Introduction

The s-block elements consist of Group 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) and Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra). Their outermost electrons occupy s-orbitals — $ns^{1}$ for Group 1 and $ns^{2}$ for Group 2. These elements are the most electropositive metals, with the largest atomic radii and lowest ionization enthalpies in their respective periods.

Group 1 — Alkali Metals

All alkali metals are soft, lustrous, low-density metals. Their defining features are:

• Electronic configuration: [Noble gas] $ns^{1}$
• Oxidation state: Exclusively +1
• Atomic and ionic radii: Largest in their period; increase down the group
• Ionization enthalpy: Very low, decreases down the group (Cs has the lowest — most easily oxidized)
• Reactivity with water: 2M + $2H_{2}O$ → 2MOH + $H_{2}$↑. Reactivity increases Li < Na < K < Rb < Cs

Flame colors are a NEET staple: Li (crimson red), Na (golden yellow), K (violet), Rb (red-violet), Cs (blue).

Oxide types in excess oxygen are one of the most tested facts: Li forms the normal oxide $Li_{2}O$, Na forms peroxide $Na_{2}O_{2}$, and K, Rb, Cs form superoxides ($KO_{2}$, $RbO_{2}$, $CsO_{2}$). The logic is lattice energy — larger cations stabilize larger, more diffuse anions.

Lithium's Anomalous Behavior (Diagonal Relationship with Mg)

Due to its exceptionally small ionic radius and highest charge density among alkali metals, Li resembles Mg more than Na:

• Both form only the normal oxide ($Li_{2}O$, MgO); neither forms peroxide or superoxide
• Both have carbonates that decompose on heating ($Li_{2}CO_{3}$ and $MgCO_{3}$); other alkali metal carbonates are thermally stable
• $LiNO_{3}$ decomposes to $Li_{2}O$ + $NO_{2}$ + $O_{2}$ (same as Mg($NO_{3}$)_{2}); other alkali metal nitrates decompose to give nitrite + $O_{2}$
• Li does not form a stable solid bicarbonate ($LiHCO_{3}$ exists only in solution)
• LiCl is soluble in organic solvents due to covalent character (Fajans' rules — small cation, large anion)

Key Sodium Compounds

NaOH (Caustic Soda): Manufactured by electrolysis of brine in the Castner-Kellner cell. The cathode is mercury; sodium forms an amalgam (Na-Hg) which then reacts with water to yield NaOH. The chlor-alkali process is a modern variant that co-produces $Cl_{2}$ and $H_{2}$.

$Na_{2}CO_{3}$ (Washing Soda): Produced via the Solvay process:

• NaCl + $NH_{3}$ + C$O_{2}$ + $H_{2}$O → $NaHCO_{3}$↓ + $NH_{4}Cl$
• $NaHCO_{3}$ precipitates because it is the least soluble species in the mixture (not because it forms first — a common NEET trap)
• 2$NaHCO_{3}$ → $Na_{2}CO_{3}$ + C$O_{2}$↑ + $H_{2}$O on heating
• $NH_{3}$ is recycled by treating $NH_{4}Cl$ with Ca(OH)_{2}

$NaHCO_{3}$ (Baking Soda): Decomposes at low temperature (2$NaHCO_{3}$ → $Na_{2}CO_{3}$ + C$O_{2}$ + $H_{2}$O), making it useful as a leavening agent and mild antacid.

Group 2 — Alkaline Earth Metals

Group 2 metals are harder, denser, and have higher melting points and ionization enthalpies than their Group 1 counterparts in the same period. They form +2 ions exclusively.

Beryllium's Anomalous Behavior (Diagonal Relationship with Al):

• Both form amphoteric oxides: BeO dissolves in both NaOH (BeO + 2NaOH → Na_{2}Be$$O_{2} + $H_{2}$O) and HCl
• Both form covalent chlorides: Be$Cl_{2}$ is linear and polymeric in solid state; $AlCl_{3}$ is dimeric
• Both have a maximum covalency of 4 (no d-orbitals available in the valence shell)
• Both form complex fluorides ($BeF_{4}^{2-}$, $AlF_{6}^{3-}$)

Key Calcium Compounds

CaO (Quicklime): Formed by strong heating of limestone: $CaCO_{3}$ →(1073 K) CaO + C$O_{2}$. Reacts exothermically with water to give slaked lime Ca(OH)_{2}.

Ca(OH)_{2} (Slaked Lime): Used in whitewashing. On exposure to air, C$O_{2}$ reacts to form $CaCO_{3}$ (white coating): Ca(OH)_{2} + C$O_{2}$ → $CaCO_{3}$↓ + $H_{2}$O.

Plaster of Paris ($CaSO_{4}$·½$H_{2}$O): Made by heating gypsum at 373 K: 2$CaSO_{4}$·$2H_{2}O$ → 2$CaSO_{4}$·½$H_{2}$O + 3$H_{2}$O. Setting reaction: $CaSO_{4}$·½$H_{2}$O + 3/$2H_{2}O$ → $CaSO_{4}$·$2H_{2}O$ (gypsum). Exothermic and slightly expansive.

Portland Cement: Contains $Ca_{2}SiO_{4}$, $Ca_{3}SiO_{5}$, and $Ca_{3}Al_{2}O_{6}$. Gypsum (2-3%) is added to retard the setting rate.

Biological Importance

• $Na^{+}$ and $K^{+}$ maintain osmotic balance and are critical for nerve impulse transmission via the $Na^{+}$-$K^{+}$ ATPase pump
• $Mg^{2+}$ is the central atom of chlorophyll (porphyrin ring) and an essential enzyme cofactor
• $Ca^{2+}$ forms hydroxyapatite [$Ca_{10}$($PO_{4}$){6}(OH){2}] in bones and teeth, participates in blood clotting, and triggers muscle contraction

NEET Examination Focus

The highest-yield facts are: (1) oxide types formed by each alkali metal in excess $O_{2}$, (2) Solvay process mechanism and why $NaHCO_{3}$ precipitates, (3) Li-Mg and Be-Al diagonal relationship properties, and (4) Plaster of Paris setting reaction with expansion.