Hydrogen and Alkali Metals: JEE Main Guide

The s-Block Elements chapter in JEE Main, covering Hydrogen and the alkali/alkaline earth metals, contributes approximately 2–4 marks per session. While lower in weight than p-Block or Coordination Chemistry, these marks are among the easiest to secure in JEE Main inorganic chemistry — almost all questions come directly from NCERT and require factual recall rather than calculations. This guide covers Hydrogen (NCERT Class 11 Chapter 9) and Alkali Metals (NCERT Class 11 Chapter 10) with the specific facts, anomalies, and compound properties that appear in JEE Main.

Hydrogen: Isotopes, Properties, and Preparation

Hydrogen has three isotopes: Protium (H or ¹H, no neutron), Deuterium (D or ²H, 1 neutron), Tritium (T or ³H, 2 neutrons, radioactive). Heavy water is D2O — used as a moderator in nuclear reactors (slows fast neutrons without absorbing them, unlike ordinary water). Ortho-hydrogen has parallel nuclear spins; para-hydrogen has antiparallel spins. At 0°C, hydrogen is approximately 75% ortho and 25% para; at very low temperatures, essentially all para. JEE Main tests these isotope properties directly. Preparation of H2: (1) displacement of H from acids by active metals: Zn + H2SO4 (dil.) → ZnSO4 + H2. (2) Electrolysis of water: 2H2O → 2H2 + O2. (3) Industrial: steam reforming of methane (CH4 + H2O → CO + 3H2, then water-gas shift: CO + H2O → CO2 + H2). (4) Water gas: C + H2O → CO + H2 (at high temperature). Syngas = CO + H2 mixture. For the transition to alkali metal chemistry that follows hydrogen in NCERT, see our Periodic Table Trends Guide which covers s-block element positioning and property trends.

Hydrides classification: ionic hydrides (NaH, CaH2 — formed by active metals; H is H⁻; reacts with water to give H2), covalent hydrides (CH4, NH3, H2O, HF — non-metals; properties vary), metallic/interstitial hydrides (TiH2, PdH0.9 — transition metals; non-stoichiometric; hydrogen atoms occupy interstitial lattice sites; used in hydrogen storage). Anomalous properties of water: high melting/boiling point (due to H-bonding), ice is less dense than liquid water (open H-bonded lattice structure in ice), high specific heat capacity. These properties of water are tested as "which of the following properties of water is due to hydrogen bonding?"

Alkali Metals: Electronic Configuration and Properties

Group 1 (alkali metals): Li, Na, K, Rb, Cs, Fr. Electronic configuration: ns¹ (n = 2–7). Low ionisation energy (highest IE in the period, but lowest among groups because only one electron to remove). Soft metals; cut with knife; low density (Li, Na, K float on water — less dense than water). Reactivity with water: Li reacts slowly, Na vigorously (catches fire in some conditions), K catches fire, Rb and Cs explosively. Reactions with O2: Li forms oxide (Li2O), Na forms peroxide (Na2O2), K/Rb/Cs form superoxide (KO2, RbO2, CsO2). This is a key JEE Main anomaly — different products with oxygen depending on the metal. Anomalous properties of lithium vs. other alkali metals (and similarity to Mg — diagonal relationship): Li forms nitride directly (Li3N), Li reacts slowly with water (unlike Na), Li2CO3 is thermally unstable (unlike Na2CO3), Li chloride is covalent (like MgCl2), LiF is insoluble (like MgF2). Test your s-block element knowledge on our JEE Main chemistry mock tests — these factual questions are solved in under 90 seconds by prepared students.

Flame colours (already mentioned in salt analysis but restated here): Li — crimson, Na — golden yellow, K — violet (lilac), Rb — red-violet, Cs — blue. The NaD line (yellow) at 589 nm is the most famous spectroscopic emission line. JEE Main tests flame colours directly and through Bunsen burner experiment concepts.

Alkaline Earth Metals: Group 2 Properties

Group 2 (alkaline earth metals): Be, Mg, Ca, Sr, Ba, Ra. Configuration: ns². Higher IE than Group 1 (two electrons to remove, higher Z_eff). Denser and harder than alkali metals. Reactivity: Be is passivated by oxide layer (reacts only with steam), Mg reacts slowly with cold water, Ca reacts vigorously with cold water (2Ca + 2H2O → Ca(OH)2 + H2↑). All dissolve in acid. Hydroxide solubility: Be(OH)2 (amphoteric), Mg(OH)2 (sparingly soluble — milk of magnesia), Ca(OH)2 (slightly soluble — lime water), Sr(OH)2 (soluble), Ba(OH)2 (freely soluble). Trend: solubility increases down the group. Carbonate thermal stability increases down the group (BaCO3 requires the highest temperature to decompose). Sulphate solubility decreases down the group: BeSO4 (soluble), MgSO4 (soluble), CaSO4 (sparingly soluble — gypsum), SrSO4 (insoluble), BaSO4 (very insoluble — used in barium meal X-ray). These solubility trends are tested directly in JEE Main.

Important calcium compounds: CaO (quicklime, formed by heating limestone CaCO3 at 900°C). Ca(OH)2 (slaked lime, formed by adding water to quicklime). CaCO3 (limestone/marble). CaSO4·2H2O (gypsum). CaSO4·½H2O (Plaster of Paris — formed by heating gypsum at 120°C; sets when water added by re-forming gypsum). Ca3(PO4)2 (calcium phosphate — found in bones and teeth). CaCl2 (anhydrous — used as drying agent). CaC2 (calcium carbide — reacts with water to give acetylene: CaC2 + 2H2O → Ca(OH)2 + C2H2). These calcium compounds appear in JEE Main as preparation, property, and use questions.

JEE Main-Specific Fact Points for s-Block

The following specific facts are directly tested in JEE Main: (1) Na is the most abundant alkali metal in Earth's crust. (2) Cs has the lowest ionisation energy among stable elements. (3) Li has the highest charge density and therefore the most hydration energy of the alkali metals — despite highest IE, it's the strongest reducing agent in solution. (4) Be is amphoteric (reacts with both acid and base) — unique in Group 2. (5) Mg ribbon burns in CO2 (Mg + CO2 → MgO + C) — hence CO2 is NOT a suitable fire extinguisher for magnesium fires. (6) Down Group 2, atomic radii, ionic radii, and metallic character increase while IE and electronegativity decrease. (7) Ba(OH)2 is the strongest base in Group 2. (8) Hard water contains Ca²⁺ and Mg²⁺ ions — temporary hardness due to bicarbonates (removed by boiling), permanent hardness due to sulphates/chlorides (removed by Clarke's process using Ca(OH)2 for temporary, or ion exchange/permutit for permanent). Create a free account on our platform for targeted s-block inorganic practice tests. Our premium subscription includes full JEE Main inorganic chemistry coverage with detailed fact sheets. For the p-block elements that follow s-block in NCERT Chapter 10 and 11, see our p-Block Elements Complete Guide.

Study tip for s-block: create two "compare and contrast" tables — one for alkali vs. alkaline earth metals (IE, reactivity with water, oxide type, flame colour, melting point trend), and one for Li vs. Mg (diagonal relationship) and Be vs. Al (diagonal relationship). These comparison tables are the exact source of JEE Main questions that ask "which statement is correct about Li and Na?"