Class 11 Hydrogen and the s-Block Elements — NCERT Chapter Guide
Hydrogen and the s-block elements sit at the very start of the Class 11 NCERT inorganic chemistry syllabus, and together they are a reliable, high-return topic for boards. Hydrogen is odd enough to deserve its own unit, and the s-block (Groups 1 and 2) is the most predictable, pattern-based part of the periodic table you will study this year. This guide covers both chapters the way they are actually examined, with the reactions, trends and worked numericals you need.
Hydrogen — position, isotopes and hydrides
Hydrogen's electronic configuration is 1s¹, which places it awkwardly: like the alkali metals it can lose one electron to form H⁺, but like the halogens it can gain one electron to form H⁻ and it exists as a diatomic gas, H₂. This dual resemblance is exactly why NCERT gives hydrogen a chapter of its own rather than folding it into Group 1.
Hydrogen has three isotopes, and their names are worth knowing precisely:
| Isotope | Symbol | Protons | Neutrons | Notes |
|---|---|---|---|---|
| Protium | ¹H | 1 | 0 | >99.98% of natural hydrogen |
| Deuterium | ²H or D | 1 | 1 | Used to make heavy water, D₂O |
| Tritium | ³H or T | 1 | 2 | Radioactive, formed in trace amounts in the upper atmosphere |
Hydrogen forms three broad classes of hydrides: ionic (saline) hydrides with s-block metals (NaH, CaH₂ — hydrogen present as H⁻), covalent (molecular) hydrides with p-block non-metals (CH₄, NH₃, H₂O, HF), and metallic (interstitial) hydrides where hydrogen occupies gaps in a d-block or f-block metal lattice (e.g. hydrides of Pd, Ti) without a fixed stoichiometry.
The s-block — Groups 1 and 2
Group 1 (alkali metals: Li, Na, K, Rb, Cs, Fr) has the general electronic configuration [noble gas]ns¹. Group 2 (alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra) has [noble gas]ns². Because both groups have only one type of valence configuration, their chemistry is unusually regular — which is exactly why examiners love trend-based questions here.
Alkali metals are softer, lower melting and far more reactive than alkaline earth metals of the same period, because losing one loosely-held ns¹ electron takes much less energy than losing two ns² electrons.
Worked example 1 — identifying a metal by flame test
A student heats an unknown alkaline earth metal chloride in a Bunsen flame and observes a brick-red colour. Which metal is it?
Flame colours are characteristic and commonly tested: Li — crimson red, Na — golden yellow, K — lilac, Ca — brick red, Sr — crimson (deeper than Li), Ba — apple/pale green. A brick-red flame identifies the metal as calcium.
Worked example 2 — writing the reaction of potassium with water
Write a balanced equation for potassium reacting with water, and state one observation.
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Observation: the reaction is so vigorous that the heat evolved ignites the hydrogen gas produced, giving a lilac flame — potassium reacts more violently than sodium because it is lower in the group, so its single valence electron is even more loosely held.
Worked example 3 — molar mass of gypsum, CaSO₄·2H₂O
Gypsum is the natural mineral form of calcium sulphate dihydrate, and heating it partially gives Plaster of Paris. Find its molar mass.
Ca = 1 × 40.078 = 40.078
S = 1 × 32.06 = 32.06
O (in SO₄) = 4 × 15.999 = 63.996
Subtotal for CaSO₄ = 40.078 + 32.06 + 63.996 = 136.134
2H₂O = 2 × (2 × 1.008 + 15.999) = 2 × 18.015 = 36.03
M(CaSO₄·2H₂O) = 136.134 + 36.03 = 172.16 g/mol
Worked example 4 — molar mass of heavy water, D₂O
Heavy water is used as a neutron moderator in some nuclear reactors. Using the atomic mass of deuterium (2.014), find M(D₂O).
D: 2 × 2.014 = 4.028
O: 1 × 15.999 = 15.999
M(D₂O) = 4.028 + 15.999 = 20.03 g/mol — about 11% heavier than ordinary
water (18.02 g/mol), which is why it is called "heavy" water.
Important compounds of the s-block
| Compound | Common name / use |
|---|---|
| NaOH | Caustic soda — made by the chlor-alkali (Castner–Kellner) process |
| Na₂CO₃·10H₂O | Washing soda — made by Solvay's process |
| NaHCO₃ | Baking soda — decomposes on mild heating to Na₂CO₃, H₂O and CO₂ |
| CaO | Quicklime — from thermal decomposition of limestone, CaCO₃ → CaO + CO₂ |
| Ca(OH)₂ | Slaked lime — used in whitewashing and in lime water (turns milky with CO₂) |
| CaSO₄·½H₂O | Plaster of Paris — sets by re-absorbing water to reform gypsum |
Sodium, potassium, magnesium and calcium also have direct biological importance: Na⁺ and K⁺ maintain nerve-impulse transmission and osmotic balance across cell membranes, Mg²⁺ is the central metal ion in chlorophyll, and Ca²⁺ is essential for bone formation and muscle contraction — a short but frequently asked point in this chapter.
Common mistakes that cost marks
- Writing H⁺ as a free ion in solution. In aqueous solution hydrogen exists as the hydronium ion, H₃O⁺, not a bare proton — examiners specifically check this in mechanism and definition answers.
- Confusing ionic and covalent hydrides by formula alone. Judge by the element bonded to hydrogen: an active metal (Na, Ca) gives an ionic hydride; a non-metal gives a covalent one.
- Assuming Group 2 is always less reactive than Group 1 across the board. True for reaction with cold water, but the comparison must be made metal-to-metal of similar position, not generalised carelessly.
- Forgetting that Be and Mg do not impart colour to a flame — their valence electrons are held too tightly for the flame's energy to excite them to a visible emission, unlike the rest of Group 2.
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 11 | Hydride classification, flame tests, industrial preparations of s-block compounds |
| JEE/NEET | Reactivity trends, solubility trends of Group 2 hydroxides and sulphates |
| IIT-JAM / CUET-PG | Diagonal relationship (Li–Mg, Be–Al), anomalous first-member behaviour |
| GATE / CSIR-NET | Complexing behaviour, industrial electrochemical processes for NaOH and Na₂CO₃ |
Practise the numericals from this chapter. Use the free calculator suite to check molar masses, percentage compositions and balanced equations while you revise this unit.
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