S-Block Elements

1) Position in Periodic Table

• Group 1: Alkali metals – Li, Na, K, Rb, Cs, Fr
• Group 2: Alkaline earth metals – Be, Mg, Ca, Sr, Ba, Ra
• Valence shell configuration:
  • Group 1: ns¹
  • Group 2: ns²

2) General Characteristics

• Highly electropositive
• Strong reducing agents
• Low ionisation enthalpy
• Large atomic and ionic radii
• Form ionic compounds

3) Physical Properties

• Soft metals (alkali metals can be cut with knife)
• Low melting and boiling points
• Density increases down the group
• Good conductors of heat and electricity

4) Chemical Properties

(a) Reaction with Water

2M + 2H₂O → 2MOH + H₂ ↑

(b) Reaction with Oxygen

4Li + O₂ → 2Li₂O

2Na + O₂ → Na₂O₂

K + O₂ → KO₂ (superoxide)

(c) Reaction with Halogens

2M + X₂ → 2MX

5) Oxides, Hydroxides & Carbonates

• Alkali metal oxides are strongly basic
• Alkali metal hydroxides are strong bases
• Carbonates of alkali metals are thermally stable
• Bicarbonates exist only for alkali metals

6) Diagonal Relationship

Elements of 2nd period show similarity with diagonally placed elements of 3rd period.

• Li ↔ Mg
• Be ↔ Al

7) Anomalous Behaviour of Lithium

• Small size and high charge density
• High polarising power
• Forms nitride: Li₃N
• Least reactive among alkali metals

8) Important Compounds of Sodium

Sodium Carbonate (Na₂CO₃·10H₂O)

• Prepared by Solvay process
• Used in glass, soap and paper industries

Sodium Bicarbonate (NaHCO₃)

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Sodium Hydroxide (NaOH)

• Prepared by electrolysis of brine
• Strong base, corrosive

9) Important Compounds of Calcium

Quicklime (CaO)

CaCO₃ → CaO + CO₂

Slaked Lime (Ca(OH)₂)

CaO + H₂O → Ca(OH)₂

Plaster of Paris

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O

10) Biological Importance

• Na⁺ and K⁺ regulate nerve impulses
• Mg²⁺ is present in chlorophyll
• Ca²⁺ essential for bones, teeth and blood clotting