s-Block Elements

The s-block elements comprise Group 1 (alkali metals, $ns^{1}$) and Group 2 (alkaline earth metals, $ns^{2}$). These elements have the largest atomic radii in their respective periods, the lowest ionization enthalpies (decreasing down each group), and exhibit predominantly +1 (Group 1) or +2 (Group 2) oxidation states.

Group 1 (Alkali Metals) are soft, low-density metals with characteristic flame colors: Li (crimson red), Na (golden yellow), K (violet), Rb (red-violet), and Cs (blue). Reactivity with water increases down the group: 2M + 2H₂O → 2MOH + H₂, ranging from Li (slow) to Cs (explosive).

Lithium shows anomalous behavior due to its small size and high polarizing power, forming a diagonal relationship with Mg. Both Li and Mg form normal oxides (Li₂O, MgO) while Na forms peroxide (Na₂O₂) and K, Rb, Cs form superoxides (KO₂). LiNO₃ decomposes to Li₂O + NO₂ + O₂ (like Mg(NO₃)₂), unlike other alkali metal nitrates which give nitrite + O₂. Li₂CO₃ decomposes on heating (others are stable), Li does not form solid bicarbonate, and LiCl is soluble in organic solvents (covalent character).

Sodium Compounds: NaOH (caustic soda) is manufactured by the Castner-Kellner cell using mercury cathode electrolysis of brine. Na₂CO₃ (washing soda) is prepared by the Solvay process: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ (precipitates because it is less soluble) → Na₂CO₃ + CO₂ + H₂O on heating. NaHCO₃ (baking soda) decomposes at low temperatures: 2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O. The chlor-alkali process electrolyzes brine to produce NaOH, Cl₂, and H₂.

Group 2 (Alkaline Earth Metals) have smaller atomic radii, higher IE, and higher melting points than Group 1 elements in the same period. Beryllium shows anomalous behavior with a diagonal relationship to Al: both form covalent chlorides (BeCl₂ is linear, polymeric), both have amphoteric oxides (BeO dissolves in both NaOH and HCl), and Be has a maximum covalency of 4 (no d-orbitals).

Calcium Compounds: CaO (quicklime) is formed by thermal decomposition of CaCO₃ at ~1073 K and reacts exothermically with water to form Ca(OH)₂ (slaked lime). Ca(OH)₂ is used in whitewashing: Ca(OH)₂ + CO₂ → CaCO₃ (white coating). CaSO₄.1/2H₂O (Plaster of Paris) sets by hydrating back to gypsum: CaSO₄.1/2H₂O + 3/2H₂O → CaSO₄.2H₂O (exothermic, slight expansion). Portland cement contains Ca₂SiO₄, Ca₃SiO₅, and Ca₃Al₂O₆; gypsum (2-3%) is added to slow the setting.

Biological Importance: Na+/K+ are essential for nerve impulse transmission and osmotic balance. $Mg^{2}$+ is the central atom in chlorophyll and an enzyme cofactor. $Ca^{2}$+ forms hydroxyapatite in bones/teeth and is needed for blood clotting and muscle contraction.

The key testable concept is the type of oxide formed by each alkali metal (Li → oxide, Na → peroxide, K/Rb/Cs → superoxide) and the Solvay process for Na₂CO₃ manufacture.