s-Block

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your JEE Advanced exam.

The s-block consists of Group 1 (alkali metals): Li, Na, K, Rb, Cs, Fr (radioactive) and Group 2 (alkaline earth metals): Be, Mg, Ca, Sr, Ba, Ra. Their outer electronic configurations are $ns^1$ and $ns^2$ respectively.

General Characteristics:

Property	Alkali Metals (Group 1)	Alkaline Earth Metals (Group 2)
Valence configuration	$ns^1$	$ns^2$
Common oxidation state	+1	+2
Ionisation energy	Low (520–376 kJ/mol for 1st IE)	Moderate (730–590 kJ/mol for 1st IE)
Atomic radius	Larger (decreases down group)	Smaller than Group 1 (same period)
Hydration enthalpy	Very high (Li⁺ > Na⁺ > K⁺…)	Very high (Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺)
Flame colour	Characteristic (Li: red, Na: yellow, K: violet, Cs: blue)	Ca: orange-red, Sr: crimson, Ba: green

Key Chemical Properties:

Reaction with water:

• Li, Na, K react with cold water: $2M + 2H_2O \to 2MOH + H_2 \uparrow$
• Ca reacts with hot water: $Ca + 2H_2O \to Ca(OH)_2 + H_2 \uparrow$
• Mg reacts with steam only: $Mg + H_2O \to MgO + H_2$
• Be does NOT react with water (oxide layer protects it)

Reaction with oxygen:

• Alkali metals form oxides, peroxides, and superoxides:
  • Li: $Li_2O$ (oxide)
  • Na: $Na_2O_2$ (peroxide)
  • K, Rb, Cs: $KO_2$ (superoxide) — $O_2^-$ ion
• Alkaline earth metals form oxides (BeO, MgO, CaO) and peroxides ($CaO_2$)

⚡ JEE Advanced exam tips:

• The solubility trend: for Group 1, most salts are soluble. Exceptions: $LiF$, $Li_3PO_4$ (sparingly soluble)
• For Group 2, solubility decreases down the group for hydroxides, fluorides, carbonates, sulphates (but increases for hydroxides for Group 1)
• $NaHCO_3$ is soluble; $Na_2CO_3$ is also soluble — carbonate solubility trend reverses between groups
• Beryllium halides ($BeCl_2$) are covalent and polymeric (chlorine bridges); Mg and Ca halides are ionic

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🟡 Standard — Regular Study (2d–2mo)

For JEE Advanced students who want genuine understanding.

Anomalous Behaviour of Lithium and Beryllium:

Lithium (diagonal relationship with Mg):

• Li is much smaller than other alkali metals — its properties are anomalous
• Li forms $Li_2O$ (like Mg, not Na which forms $Na_2O_2$)
• LiOH decomposes on heating: $2LiOH \to Li_2O + H_2O$ (like Mg, unlike other alkali hydroxides)
• $Li_2CO_3$ is sparingly soluble (like MgCO_3$, unlike other alkali carbonates)
• Li reacts with $N_2$ to form $Li_3N$ (like Mg)
• Li shows covalent character in some compounds (unlike purely ionic Na, K)

Beryllium (diagonal relationship with Al):

• BeCl₂ is covalent and polymeric (solid) — like $AlCl_3$ (unlike ionic MgCl₂)
• Be does not form a peroxide
• BeO is amphoteric (like $Al_2O_3$) — reacts with both acids and bases $BeO + 2HCl \to BeCl_2 + H_2O$; $BeO + NaOH \to Na_2[BeO_2] + H_2O$
• Be does not react with water; Al does not either (due to oxide layer)

Important Compounds:

Sodium carbonate ($Na_2CO_3 \cdot 10H_2O$ — washing soda): Solvay process: $2NH_3 + CO_2 + H_2O \to (NH_4)_2CO_3$; $(NH_4)_2CO_3 + NaCl \to NaHCO_3 \downarrow + NH_4Cl$; $2NaHCO_3 \to Na_2CO_3 + H_2O + CO_2$

Sodium hydroxide (NaOH — caustic soda): Manufactured by Castner-Kellner process (electrolysis of NaCl solution using mercury cathode). $2NaCl + 2H_2O \xrightarrow{\text{electrolysis}} 2NaOH + Cl_2 + H_2$

Portland cement contains: $CaO$ (~60-67%), $SiO_2$ (~17-25%), $Al_2O_3$ (~3-8%), $Fe_2O_3$ (~0.5-6%), plus $MgO$, alkalis, gypsum.

Hardness of Water:

Temporary hardness (bicarbonates of Ca and Mg) — removed by boiling: $Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3 \downarrow + CO_2 + H_2O$

Permanent hardness (chlorides and sulphates of Ca and Mg) — removed by:

1. Washing soda: $CaCl_2 + Na_2CO_3 \to CaCO_3 \downarrow + 2NaCl$
2. Ion exchange resins (zeolites): $Ca^{2+} \to Na^+$ exchange
3. Complexometric: EDTA titrations for quantitative determination

⚡ Common student mistakes:

1. Confusing the products of burning alkali metals with oxygen
2. Forgetting that Be and Mg do not follow the typical alkaline earth metal chemistry
3. Not knowing the exact diagonal relationship elements: Li↔Mg, Be↔Al
4. Confusing $Na_2O_2$ (peroxide, deep yellow) with $Na_2O$ (oxide, pale yellow)

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for JEE Advanced mastery of s-block chemistry.

Hydration Enthalpy and Ionic Radii:

The hydration enthalpy (energy released when gaseous ion is hydrated) is inversely proportional to ionic radius: $$|\Delta H_{hyd}| \propto \frac{1}{r_{ionic}}$$

For Group 1 cations at 298 K: $Li^+$: 520 kJ/mol, $r ≈ 76$ pm $Na^+$: 406 kJ/mol, $r ≈ 102$ pm $K^+$: 322 kJ/mol, $r ≈ 138$ pm

Smaller ions have higher hydration enthalpy — this affects solubility, mobility, and the quality of aqueous solutions.

Why Lithium Salts Are Often Soluble Differently:

Despite $Li^+$ being small and highly polarising (high charge density), the lattice energy of LiF is very high due to small $Li^+$ size — making $LiF$ insoluble. Meanwhile $LiCl$ is soluble because the lattice energy is overcome by hydration enthalpy. This is the key principle: lattice energy vs. hydration enthalpy competition determines solubility.

The Solubility Product Principle:

For sparingly soluble salts like $CaCO_3$: $$CaCO_3(s) \rightleftharpoons Ca^{2+}(aq) + CO_3^{2-}(aq)$$ $$K_{sp} = [Ca^{2+}][CO_3^{2-}]$$

The solubility product of Group 2 carbonates DECREASES down the group: $BeCO_3$: $K_{sp} ≈ 1 × 10^{-7}$, $MgCO_3$: $K_{sp} ≈ 1 × 10^{-5}$, $CaCO_3$: $K_{sp} ≈ 1 × 10^{-8}$ (dolomite), $BaCO_3$: $K_{sp} ≈ 1 × 10^{-9}$

This trend is due to decreasing hydration enthalpy (less able to stabilise ions in solution) and decreasing lattice enthalpy — the net effect is decreasing solubility.

Complexation in s-Block:

While d-block metals form extensive coordination complexes, s-block metals form FEW complexes:

• Crown ethers (e.g., 18-crown-6) selectively complex $K^+$ (ionic radius 138 pm matches the 18-crown-6 cavity size of ~260-320 pm)
• Cryptands (e.g., [2.2.2]-cryptand) complex $K^+$ even more selectively
• This selectivity is the basis of ion-selective electrodes

Photochemical Behaviour:

Sodium in a bunsen burner flame: the $3p \to 3s$ transition emits at 589 nm (D-lines, yellow) — actually two very close lines: $Na \cdot 589.0$ nm and $Na \cdot 589.6$ nm (fine structure from spin-orbit coupling, $^2P_{3/2}$ and $^2P_{1/2}$).

Biological Importance:

• Na⁺/K⁺ pump (Na⁺/K⁺-ATPase): maintains 10× more Na⁺ outside cells and 10× more K⁺ inside cells — essential for nerve impulse transmission, active transport
• Ca²⁺: muscle contraction, bone structure ($Ca_5(PO_4)_3OH$ = hydroxyapatite), blood clotting, enzyme cofactor
• Mg²⁺: central atom in chlorophyll (photosynthesis), ATP binding, enzyme cofactor
• Mg²⁺ is essential for all cells; deficiency causes muscle spasms, cardiovascular problems

Lattice Energy — Born-Haber Cycle:

Lattice energy for ionic compounds like $NaCl$: $$U = -\frac{N_A z_+ z_- e^2}{r_0}\left(1 - \frac{1}{n}\right)$$ where $n$ = Born exponent (typically 9 for NaCl-type), $r_0$ = interionic distance.

The Kapustinskii equation extends this to any ionic compound: $$U = -\frac{120.2 \times z_+ z_- \nu}{r_0}\left(1 - \frac{0.345}{r_0}\right) \text{ kJ/mol}$$ where $\nu$ = number of ions per formula unit.

JEE Advanced Previous Year Patterns:

• Anomalous behaviour of Li and Be: very common
• Solubility trends: common
• Flame tests: common
• Reaction with oxygen/water: common
• Diagonal relationship: common
• s-block in biological systems: occasionally tested

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