burns to BeO & Be₃N₂; Mg burns to MgO & Mg₃N₂).

·       Ca, Sr, Ba: readily form MO + M₃N₂ in air; react with cold water → M(OH)₂ + H₂ (vigour ↑ down group).

Halogens, Hydrogen, Acids, Ammonia

·       Halogens (Δ): M + X₂ → MX₂ (BeX₂ is covalent; others mostly ionic).

·       Hydrogen (Δ): MH₂ (BeH₂ prepared via BeCl₂ + LiAlH₄).

·       Acids: M + 2HCl → MCl₂ + H₂.

·       Liquid NH₃: deep blue/black solutions; ammoniates like [M(NH₃)₆]²⁺ can be isolated.

4) General Characteristics of Compounds

Oxides & Hydroxides

·       Oxides: MO (rock‑salt) except BeO (covalent, amphoteric).

·       Hydroxides: MO + H₂O → M(OH)₂ (sparingly soluble).

·       Trend (Mg → Ba): basicity ↑, thermal stability ↑, and solubility ↑ for M(OH)₂.

·       Be(OH)₂ amphoteric: Be(OH)₂ + 2OH⁻ → [Be(OH)₄]²⁻; also dissolves in acids.

Halides

·       BeX₂: polymeric chains (solid), bridged dimers (vapour); strong Lewis acid; soluble in organics.

·       Mg–Ba halides: predominantly ionic; hydration of chlorides decreases down group (MgCl₂·8H₂O → BaCl₂·2H₂O).

·       Fluorides least soluble (high lattice energy).

·       Heating hydrates: Be/Mg-halide hydrates hydrolyse; Ca/Sr/Ba hydrates dehydrate to anhydrous salts.

Carbonates, Sulphates, Nitrates

·       Carbonates (MCO₃): insoluble in water; solubility ↓ down group; all decompose on heating → MO + CO₂ (stability ↑ down group). BeCO₃ unstable (kept under CO₂).

·       Sulphates (MSO₄): BeSO₄, MgSO₄ soluble; solubility decreases Ca → Ba.

·       Nitrates (M(NO₃)₂): hydration decreases down group; on heating: 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.

5) Solubility Logic (Why the Trends?)

·       Hydroxides: lattice enthalpy drops markedly with bigger M²⁺, outweighing decreased hydration → solubility ↑ down group.

·       Carbonates/Sulphates: large anions → lattice enthalpy roughly constant across group; hydration ↓ down group → solubility ↓ down group.

6) Beryllium: Anomalous Behaviour & Diagonal Relationship with Aluminium

·       Tiny Be²⁺, high ionisation enthalpy → high polarising power → covalent bonding & hydrolysis.

·       Coordination number generally ≤ 4 (no low‑lying d orbitals in valence shell).

·       Amphoteric BeO and Be(OH)₂.

·       Diagonal relationship (Be ↔ Al): passive to acids (oxide film), amphoteric hydroxides forming [Be(OH)₄]²⁻/[Al(OH)₄]⁻; chloride Lewis acidity; complexes [BeF₄]²⁻/[AlF₆]³⁻.

7) Core Calcium Compounds (Industrial Importance)

Quicklime (CaO)

·       Manufacture: CaCO₃ (1070–1270 K) → CaO + CO₂ (continuous CO₂ removal).

·       Reactions: CaO + H₂O → Ca(OH)₂ (slaking); CaO + SiO₂ → CaSiO₃; 6CaO + P₄O₁₀ → 2Ca₃(PO₄)₂.

·       Uses: cement, cheap alkali, sugar refining, dyestuffs; getter with Ba.

Slaked lime (Ca(OH)₂)

·       Lime water (solution), milk of lime (suspension).

·       CO₂ test: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O; excess CO₂: Ca(HCO₃)₂ (ppt dissolves).

·       Bleaching powder step: 2Ca(OH)₂ + 2Cl₂ → CaCl₂ + Ca(OCl)₂ + 2H₂O.

·       Uses: mortar/whitewash, glass, tanning, bleaching powder, sugar purification.

Calcium Carbonate (CaCO₃)

·       Prep: Ca(OH)₂ + CO₂ → CaCO₃; CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl (avoid excess CO₂).

·       Thermal: CaCO₃ (≈1200 K) → CaO + CO₂; acids liberate CO₂.

·       Uses: building marble, quicklime source, flux in metallurgy, precipitated CaCO₃ for paper, antacid, toothpaste abrasive, fillers.

Plaster of Paris (CaSO₄·½H₂O)

·       From gypsum at ~393 K: CaSO₄·2H₂O → 2(CaSO₄·½H₂O) + 3H₂O; above 393 K → anhydrous (dead‑burnt).

·       Sets with water to a hard mass in 5–15 min; uses: casts, immobilising fractures, dentistry, ornamentation.

Cement (Portland)

·       Clinker (lime + clay heated) ground with 2–3% gypsum.

·       Typical composition: CaO 50–60%; SiO₂ 20–25%; Al₂O₃ 5–10%; MgO 2–3%; Fe₂O₃ 1–2%; SO₃ 1–2%.

·       Quality ratios: SiO₂/Al₂O₃ = 2.5–4; CaO/(SiO₂+Al₂O₃+Fe₂O₃) ≈ 2.

·       Main phases: C₃S ~51%, C₂S ~26%, C₃A ~11%; gypsum retards setting (hydration–rearrangement).

8) Elemental Uses & Biological Roles

Uses (Elements)

·       Be: Cu–Be springs (high strength); X‑ray tube windows.

·       Mg: light alloys (Al/Zn/Mn/Sn) for aircraft; flash/bombs/signals; milk of magnesia; MgCO₃ in toothpaste.

·       Ca: reductant for refractory oxides; Ca/Ba as getters in vacuum tubes.

·       Ra: radiotherapy salts.

Biological Importance

·       Body stores: ~25 g Mg; ~1200 g Ca; daily need ≈ 200–300 mg.

·       Mg²⁺: cofactor for ATP‑dependent enzymes; central to chlorophyll; roles in nerve & muscle function.

·       Ca²⁺: ~99% in bones/teeth; neuromuscular transmission, membrane integrity, blood coagulation; plasma ~100 mg L⁻¹ regulated by calcitonin & PTH.

10) Must‑Remember Equations

·       Air/N₂: 2Mg + O₂ → 2MgO; 3Ca + N₂ → Ca₃N₂.

·       Water: M + 2H₂O → M(OH)₂ + H₂ (Be no rxn; Mg hot; Ca/Sr/Ba cold).

·       Halogens: M + X₂ → MX₂.

·       Hydrides: M + H₂ (Δ) → MH₂; BeCl₂ + LiAlH₄ → BeH₂ + LiCl + AlCl₃.

·       Be amphoterism: Be(OH)₂ + 2OH⁻ → [Be(OH)₄]²⁻.

·       Limewater: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O; +CO₂ → Ca(HCO₃)₂.

·       Nitrates (Δ): 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.

·       Cement raw step: CaCO₃ → CaO + CO₂; clinker + gypsum → cement.