s-Block and p-Block Elements

Periodicity in the s-block

Group 1 (alkali metals: Li, Na, K, Rb, Cs) and Group 2 (alkaline earth: Be, Mg, Ca, Sr, Ba) share the defining feature of an outermost ns¹ or ns² configuration. Atomic radius increases down the group, ionisation energy decreases, and metallic character intensifies. The FSc Punjab Textbook Chemistry XII Chapter 1 stresses that Li and Be are anomalous because of their small size and high charge density: Li resembles Mg, and Be resembles Al — the famous diagonal relationship. Alkali metals react vigorously with water (2Na + 2H₂O → 2NaOH + H₂), with reactivity increasing K > Na > Li in terms of vigour, though Li has the most negative E° due to high hydration energy.

Sodium, the workhorse of Group 1

Sodium is extracted by Down's cell electrolysis of molten NaCl with CaCl₂ added to lower the melting point from 801 °C to about 600 °C. At the cathode Na⁺ + e⁻ → Na; at the anode 2Cl⁻ → Cl₂ + 2e⁻. NaOH is manufactured by the chlor-alkali (Castner-Kellner or membrane) process. Sodium peroxide Na₂O₂ forms in air and is used in submarines to absorb CO₂: 2Na₂O₂ + 2CO₂ → 2Na₂CO₃ + O₂. The Solvay process for Na₂CO₃ uses NH₃, CO₂, and brine; ammonia is recovered, making it efficient.

Group 13 and 14 highlights

Aluminium (Group 13) is amphoteric: it dissolves in NaOH to give sodium aluminate (2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂) and in HCl to give AlCl₃. Its passivation by a thin Al₂O₃ layer underlies anodising. Boron, the lightest member, is a non-metal forming covalent BX₃ Lewis acids — central to many organic reactions. In Group 14, the inert pair effect explains why Pb prefers the +2 state while C and Si are firmly +4. CO₂ is a discrete linear molecule; SiO₂ is a giant covalent solid — a textbook contrast cited in Atkins Inorganic Chemistry.

Group 15, 16, 17 and oxidising power

Nitrogen exists as N≡N with a bond enthalpy of 945 kJ/mol, explaining its inertness. Phosphorus shows allotropy: white (P₄, reactive), red (polymeric, stable). HNO₃ is manufactured by Ostwald's process: catalytic oxidation of NH₃ over Pt-Rh at 850 °C. H₂SO₄ via the Contact process uses V₂O₅ at 450 °C. Halogens (Group 17) are the most electronegative non-metals: oxidising power F₂ > Cl₂ > Br₂ > I₂. F₂ displaces all other halides; Cl₂ displaces Br⁻ and I⁻. Bond dissociation: Cl–Cl (242) > Br–Br (193) > F–F (158, anomalously low due to lone-pair repulsion in the small F₂ molecule) > I–I (151).

Diagonal relationships and anomalies

Li-Mg: both form nitrides directly with N₂, both have covalent halides soluble in organic solvents, and both have carbonates that decompose on heating (unlike Na₂CO₃). Be-Al: both amphoteric oxides, both form covalent chlorides (BeCl₂, AlCl₃ dimerise as Be₂Cl₄ and Al₂Cl₆). B-Si: both form acidic oxides, both make hydrides that hydrolyse. These patterns dominate MDCAT trend questions, and Cotton & Wilkinson Advanced Inorganic Chemistry remains the canonical reference.