The s-block elements (supplementary)

Group 1 — alkali metals

Supplementary note: some CBSE Class 11 schedules no longer examine this full chapter. Use only if your school still teaches it.

Li, Na, K, Rb, Cs, Fr: one valence electron, +1 ions, soft metals, low densities (Li, Na, K float on water with caution — violent reaction).

Trends down the group: atomic radius increases, ionisation enthalpy decreases, reactivity with water and oxygen increases. Flame colours (Na yellow, K lilac) come from excited electrons — a qualitative test link.

Important compounds: NaOH (Castner–Kellner / chlor-alkali idea), Na₂CO₃, NaHCO₃ (baking soda, antacid), K₂SO₄ fertiliser context. Anomalous Li: small size, polarising, forms covalent compounds more readily than other alkali metals.

Group 2 — alkaline earth metals

Be, Mg, Ca, Sr, Ba, Ra: two valence electrons, +2 ions. Be is less typical (covalent, amphoteric BeO); exam focus is often Mg and Ca.

Trends: radius up, ionisation down, reactivity with water up (Mg needs hot water or steam; Ca reacts with cold water).

Compounds: CaO (quicklime), Ca(OH)₂ (slaked lime), CaCO₃ (limestone, marble), plaster of Paris (CaSO₄·½H₂O) from gypsum — know one-line uses.

Reactions to compare

With oxygen: oxides form; peroxides and superoxides appear for heavier alkali metals (Na₂O₂, KO₂). Alkaline earth oxides are basic (except BeO).

With water: alkali metals → MOH + H₂; alkaline earth → M(OH)₂ + H₂ with increasing vigour down the group.

Diagonal relationship (Li with Mg, Be with Al): similar charge/radius ratios give similar properties — LiCl partly covalent, Be amphoteric, etc.