s-Block Elements & Hydrogen

The s-block elements comprise Groups 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) and Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra), along with hydrogen. Their chemistry is governed by the $ns^{1}$ (Group 1) and $ns^{2}$ (Group 2) valence configuration, leading to predominantly ionic chemistry with characteristic +1 and +2 oxidation states respectively.

Hydrogen — The Unique Element

Hydrogen ($1s^{1}$) occupies a unique position: it resembles both alkali metals ($ns^{1}$, forms H⁺) and halogens (one electron short of noble gas, forms H^-). It is placed in Group 1 but differs fundamentally — high IE (1312 kJ/mol), forms covalent bonds predominantly, and exists as diatomic H₂.

Types of hydrides: (1) Ionic/saline hydrides (NaH, CaH₂) — formed by electropositive metals, react vigorously with water producing H₂. (2) Covalent/molecular hydrides (H₂O, NH₃, CH₄) — formed by p-block elements. (3) Metallic/interstitial hydrides (TiH, PdH) — non-stoichiometric, H occupies interstitial sites, good conductors. Pd absorbs 900 times its volume of H₂.

Water: Amphoteric solvent, high dielectric constant (80.4), anomalous density maximum at 4 degrees C (ice floats). Hard water contains $Ca^{2}$+/$Mg^{2}$+ with HCO₃^- (temporary, removed by boiling) or $SO4^{2}$-/Cl^- (permanent, removed by Na₂CO₃/zeolite/ion exchange). Heavy water D₂O is used as moderator in nuclear reactors.

H₂O₂: Oxidation state of O is -1. Open-book structure with dihedral angle 111.5 degrees (gas). Acts as both oxidiser ($2Fe^{2}$+ + H₂O₂ + 2H⁺ → $2Fe^{3}$+ + 2H₂O) and reducer (2MnO₄^- + 5H₂O₂ + 6H⁺ → $2Mn^{2}$+ + 5O₂ + 8H₂O). Decomposes: 2H₂O₂ → 2H₂O + O₂. Stored in wax-coated dark bottles. Volume strength: "x volume" H₂O₂ releases x volumes of O₂ per volume of solution at STP.

Group 1 — Alkali Metals

Configuration: [noble gas] $ns^{1}$. Largest atoms in their period. Lowest IE (decreases down: Li > Na > K > Rb > Cs). Always +1 OS. Strong reducing agents. All react with water: 2M + 2H₂O → 2MOH + H₂ (vigour increases down the group: Li slow, Na vigorous, K ignites, Rb/Cs explosive).

Anomalous behaviour of Li: (1) Very small size, high charge density. (2) High IE compared to others. (3) LiF, Li₂CO₃, Li₃PO₄ are insoluble (unlike other Group 1 salts). (4) LiCl is covalent and soluble in organic solvents (Fajans' rules — small cation polarises anion). (5) Li₂CO₃ decomposes on heating (like MgCO₃). (6) Li forms nitride Li₃N directly (like Mg). (7) Diagonal relationship with Mg.

Compounds: NaOH (caustic soda) — Castner-Kellner process (Hg cathode). Na₂CO₃ (soda ash) — Solvay process (NaCl + NH₃ + CO₂ + H₂O → NaHCO₃, then heated). NaHCO₃ (baking soda) — mild base, used in antacids, fire extinguishers. KO₂ (potassium superoxide) — used in space/submarines: 4KO₂ + 2CO₂ → 2K₂CO₃ + 3O₂.

Oxides: Li forms Li₂O (normal oxide), Na forms Na₂O₂ (peroxide), K/Rb/Cs form superoxides MO₂. This trend reflects increasing cation size stabilising larger anions.

₂: Cation Size Effect Li₂O Normal oxide → Na₂O₂ Peroxide → KO₂, RbO₂, CsO₂ Superoxides Larger cation → stabilises larger anion (O₂- < O₂₂- < O₂-) Li+ too small to stabilise large O₂₂- or O₂-

Group 2 — Alkaline Earth Metals

Configuration: [noble gas] $ns^{2}$. Smaller than corresponding Group 1 elements. Higher IE (but still low). Always +2 OS. Form ionic compounds but with more covalent character than Group 1 (higher charge density).

Anomalous behaviour of Be: (1) Very small size, very high charge density. (2) Does not react with water (oxide layer). (3) BeCl₂ is covalent, polymeric (sp3 bridging), soluble in organic solvents. (4) BeO and Be(OH)₂ are amphoteric (unlike other Group 2 oxides). (5) Be does not form peroxide. (6) Be shows max covalence of 4 (no d-orbitals in 2nd period). (7) Diagonal relationship with Al: both form amphoteric oxides, covalent chlorides, [M(OH)₄]^- in excess NaOH.

Thermal stability of carbonates: BeCO₃ < MgCO₃ < CaCO₃ < SrCO₃ < BaCO₃ (increases down — larger cation stabilises large $CO3^{2}$- anion, harder to polarise and decompose). Same trend for hydroxides, nitrates, sulphates.

Solubility trends: Hydroxides — solubility increases down (Mg(OH)₂ insoluble, Ba(OH)₂ soluble). Sulphates — solubility decreases down (MgSO₄ soluble, BaSO₄ insoluble). This is due to interplay of lattice energy and hydration energy.

Important compounds: CaO (quicklime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone/marble/chalk), CaSO₄.2H₂O (gypsum), CaSO₄.1/2H₂O (Plaster of Paris — sets by hydration back to gypsum). MgSO₄.7H₂O (Epsom salt). BaSO₄ (X-ray barium meal, insoluble).

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