ChemistryCHEM 06
s-Block Elements & Hydrogen
Alkali metals, alkaline earth metals, hydrogen properties, water, anomalous behaviour of Li/Be
NCERT XI Ch.8+10
NCERT Ref
Concept Core
Essential theory — everything NCERT tests on s-Block Elements & Hydrogen
GROUP 1 — ALKALI METALS
Li, Na, K, Rb, Cs, Fr. Config: [noble gas]ns¹. Softest metals, lowest IE. Reactivity increases down group (Li least reactive, Cs most).
With water: 2M + 2H₂O → 2MOH + H₂. Li reacts slowly; Cs explosively. With O₂: Li→Li₂O; Na→Na₂O₂; K,Rb,Cs→MO₂ (superoxide).
ANOMALOUS BEHAVIOUR OF Li
Li resembles Mg (diagonal relationship) due to similar charge/radius ratio.
Li uniquely: forms Li₃N (nitride) with N₂. Li₂O (normal oxide, not peroxide). Lithium salts less soluble. LiCl is covalent. Li forms organolithium compounds.
GROUP 2 — ALKALINE EARTH METALS
Be, Mg, Ca, Sr, Ba, Ra. Config: [noble gas]ns². Higher IE than Group 1. Harder, higher mp. Reactivity increases down group.
Anomalous Be: diagonal with Al. BeCl₂ is covalent, polymeric. Be doesn't react with water at room temp. BeSO₄ and BeCO₃ soluble (unlike other sulfates/carbonates).
HYDROGEN
Protium (¹H), Deuterium (²H, D), Tritium (³H, T, radioactive). D₂O = heavy water (moderator in nuclear reactors).
Resembles alkali metals (loses e) AND halogens (gains e). Hydrides: ionic (NaH), covalent (HCl), metallic/interstitial (Pd with H₂).
H₂O: universal solvent, high bp (H-bonding), anomalous expansion at 4°C (density max).
COMPOUNDS OF Na & Ca
NaOH: manufactured by chlor-alkali process. Na₂CO₃ (washing soda): Solvay process. NaHCO₃ (baking soda). Na₂SO₄·10H₂O: Glauber's salt.
CaO (quicklime). Ca(OH)₂ (slaked lime). CaCO₃ (limestone/marble). CaSO₄·½H₂O: Plaster of Paris. CaC₂: calcium carbide (produces acetylene).
Fact & Formula Vault
High-yield facts, numbers, and formulas
Alkali Metal Reactions
Li + N₂ → Li₃N (only Li!)
Na + O₂ → Na₂O₂ (peroxide)
K/Rb/Cs + O₂ → MO₂ (superoxide)
Li: diagonal relationship with Mg
Hydrogen Facts
Protium: ¹H (99.985%)
Deuterium: ²H (0.015%)
Tritium: ³H (radioactive)
D₂O: heavy water, nuclear moderator
Key Compounds
Na₂CO₃: Solvay process
CaO + H₂O → Ca(OH)₂ (slaked lime)
CaSO₄·½H₂O: Plaster of Paris
CaC₂ + H₂O → C₂H₂ (acetylene)
Worked Examples
NEET-style questions solved step-by-step
EASYWhich alkali metal forms superoxide when burnt in excess air?▾
Which alkali metal forms superoxide when burnt in excess air?
K, Rb, Cs form superoxides (MO₂). Na forms peroxide (Na₂O₂). Li forms normal oxide (Li₂O). Reactivity and oxide type increases down Group 1.
MEDIUMThe moderator used in nuclear reactors to slow neutrons is:▾
The moderator used in nuclear reactors to slow neutrons is:
Heavy water (D₂O). Deuterium nucleus (deuteron) has mass close to neutron, making it an effective moderator. Also: graphite is used as moderator.
HARDBeCl₂ differs from MgCl₂ because it is:▾
BeCl₂ differs from MgCl₂ because it is:
BeCl₂ is covalent (polymeric in solid, exists as BeₙCl₂ₙ chains, monomeric at high temp). MgCl₂ is ionic. Be's small size + high charge density → polarises Cl → covalent character (Fajan's rules).
Mistake DNA
Common NEET traps for this chapter
⚠ Li vs Na oxide products
Li → Li₂O (normal). Na → Na₂O₂ (peroxide). K,Rb,Cs → MO₂ (superoxide). Must memorise all three.
✓ Fix: Li=normal, Na=peroxide, K/Rb/Cs=superoxide
⚠ Anomalous elements
Li resembles Mg. Be resembles Al. These diagonal relationships are tested directly.
✓ Fix: Li↔Mg diagonal. Be↔Al diagonal. Both anomalous in their groups.
⚠ Hard vs Soft water
Temporary hardness (Ca/Mg bicarbonate) removed by boiling. Permanent hardness (sulfate/chloride) removed by washing soda (Na₂CO₃).
✓ Fix: Temporary = boiling. Permanent = Na₂CO₃ or ion exchange.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. Li anomaly (Li₃N, Li₂O), Na oxide (peroxide), K superoxide, D₂O as moderator, Plaster of Paris, Solvay process.
High-Yield
Li forms Li₃N uniquely. Na→Na₂O₂. K→KO₂. BeCl₂ is covalent. D₂O = heavy water = moderator. CaSO₄·½H₂O = Plaster of Paris.
Strategy
Memorise oxide types for alkali metals. Know anomalous Li and Be properties. Industrial preparations: Solvay (Na₂CO₃), chlor-alkali (NaOH).
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