Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
The s-Block Elements comprise Group 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) and Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra) of the periodic table. These are the most electropositive elements, forming +1 and +2 cations respectively. CBSE Class 11 Chemistry Chapter 10 covers their electronic configurations, trends, anomalous behaviours, diagonal relationships, important compounds, biological significance, and Portland cement.
This chapter has significant overlap with competitive exam syllabi (NEET, JEE) and requires thorough memorization of trends and compound-specific details.
Group 1: Alkali Metals (Li, Na, K, Rb, Cs, Fr)
Electronic Configuration
| Element | Configuration | Atomic Radius (pm) |
|---|---|---|
| Li | [He] 2s¹ | 152 |
| Na | [Ne] 3s¹ | 186 |
| K | [Ar] 4s¹ | 227 |
| Rb | [Kr] 5s¹ | 248 |
| Cs | [Xe] 6s¹ | 265 |
All have ns¹ configuration — one electron in the outermost shell, which they readily lose to form M⁺ ions.
Trends in Physical Properties
| Property | Trend down the group | Reason |
|---|---|---|
| Atomic radius | Increases | New shells added |
| Ionic radius | Increases | Same as atomic radius trend |
| Ionization enthalpy | Decreases | Atomic size increases, nuclear pull decreases |
| Hydration enthalpy | Decreases (Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺) | Smaller ions are more hydrated |
| Density | Increases (Li < Na < K < Rb < Cs) | Mass increase outweighs volume increase; K is an exception (K < Na in density) |
| Melting point | Decreases | Metallic bonding weakens as size increases |
| Flame colour | Li = crimson, Na = yellow, K = lilac, Rb = red violet, Cs = blue | Electron excitation and emission |
Trends in Chemical Properties
1. Reactivity increases down the group: Cs is the most reactive alkali metal. 2. Reaction with water: 2M + 2H₂O → 2MOH + H₂ (vigorous; K, Rb, Cs react explosively). 3. Reaction with oxygen: Li₂O (oxide), Na₂O₂ (peroxide), KO₂ (superoxide). Larger ions stabilize larger anions (O²⁻ < O₂²⁻ < O₂⁻). 4. Reaction with hydrogen: 2M + H₂ → 2MH (ionic hydrides; reactivity increases down group). 5. Reaction with halogens: 2M + X₂ → 2MX (vigorous; reactivity increases down group). 6. Reaction with nitrogen: Only Li reacts: 6Li + N₂ → 2Li₃N (Li is unique among alkali metals). 7. Reducing power: Increases down the group (Cs is strongest reducing agent).
Anomalous Behaviour of Lithium
Lithium differs from other alkali metals due to its very small size, high charge/size ratio, and high polarization power.
| Property | Lithium | Other Alkali Metals |
|---|---|---|
| Hardness | Hardest | Softer |
| Melting point | Highest (180 °C) | Lower |
| Ionization enthalpy | Highest | Lower |
| Reaction with N₂ | Forms Li₃N | Do not react |
| Halides | Partially covalent | Predominantly ionic |
| Carbonate | Li₂CO₃ decomposes on heating | Stable to heat |
| Hydroxide | LiOH decomposes on heating | Stable |
| Bicarbonate | LiHCO₃ does not exist as solid | Exist as solids |
| Hydration | Maximum hydration enthalpy | Decreases down group |
Diagonal Relationship: Li and Mg
Li (Group 1, Period 2) resembles Mg (Group 2, Period 3) due to similar charge/size ratio.
| Similarity | Explanation |
|---|---|
| Both form nitrides | 6Li + N₂ → 2Li₃N; 3Mg + N₂ → Mg₃N₂ |
| Both form normal oxides on burning | 4Li + O₂ → 2Li₂O; 2Mg + O₂ → 2MgO |
| Hydroxides decompose on heating | 2LiOH → Li₂O + H₂O; Mg(OH)₂ → MgO + H₂O |
| Carbonates decompose on heating | Li₂CO₃ → Li₂O + CO₂; MgCO₃ → MgO + CO₂ |
| Chlorides are deliquescent | LiCl and MgCl₂ absorb moisture |
| Both are harder and lighter | Higher melting points than other group members |
Important Compounds of Sodium
Sodium Hydroxide (NaOH) — Caustic Soda
Preparation (Castner-Kellner cell / Chlor-alkali process):
- Electrolysis of brine: 2NaCl + 2H₂O → 2NaOH + H₂↑ + Cl₂↑
- NaOH at cathode, H₂ at cathode, Cl₂ at anode.
Properties: White crystalline solid, deliquescent, soluble in water (exothermic), strong base.
Uses: Soap making, paper industry, petroleum refining, rayon manufacture, drain cleaner.
Sodium Carbonate (Na₂CO₃·10H₂O) — Washing Soda
Preparation (Solvay process): 1. NH₃ + H₂O + NaCl + CO₂ → NaHCO₃↓ + NH₄Cl 2. 2NaHCO₃ →(heat) Na₂CO₃ + H₂O + CO₂
Properties: White crystalline solid, efflorescent (loses water of crystallization), alkaline in solution.
Uses: Water softening, glass making, soap and detergent industry, laundry.
Sodium Bicarbonate (NaHCO₃) — Baking Soda
Preparation: Intermediate in Solvay process (see above).
Properties: White crystalline solid, mild antacid, decomposes on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Uses: Baking (with acid generates CO₂ for leavening), antacid, fire extinguisher.
Sodium Chloride (NaCl) — Common Salt
Occurrence: Sea water (2.7% NaCl), rock salt deposits.
Uses: Table salt, raw material for NaOH, Na₂CO₃, Cl₂, H₂ (chlor-alkali industry), food preservation.
Group 2: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)
Electronic Configuration
| Element | Configuration | Atomic Radius (pm) |
|---|---|---|
| Be | [He] 2s² | 111 |
| Mg | [Ne] 3s² | 160 |
| Ca | [Ar] 4s² | 197 |
| Sr | [Kr] 5s² | 215 |
| Ba | [Xe] 6s² | 217 |
All have ns² configuration — they lose two electrons to form M²⁺ ions.
Trends in Physical and Chemical Properties
| Property | Trend down the group | Reason |
|---|---|---|
| Atomic/ionic radius | Increases | New shells added |
| Ionization enthalpy | Decreases | Size increases, nuclear pull decreases |
| Density | Increases (exception: Ca < Mg) | Mass increases more than volume |
| Melting point | No clear trend (Be highest at 1283 °C) | Varied crystal structures |
| Flame colour | Ca = brick red, Sr = crimson, Ba = apple green | Be and Mg have no characteristic flame colour |
| Reactivity with water | Increases (Be < Mg < Ca < Sr < Ba) | Easier electron loss down group |
| Basic character of hydroxides | Increases | Ionic character increases |
| Solubility of sulphates | Decreases | Lattice energy decreases less than hydration energy |
Reaction with Water
| Element | Reaction |
|---|---|
| Be | No reaction with water (oxide layer protects) |
| Mg | Reacts with steam: Mg + 2H₂O → Mg(OH)₂ + H₂ |
| Ca | Reacts with cold water: Ca + 2H₂O → Ca(OH)₂ + H₂ |
| Sr, Ba | Vigorous reaction with cold water |
Anomalous Behaviour of Beryllium
Be differs from other Group 2 elements due to its very small size, high charge/size ratio, and high ionization enthalpy.
| Property | Beryllium | Other Group 2 Elements |
|---|---|---|
| Coordination number | Maximum 4 | Higher (6) |
| Oxide/hydroxide | Amphoteric (BeO, Be(OH)₂) | Basic |
| Halides | Covalent (BeCl₂ polymeric) | Ionic |
| Reaction with water | No reaction | React (Mg with steam, others with cold water) |
| Hydride | Covalent (BeH₂ polymeric) | Ionic |
| Carbonate | Unstable at room temperature | Stable |
Diagonal Relationship: Be and Al
Be (Group 2, Period 2) resembles Al (Group 13, Period 3).
| Similarity | Explanation |
|---|---|
| Both form amphoteric oxides | BeO and Al₂O₃ dissolve in both acid and base |
| Both form covalent halides | BeCl₂ and AlCl₃ are covalent, act as Lewis acids |
| Both resist acid attack | Due to oxide layer |
| Both form complex ions | [BeF₄]²⁻ and [AlF₆]³⁻ |
| Both have no characteristic flame colour | Be and Al do not impart colour to flame |
| Hydroxides are amphoteric | Be(OH)₂ and Al(OH)₃ dissolve in both acid and base |
Important Compounds of Calcium
Calcium Oxide (CaO) — Quicklime
Preparation: CaCO₃ →(heat) CaO + CO₂ (limestone decomposition)
Properties: White solid, mp 2570 °C, reacts vigorously with water (slaking): CaO + H₂O → Ca(OH)₂ + heat.
Uses: Manufacture of cement, steel (flux), soda lime, soil treatment, white paint (whitewash).
Calcium Hydroxide (Ca(OH)₂) — Slaked Lime
Preparation: CaO + H₂O → Ca(OH)₂ (slaking of quicklime)
Properties: White powder, sparingly soluble in water (solution called limewater).
Uses: White washing, sugar refining, leather industry, soil treatment (reduces acidity).
Calcium Carbonate (CaCO₃) — Limestone
Occurrence: Limestone, marble, chalk, calcite.
Properties: Reacts with acids: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Decomposes on heating: CaCO₃ → CaO + CO₂.
Uses: Cement manufacture, building material, antacid, chalk.
Calcium Sulphate (CaSO₄)
- Anhydrite: CaSO₄ (anhydrous)
- Gypsum: CaSO₄·2H₂O — used in plaster of Paris
- Plaster of Paris (POP): CaSO₄·½H₂O — prepared by heating gypsum at 373 K: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
- Uses of POP: Casts for broken bones, statues, casts, surgical bandages.
Important: If gypsum is heated above 373 K, it forms dead burnt plaster (anhydrous CaSO₄) which does not set with water.
Biological Importance of s-Block Elements
| Element | Biological Role |
|---|---|
| Na⁺ | Major extracellular cation; maintains osmotic pressure, acid-base balance, nerve impulse transmission |
| K⁺ | Major intracellular cation; regulates heart function, muscle contraction, nerve impulses; component of Na⁺/K⁺ pump |
| Mg²⁺ | Cofactor for enzymes (hexokinase, DNA polymerase); component of chlorophyll; stabilizes ribosomes |
| Ca²⁺ | Component of bones and teeth (hydroxyapatite: Ca₁₀(PO₄)₆(OH)₂); blood clotting; muscle contraction; nerve function |
| Na⁺/K⁺ pump | Active transport: 3Na⁺ out + 2K⁺ in per ATP; maintains electrochemical gradient |
Deficiency diseases:
- Na⁺ deficiency: Low blood pressure, muscle cramps
- K⁺ deficiency: Heart irregularities, muscle weakness
- Mg²⁺ deficiency: Growth retardation, chlorosis in plants
- Ca²⁺ deficiency: Osteoporosis, rickets, tetany
Portland Cement
Composition
| Component | Formula | Percentage |
|---|---|---|
| Tricalcium silicate | 3CaO·SiO₂ (C₃S) | 45–50% |
| Dicalcium silicate | 2CaO·SiO₂ (C₂S) | 25–30% |
| Tricalcium aluminate | 3CaO·Al₂O₃ (C₃A) | 8–12% |
| Tetracalcium aluminoferrite | 4CaO·Al₂O₃·Fe₂O₃ (C₄AF) | 8–12% |
Raw Materials
- Calcareous: Limestone (CaCO₃)
- Argillaceous: Clay (provides SiO₂, Al₂O₃, Fe₂O₃)
Manufacture Process
1. Mixing: Limestone and clay are crushed and mixed in correct proportions. 2. Burning (calcination): Mixture heated in rotary kiln at 1500–1600 °C. Product = clinker. 3. Grinding: Clinker mixed with 2–3% gypsum (CaSO₄·2H₂O) to prevent rapid setting.
Role of gypsum: Retards the setting time of cement (prevents flash setting by C₃A).
Setting of Cement
- Initial setting: C₃A reacts with water rapidly (gypsum slows this).
- Hardening: C₃S and C₂S react with water to form a gel of calcium silicate hydrate, providing strength.
Alkali vs Alkaline Earth Metals — Comparison
| Property | Alkali Metals (Group 1) | Alkaline Earth Metals (Group 2) |
|---|---|---|
| Valence electrons | 1 (ns¹) | 2 (ns²) |
| Oxidation state | +1 | +2 |
| Ionization enthalpy | Lower | Higher (removing 2 electrons) |
| Reactivity | More reactive | Less reactive |
| Reaction with water | Vigorous (K, Rb, Cs explosive) | Less vigorous (Be no reaction, Mg with steam) |
| Oxides | Li₂O, Na₂O₂, KO₂ | MO (mostly oxides) |
| Hydroxides | Stronger bases MOH | Less basic M(OH)₂ |
| Carbonates | M₂CO₃ (stable except Li₂CO₃) | MCO₃ (decompose down group) |
| Flame colour | All show characteristic colours | Ca, Sr, Ba show; Be, Mg do not |
| Density | Lower (Li floats on water) | Higher |
Common Mistakes
1. Confusing oxide, peroxide, and superoxide formation: Li forms only Li₂O (oxide), Na forms Na₂O₂ (peroxide), K, Rb, Cs form MO₂ (superoxide). The trend is due to larger cations stabilizing larger anions through lattice energy.
2. Li is NOT stored in kerosene: Li is less reactive and is stored wrapped in paraffin wax. Na and K are stored in kerosene. Students often write "all alkali metals stored in kerosene."
3. Be(OH)₂ is amphoteric, not basic: Unlike Ca(OH)₂ and Ba(OH)₂, Be(OH)₂ dissolves in both acid and base. This is a common exam trap.
4. Gypsum temperature: Plaster of Paris is formed at 373 K. Heating above 373 K gives dead burnt plaster (useless). Students confuse the temperature.
5. Solvay process limitations: Solvay process cannot be used for K₂CO₃ because KHCO₃ is too soluble to precipitate. This is a specific detail asked in exams.
6. Diagonal relationship direction: Li resembles Mg (not Na); Be resembles Al (not B). The diagonal goes from top-left to bottom-right.
7. Flame colours: Na = yellow, K = lilac (violet), Ca = brick red, Sr = crimson, Ba = apple green. Students often mix up Sr and Ba.
Exam Tips
1. Trend-based questions (2–3 marks): Practice writing trends down Group 1 and Group 2 with reasons. Atomic radius, ionization enthalpy, reactivity, and hydration enthalpy are the most asked.
2. Anomalous behaviour (3 marks): Li's anomalous behaviour and Be's anomalous behaviour are frequently asked. List at least 4–5 points with brief reasons.
3. Diagonal relationship (2–3 marks): Explain Li-Mg or Be-Al similarity with 4 examples. This is a standard question.
4. Compound preparation (3 marks): Solvay process for Na₂CO₃, Castner-Kellner cell for NaOH, and plaster of Paris from gypsum. Write balanced equations.
5. Biological importance (2 marks): Know the roles of Na, K, Mg, Ca. Na⁺/K⁺ pump is a commonly asked detail.
6. Portland cement (2–3 marks): Composition (4 compounds with formulas), role of gypsum, setting process. Numericals on percentage composition may appear.
7. Flame colours (1 mark): Memorize the flame colours for all alkali and alkaline earth metals.
Quick Revision Notes
- Group 1 (ns¹): Li, Na, K, Rb, Cs, Fr. Highly reactive, stored in kerosene (Li in paraffin), form +1 ions.
- Group 2 (ns²): Be, Mg, Ca, Sr, Ba, Ra. Less reactive than Group 1, form +2 ions.
- Trends down Group 1: Size ↑, IE ↓, reactivity ↑, hydration enthalpy ↓, density ↑, melting point ↓.
- Anomalous Li: Hard, highest IE, forms Li₃N, covalent halides, Li₂CO₃ decomposes, diagonal relationship with Mg.
- Anomalous Be: Small, high IE, amphoteric oxide/hydroxide, covalent halides, diagonal relationship with Al.
- Oxide/peroxide/superoxide: Li → Li₂O, Na → Na₂O₂, K/Rb/Cs → MO₂.
- NaOH: Castner-Kellner cell (electrolysis of brine). Uses: soap, paper, petroleum.
- Na₂CO₃: Solvay process. Uses: glass, water softening, soap.
- NaHCO₃: Baking soda, antacid. Decomposes on heating.
- CaO: Quicklime. Ca(OH)₂: Slaked lime. CaSO₄·½H₂O: Plaster of Paris (at 373 K).
- Biological importance: Na⁺ (extracellular), K⁺ (intracellular), Mg²⁺ (chlorophyll, enzymes), Ca²⁺ (bones, blood clotting).
- Portland cement: C₃S (45–50%), C₂S (25–30%), C₃A (8–12%), C₄AF (8–12%). Gypsum prevents flash setting.
- Flame colours: Li = crimson, Na = yellow, K = lilac, Ca = brick red, Sr = crimson, Ba = apple green.
- Solvay process: NH₃ + CO₂ + NaCl + H₂O → NaHCO₃ → Na₂CO₃. Cannot be used for K₂CO₃.