Chapter 10 — The s-Block Elements
Welcome to HSLC Guru! This English-medium guide is prepared exclusively for ASSEB Class 11 Chemistry learners. Chapter 10 — The s-Block Elements introduces you to the alkali metals (Group 1) and the alkaline earth metals (Group 2). These elements are the most reactive metals of the periodic table, and their chemistry is central to industry, biology and everyday life. This compact resource gives you a structured Summary, exam-ready Question and Answer practice (1-mark, 2–3 mark and 5–7 mark), MCQs, Fill-in-the-blanks, True/False questions, and a Glossary — fully aligned with the ASSEB syllabus.
Summary
The s-block consists of elements in which the last electron enters an s-orbital. Group 1 (alkali metals) — lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and the radioactive francium (Fr) — share the general valence configuration ns¹. They are soft, silvery-white, low-melting metals with low ionisation enthalpy and a strong tendency to lose one electron to form M⁺ ions. Down the group, atomic and ionic radii increase, ionisation enthalpy decreases, density increases (with anomalies), and reducing character increases (Cs is among the strongest reducers; in aqueous solution, however, Li shows the most negative E° due to its very high hydration enthalpy). Alkali metals tarnish in air, react vigorously with water giving MOH and H₂, burn in oxygen to form different oxides (Li → monoxide Li₂O; Na → peroxide Na₂O₂; K, Rb, Cs → superoxides MO₂), and impart characteristic flame colours (Li-crimson, Na-golden yellow, K-violet, Rb-red violet, Cs-blue).
Anomalous behaviour of lithium arises from its very small size, high polarising power and high charge/radius ratio. Unlike other alkali metals, Li forms a normal oxide (Li₂O) only, its carbonate, hydroxide and nitrate decompose easily, LiF and Li₂CO₃ are sparingly soluble, and Li reacts with N₂ to form Li₃N. Diagonal relationship of Li with Mg (Group 2) is observed because the two elements have similar charge/radius ratios and electronegativities. Both react with N₂, form normal oxides, have hard chlorides, and their carbonates decompose on heating. Group 2 (alkaline earth metals) — beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radioactive radium (Ra) — have valence configuration ns². They are harder, denser and higher melting than Group 1. They form M²⁺ ions; reactivity, basic strength of oxides/hydroxides, and solubility of hydroxides increase down the group while solubility of sulphates and carbonates decreases. Flame colours: Ca-brick red, Sr-crimson, Ba-apple green (Be and Mg do not impart colour due to high ionisation enthalpy of their tightly held electrons).
Anomalous Be: small size, high ionisation enthalpy and high electronegativity make Be largely covalent. BeO and Be(OH)₂ are amphoteric, BeCl₂ is covalent and polymeric in solid state, Be does not react with water, and Be carbide gives methane on hydrolysis (other Group 2 carbides give acetylene). Diagonal relationship of Be with Al: similar charge/radius ratio, both form covalent compounds, amphoteric oxides/hydroxides, complexes such as BeF₄²⁻ and AlF₆³⁻, and chlorides that act as Lewis acids. Industrially important compounds: Sodium obtained by Down’s process (electrolysis of fused NaCl with CaCl₂); used as coolant in fast breeder reactors and in Na vapour lamps. NaOH (caustic soda) made by Castner–Kellner / membrane cell electrolysis of brine; used in soap, paper, rayon, petroleum refining. Na₂CO₃ (washing soda) produced by the Solvay (ammonia–soda) process; used in glass, detergents, paper, water softening. NaHCO₃ (baking soda) mild antacid and leavening agent. Magnesium obtained by Dow process; used in Mg–Al alloys, flash photography, Grignard reagents. Calcium made by electrolysis of fused CaCl₂; used as a deoxidiser. Plaster of Paris, CaSO₄·½H₂O, obtained by heating gypsum to ~120°C; used in surgical casts, statues, dental work. Cement (Portland) is a complex calcium aluminosilicate produced from limestone and clay; major phases — alite (Ca₃SiO₅), belite (Ca₂SiO₄), aluminate (Ca₃Al₂O₆) and ferrite (Ca₄Al₂Fe₂O₁₀); setting involves hydration.
Biological importance: Sodium and potassium are essential extracellular and intracellular cations respectively; the Na⁺/K⁺ pump maintains nerve impulses, osmotic balance, muscle contraction and glucose transport. Magnesium is the central atom of chlorophyll, activates many enzymes (kinases) and is required for ATP utilisation. Calcium is the principal component of bones and teeth (as hydroxyapatite), is essential for blood clotting, muscle contraction, neurotransmitter release and intracellular signalling. Deficiencies and excesses of these ions cause well-defined physiological disorders, underscoring why s-block chemistry is taught alongside life-science topics in the ASSEB curriculum.
1-Mark Questions and Answers
Q1. What is the general valence shell electronic configuration of alkali metals?
Answer: ns¹.
Q2. Name the alkali metal that imparts a golden yellow colour to the flame.
Answer: Sodium (Na).
Q3. Which alkali metal is the strongest reducing agent in aqueous solution?
Answer: Lithium, due to its very high hydration enthalpy giving the most negative E° value.
Q4. Write the chemical formula of Plaster of Paris.
Answer: CaSO₄·½H₂O (calcium sulphate hemihydrate).
Q5. Name the process used for the manufacture of sodium carbonate.
Answer: Solvay (ammonia–soda) process.
Q6. Which oxide is formed when potassium burns in excess oxygen?
Answer: Potassium superoxide, KO₂.
Q7. Why is BeCl₂ covalent in nature?
Answer: Be²⁺ has a very small size and high charge density, leading to high polarising power, hence Be–Cl bond is largely covalent.
Q8. Which metal ion is present in chlorophyll?
Answer: Magnesium (Mg²⁺).
Q9. Name the radioactive element of Group 1.
Answer: Francium (Fr).
Q10. What is the principal calcium compound of bones and teeth?
Answer: Hydroxyapatite, Ca₅(PO₄)₃OH.
Q11. Why is potassium kept immersed in kerosene oil?
Answer: Because potassium reacts violently with the moisture and oxygen of air, kerosene (an unreactive hydrocarbon) keeps it isolated from both.
Q12. Name the metal used as a coolant in fast breeder nuclear reactors.
Answer: Liquid sodium (often as a Na–K alloy).
2–3 Marks Questions and Answers
Q1. Why are alkali metals not found in free state in nature?
Answer: Alkali metals have very low ionisation enthalpies and a strong tendency to lose their single ns¹ electron. Hence they are extremely reactive — they readily react with moisture, oxygen and CO₂ of the atmosphere. As a result, they always occur in combined state, mostly as halides, carbonates, sulphates and silicates (e.g., NaCl, KCl, Na₂CO₃·10H₂O, K₂SO₄, etc.).
Q2. Explain the diagonal relationship of lithium with magnesium with three points.
Answer: Despite being in different groups and periods, Li and Mg show several similarities because of comparable charge/radius ratios and electronegativities. (i) Both react directly with nitrogen forming nitrides — Li₃N and Mg₃N₂. (ii) Their carbonates decompose on heating: Li₂CO₃ → Li₂O + CO₂ and MgCO₃ → MgO + CO₂. (iii) Both form normal oxides only (Li₂O, MgO) when burnt in air, unlike the other alkali metals. (iv) Their chlorides (LiCl, MgCl₂) are deliquescent and soluble in alcohol, indicating partial covalent character.
Q3. What happens when sodium reacts with (i) oxygen, (ii) water, (iii) hydrogen?
Answer: (i) In air at room temperature Na forms Na₂O; in excess O₂ it gives sodium peroxide: 2Na + O₂ → Na₂O₂. (ii) With water Na reacts vigorously, often catching fire: 2Na + 2H₂O → 2NaOH + H₂↑. (iii) On heating with H₂ at ~673 K, sodium hydride is formed: 2Na + H₂ → 2NaH (an ionic, saline hydride).
Q4. Give two industrial uses each of NaOH and Na₂CO₃.
Answer: NaOH: (i) manufacture of soaps and detergents through saponification of fats; (ii) manufacture of paper, rayon, dyes, drugs and in petroleum refining (mercerisation of cotton, removal of sulphur from petroleum). Na₂CO₃: (i) manufacture of glass, soap, paper, borax and caustic soda; (ii) softening of hard water and laundering of clothes (washing soda).
Q5. Why does Be show anomalous behaviour? Mention any three points.
Answer: Beryllium differs from the other Group 2 elements because of its very small atomic and ionic size, very high ionisation enthalpy, high electronegativity and absence of d-orbitals in its valence shell. (i) Be does not react with water even at high temperature, while Mg–Ba decompose water. (ii) BeO and Be(OH)₂ are amphoteric, while oxides/hydroxides of Mg–Ba are basic. (iii) BeCl₂ is covalent and forms a polymeric chain in solid state, whereas other chlorides MCl₂ are ionic. (iv) Be carbide (Be₂C) gives methane on hydrolysis, while CaC₂ gives ethyne.
Q6. Describe two important uses of plaster of Paris and write the equation for its setting.
Answer: Uses: (i) for making surgical casts to immobilise broken bones in orthopaedics; (ii) for making moulds, statues and decorative items, and in dentistry. Setting reaction: When mixed with water it sets to a hard mass by absorbing water and changing back to gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O. The reaction is slightly exothermic and accompanied by a small expansion, which is useful in mould-making.
Q7. What is the cause of the diagonal relationship between Be and Al? Mention any three similarities.
Answer: The diagonal relationship is mainly due to the very similar charge/radius ratios and electronegativities of Be²⁺ (~3.0/0.31) and Al³⁺ (~3.0/0.50). Important similarities are: (i) BeO and Al₂O₃ are both amphoteric and high-melting; (ii) BeCl₂ and AlCl₃ are covalent Lewis acids that form bridged structures and dimers in vapour phase; (iii) both metals form complex fluoro-anions, BeF₄²⁻ and AlF₆³⁻, and both are passivated by concentrated nitric acid.
Q8. Why does the solubility of alkaline earth metal hydroxides increase, while that of their sulphates decrease, on moving down the group?
Answer: Solubility depends on lattice enthalpy and hydration enthalpy. For hydroxides, the hydroxide ion is small, so the lattice enthalpy decreases sharply down the group as M²⁺ size grows, while the hydration enthalpy of M²⁺ decreases more gradually — net free-energy of dissolution becomes more negative, hence solubility increases (Be(OH)₂ insoluble < Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂). For sulphates, the sulphate ion is large, so the lattice enthalpy hardly changes down the group, but the hydration enthalpy of M²⁺ falls steadily; therefore solubility decreases (BeSO₄, MgSO₄ very soluble > CaSO₄ slightly > SrSO₄ > BaSO₄ insoluble).
5–7 Marks Questions and Answers
Q1. Discuss the trends in atomic radii, ionisation enthalpy, density, melting point and reactivity of alkali metals on moving down the group, with reasons.
Answer: (a) Atomic and ionic radii increase from Li to Cs because each successive element has an additional principal energy shell, and the screening by inner electrons outweighs the small increase in nuclear charge. (b) Ionisation enthalpy decreases down the group: the outermost ns¹ electron lies farther from the nucleus and is better shielded, so it is held less tightly. Consequently, the metallic and reducing character increases down the group. (c) Density generally increases (Li < Na < K < Rb < Cs), but K is lighter than Na — an anomaly arising from a sharper increase in atomic volume between Na and K. All alkali metals are lighter than water except Rb and Cs. (d) Melting and boiling points are very low and decrease down the group because of weak metallic bonding caused by only one valence electron and increasing atomic size. (e) Chemical reactivity increases down the group with respect to oxygen, water and halogens because of decreasing ionisation enthalpy. However, in aqueous medium Li is the strongest reducing agent due to its very high hydration enthalpy that overrides its larger ionisation enthalpy.
Q2. Describe the manufacture of sodium hydroxide by the Castner–Kellner mercury cathode process. Mention any three uses.
Answer: The Castner–Kellner cell consists of a rectangular tank divided into compartments. A saturated solution of brine (NaCl) is electrolysed using graphite anodes and a flowing layer of mercury at the bottom acting as cathode. At the anode chloride ions are discharged: 2Cl⁻ → Cl₂ + 2e⁻. At the mercury cathode Na⁺ is preferentially discharged (because of high overpotential of H₂ on Hg) and dissolves in mercury forming sodium amalgam: Na⁺ + e⁻ → Na (in Hg). The amalgam flows into a second compartment (denuder) where it reacts with water on graphite blocks: 2Na(Hg) + 2H₂O → 2NaOH + H₂↑ + 2Hg. The freed mercury returns to the cathode compartment, while pure NaOH solution is drawn off. Uses: (i) in soap, detergent, paper, rayon and textile industries; (ii) in petroleum refining and mercerisation of cotton; (iii) as a laboratory reagent and in the preparation of pure fats, dyes and drugs.
Q3. Outline the Solvay process for the manufacture of sodium carbonate. Why cannot the same process be used for K₂CO₃?
Answer: In the Solvay (ammonia–soda) process, ammonia is first absorbed in saturated brine to give ammoniated brine. Carbon dioxide (obtained by heating limestone) is then passed into this solution. The reactions are: NH₃ + H₂O + CO₂ → NH₄HCO₃; NH₄HCO₃ + NaCl → NaHCO₃↓ + NH₄Cl. The sparingly soluble sodium bicarbonate is filtered out and ignited to give sodium carbonate: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Ammonia is recovered by treating the filtrate with Ca(OH)₂ obtained from CaO (from limestone): 2NH₄Cl + Ca(OH)₂ → 2NH₃ + CaCl₂ + 2H₂O. Thus NH₃ is recycled and the only waste is CaCl₂. The process cannot be used for K₂CO₃ because potassium bicarbonate (KHCO₃) is too soluble in water and does not precipitate, so it cannot be separated by filtration.
Q4. Compare and contrast the chemistry of Group 1 and Group 2 elements with respect to (a) electronic configuration, (b) hardness and density, (c) flame colours, (d) nature of oxides, (e) solubility of carbonates and (f) reaction with water.
Answer: (a) Group 1 has ns¹ and forms +1 ions; Group 2 has ns² and forms +2 ions, so Group 2 elements have higher charge densities. (b) Group 2 metals are harder and denser than Group 1 because they have two valence electrons, leading to stronger metallic bonding and smaller atomic radii. (c) Heavier Group 1 metals (Na, K, Rb, Cs) and Group 2 metals (Ca, Sr, Ba) impart characteristic flame colours; Be and Mg do not because their electrons are tightly bound (high ionisation enthalpies) and the energy required for excitation falls outside the visible region. (d) Group 1 oxides are strongly basic; Group 2 oxides are basic but BeO is amphoteric and MgO is feebly basic. (e) Carbonates of Group 1 (except Li₂CO₃) are soluble and stable to heat; carbonates of Group 2 are insoluble in water and decompose on heating, with thermal stability increasing down the group. (f) All Group 1 metals react vigorously with cold water giving MOH and H₂; in Group 2, Be does not react, Mg reacts with hot water/steam, while Ca, Sr and Ba react with cold water like alkali metals.
Q5. Discuss the biological importance of sodium, potassium, magnesium and calcium ions in the human body.
Answer: Sodium (Na⁺) is the principal extracellular cation. It maintains osmotic pressure of body fluids, regulates blood pressure and is essential for the conduction of nerve impulses through the Na⁺/K⁺ pump (an ATP-driven active transport that pushes Na⁺ out and K⁺ into cells). It also assists in glucose absorption in the intestine. Potassium (K⁺) is the major intracellular cation. It activates many enzymes, helps in protein synthesis and, together with Na⁺, generates the membrane potential required for nerve transmission and muscle contraction (including the heartbeat). Magnesium (Mg²⁺) is the central metal ion of chlorophyll, making photosynthesis possible. In animals it is a cofactor for over 300 enzymes (kinases, hexokinase, ATPase) and stabilises ATP and DNA structures. Calcium (Ca²⁺) is the most abundant metal ion in the human body, mainly present in bones and teeth as hydroxyapatite, Ca₅(PO₄)₃OH. It is essential for blood clotting (activates prothrombin to thrombin), muscle contraction, transmission of nerve impulses, secretion of hormones and as an intracellular second messenger. Deficiencies of these ions lead to muscle cramps (Mg, K), weak bones/rickets (Ca), and disturbances in nerve and heart function (Na, K).
Multiple Choice Questions (MCQ)
Q1. The general electronic configuration of alkaline earth metals is —
(a) ns¹ (b) ns²np¹ (c) ns² (d) (n−1)d¹ns²
Answer: (c) ns².
Q2. Which of the following imparts a brick-red colour to the flame?
(a) Na (b) K (c) Ca (d) Ba
Answer: (c) Ca.
Q3. The most abundant alkali metal in the earth’s crust is —
(a) Li (b) Na (c) K (d) Cs
Answer: (b) Na.
Q4. Sodium is obtained commercially by the —
(a) Solvay process (b) Down’s process (c) Castner process (d) Dow process
Answer: (b) Down’s process.
Q5. Plaster of Paris is —
(a) CaSO₄·2H₂O (b) CaSO₄·½H₂O (c) CaSO₄ (d) Ca(OH)₂
Answer: (b) CaSO₄·½H₂O.
Q6. Which alkali metal forms a superoxide on burning in air?
(a) Li (b) Na (c) K (d) Be
Answer: (c) K.
Q7. The element showing diagonal relationship with Al is —
(a) Li (b) Mg (c) Be (d) Ca
Answer: (c) Be.
Q8. Which compound is the central ion in chlorophyll?
(a) Fe²⁺ (b) Mg²⁺ (c) Ca²⁺ (d) Cu²⁺
Answer: (b) Mg²⁺.
Q9. Among LiCl, NaCl, KCl, RbCl, the most covalent character is shown by —
(a) LiCl (b) NaCl (c) KCl (d) RbCl
Answer: (a) LiCl.
Q10. Sodium bicarbonate is used as —
(a) coolant (b) antacid (c) bleaching agent (d) drying agent
Answer: (b) antacid.
Fill in the Blanks
Q1. The general valence configuration of Group 1 elements is __________.
Answer: ns¹.
Q2. Caesium imparts __________ colour to the flame.
Answer: blue/violet.
Q3. __________ is known as washing soda.
Answer: Na₂CO₃·10H₂O.
Q4. The major component of cement responsible for early strength is __________.
Answer: alite (Ca₃SiO₅).
Q5. Beryllium carbide on hydrolysis gives __________.
Answer: methane (CH₄).
True / False
Q1. Lithium is the strongest reducing agent in aqueous solution. Answer: True.
Q2. Beryllium reacts vigorously with cold water like calcium. Answer: False (Be does not react with water even at high temperature).
Q3. Sodium peroxide is used as an oxygen source in submarines. Answer: True.
Q4. The Solvay process can be used to manufacture potassium carbonate. Answer: False (KHCO₃ is too soluble to precipitate).
Q5. Magnesium is the central metal of haemoglobin. Answer: False (it is iron; Mg is the central metal of chlorophyll).
Glossary
| Term | Meaning |
|---|---|
| s-block elements | Elements in which the last electron enters the s-orbital; comprise Groups 1 and 2. |
| Alkali metals | Group 1 elements (Li, Na, K, Rb, Cs, Fr) with valence configuration ns¹. |
| Alkaline earth metals | Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra) with valence configuration ns². |
| Ionisation enthalpy | Energy required to remove the most loosely bound electron from a gaseous atom. |
| Hydration enthalpy | Energy released when one mole of gaseous ions is dissolved in water; very high for small ions like Li⁺. |
| Diagonal relationship | Similarity in properties between an element of period 2 and the next-group element of period 3 (e.g., Li–Mg, Be–Al). |
| Down’s process | Electrolysis of fused NaCl (with CaCl₂ added to lower melting point) to obtain metallic sodium. |
| Solvay process | Industrial method of making Na₂CO₃ from brine, ammonia and CO₂. |
| Castner–Kellner cell | Mercury-cathode electrolytic cell used to manufacture NaOH from brine. |
| Plaster of Paris | Calcium sulphate hemihydrate (CaSO₄·½H₂O) obtained by heating gypsum to ~120 °C. |
| Cement | Mixture of calcium silicates and aluminates produced from limestone and clay; sets on hydration. |
| Hydroxyapatite | Ca₅(PO₄)₃OH; chief mineral of bones and teeth. |
| Na⁺/K⁺ pump | ATP-driven membrane protein that maintains the Na⁺ and K⁺ gradients in living cells. |
| Amphoteric oxide | Oxide that reacts with both acids and bases (e.g., BeO, Al₂O₃). |
| Superoxide | Compound containing the O₂⁻ ion, e.g., KO₂, RbO₂, CsO₂. |
Prepared by HSLC Guru — ASSEB Class 11 Chemistry, English Medium. Keep practising and good luck with your examination!