Last Updated: May 2026
The s-Block Elements chapter (NCERT Class 11 Chapter 9 in the rationalised syllabus) covers Group 1 (alkali metals) and Group 2 (alkaline earth metals). It contributes 1–2 questions on average to JEE Main 2027, and the chapter is a goldmine for students who memorise five anomalous-property comparisons and ten signature reactions.
Quick Reference Table — Group 1 vs Group 2
| Property | Group 1 (Alkali) | Group 2 (Alkaline Earth) |
|---|---|---|
| Outer config | ns¹ | ns² |
| Common oxidation state | +1 | +2 |
| Atomic radius (down group) | Increases (Li < Na < K < Rb < Cs) | Increases (Be < Mg < Ca < Sr < Ba) |
| Ionisation energy (down group) | Decreases | Decreases |
| Hydration enthalpy | Li > Na > K > Rb > Cs (ionic radius rises) | Be > Mg > Ca > Sr > Ba |
| Reaction with water | All vigorous; Cs explosive | Be does not react; Mg slow; Ca-Ba vigorous |
| Solubility of hydroxides (down group) | Increases | Increases |
| Solubility of carbonates / sulphates (down group) | — | Decreases |
Anomalous Behaviour of Lithium and Beryllium
Both first-period s-block elements deviate from group trends because of their small size, high charge density, and high polarising power. Six classic Li/Be anomalies:
- Li reacts only with N₂ to form Li₃N (other alkali metals do not)
- Li hydroxide is sparingly soluble; LiF is sparingly soluble in water (lattice energy beats hydration)
- Be does not react with water even on heating; Mg reacts only on heating
- Be forms covalent compounds (BeCl₂ is covalent, polymeric in solid)
- Be compounds are amphoteric (BeO + Be(OH)₂ react with both acids and bases)
- Be shows diagonal relationship with Al; Li with Mg
Diagonal Relationship — Two Pairs to Memorise
| Diagonal Pair | Shared Behaviour |
|---|---|
| Li and Mg | Both burn in N₂; both form normal oxides only; both do not form solid bicarbonates |
| Be and Al | Both amphoteric; both form covalent halides; both passivated by HNO₃ |
Ten Signature Reactions
- 4 Na + O₂ → 2 Na₂O ; 2 Na + O₂ → Na₂O₂ ; K, Rb, Cs → superoxide MO₂
- 2 Na + 2 H₂O → 2 NaOH + H₂↑ (vigorous; with K, Rb, Cs explosive)
- Mg + 2 HCl → MgCl₂ + H₂↑
- Solvay process: 2 NaCl + CaCO₃ → Na₂CO₃ + CaCl₂ (uses NH₃ as catalyst, intermediate is NaHCO₃)
- Castner-Kellner process: electrolysis of NaCl brine → NaOH + Cl₂ + H₂
- CaO (quick lime) + H₂O → Ca(OH)₂ (slaked lime) — exothermic
- CaCO₃ + heat → CaO + CO₂↑ (limestone calcination)
- Mg + N₂ → Mg₃N₂ (on burning)
- Be(OH)₂ + 2 NaOH → Na₂[Be(OH)₄] (amphoteric)
- 2 Mg + CO₂ → 2 MgO + C (Mg burns even in CO₂; never use CO₂ extinguisher on Mg fire)
Industrial Compounds — Memorise Formulae and Uses
- Na₂CO₃·10H₂O — washing soda; produced by Solvay process
- NaHCO₃ — baking soda; antacid; fire extinguisher
- NaOH — caustic soda; soap, paper, textile industries
- CaO — quick lime; steel-making, cement
- Ca(OH)₂ — slaked lime; mortar, whitewash
- CaSO₄·½H₂O — Plaster of Paris; surgical casts
- MgSO₄·7H₂O — Epsom salt; medicinal laxative
Biological Importance
- Na — extracellular cation; nerve transmission
- K — intracellular cation; muscle contraction; ATP synthesis
- Mg — chlorophyll cofactor; ribosome assembly
- Ca — bones, teeth, blood-clotting cascade, muscle contraction
Flame Test Colours
| Element | Flame Colour |
|---|---|
| Li | Crimson red |
| Na | Golden yellow |
| K | Lilac (pale violet) |
| Rb | Red-violet |
| Cs | Blue |
| Ca | Brick red |
| Sr | Crimson |
| Ba | Apple green |
35 Practice MCQs — JEE Main s-Block
[cg_quiz id=”cg-jee-s-block-2027″]
Frequently Asked Questions
Why does Li form Li₃N but Na does not?
Li is small with high charge density, giving it sufficient lattice energy to stabilise Li₃N. Na is larger, the lattice energy is lower, and the reaction with N₂ is not thermodynamically favoured.
Why is Be(OH)₂ amphoteric while Mg(OH)₂ is basic?
Be is small with high polarising power and forms covalent bonds. Be(OH)₂ donates and accepts protons. Mg, being larger, forms ionic Mg(OH)₂ that is purely basic.
What is the diagonal relationship?
First-row elements show similar properties to the second-row, second-group elements (Li-Mg, Be-Al, B-Si). The similarity arises from comparable charge-to-size ratios and electronegativities.
Why is hydration enthalpy of Li⁺ highest among alkali metals?
Li⁺ has the smallest ionic radius and therefore the highest charge density, attracting water molecules most strongly. This is why Li is the strongest reducing agent in aqueous solution despite having a relatively higher ionisation energy.
Continue Your JEE 2027 Prep
- JEE Main p-Block Elements 2027
- JEE Main d and f Block Elements 2027
- JEE Gurukul Courses
- JEE Free Mock Test
Bottom line: Memorise the diagonal relationship pairs, six Li/Be anomalies, ten reactions, eight flame-test colours and seven industrial compounds. JEE Main rewards specifics — formulas of Solvay intermediates, polymerism of BeCl₂, anomaly of Mg burning in CO₂.