Hydrogen covers the lightest element and why its anomalous 1s1 configuration lets it resemble both alkali metals and halogens, its three isotopes, and the preparation, properties and uses of dihydrogen. It then builds out the chemistry around hydrogen: ionic, covalent and metallic hydrides, the structure and hardness of water, and the dual redox behaviour of hydrogen peroxide. It is a high-yield, mostly factual chapter for NEET where most questions reward clear recall of these classifications and reactions.
Table of Contents
- Key Concepts - Position of hydrogen, dihydrogen, hydrides, water, H₂O₂, heavy water, hydrogen as a fuel
- Weightage in Board & Entrance Exams
- Important Definitions
- Solved Examples
- Important Questions for Board Exams
- Quick Revision Points
Key Concepts
1. Position of Hydrogen in the Periodic Table
Hydrogen has the electronic configuration 1s¹ - just one electron. This single electron makes it behave like two completely different families, so its position is genuinely unique.
Like the alkali metals (Group 1), hydrogen has one valence electron, forms a unipositive ion (H⁺), and combines with halogens, oxygen, and sulphur. Like the halogens (Group 17), it is one electron short of a noble-gas configuration, exists as a diatomic molecule (H₂), and forms the hydride ion (H⁻).
Key idea: Because hydrogen resembles both Group 1 and Group 17 but matches neither perfectly, it is given a unique, isolated position at the top of the periodic table.
2. Isotopes of Hydrogen
Hydrogen has three isotopes that differ only in the number of neutrons. They have the same chemical properties but differ in physical properties such as rate of reaction.
| Isotope | Symbol | Protons | Neutrons | Abundance |
|---|---|---|---|---|
| Protium | ₁H¹ | 1 | 0 | 99.985% |
| Deuterium | ₁H² (D) | 1 | 1 | 0.015% |
| Tritium | ₁H³ (T) | 1 | 2 | radioactive, trace |
Note: Tritium is radioactive and emits β-particles with a half-life of about 12.3 years.
3. Dihydrogen - Occurrence and Preparation
Dihydrogen (H₂) is the most abundant element in the universe (≈70% by mass) but is rare in the free state on Earth, occurring mainly in combined form as water, acids, and hydrocarbons.
Laboratory Preparation
By the action of dilute hydrochloric acid or sulphuric acid on granulated zinc:
Zn + 2HCl → ZnCl₂ + H₂↑
Commercial / Industrial Preparation
- Electrolysis of water (with a trace of acid/base) gives very pure dihydrogen: 2H₂O → 2H₂ + O₂.
- Water-gas / syngas: steam passed over red-hot coke gives water gas: C + H₂O → CO + H₂ (at 1270 K).
- Water-gas shift reaction: CO + H₂O → CO₂ + H₂, with CO₂ removed under pressure to leave pure H₂.
4. Properties and Uses of Dihydrogen
Dihydrogen is a colourless, odourless, tasteless, highly combustible gas that is lighter than air and almost insoluble in water.
Chemical Properties
- With metals: forms ionic hydrides, e.g. 2Na + H₂ → 2NaH.
- With non-metals: 3H₂ + N₂ → 2NH₃ (Haber process); H₂ + Cl₂ → 2HCl.
- Reducing agent: CuO + H₂ → Cu + H₂O - reduces metal oxides to metals.
- Hydrogenation: adds across C=C bonds to convert oils into fats (vanaspati ghee) using a Ni catalyst.
Uses
- Manufacture of ammonia (Haber process) and hence fertilisers.
- Hydrogenation of vegetable oils into vanaspati.
- Manufacture of methanol and HCl.
- As rocket fuel and in fuel cells; in oxy-hydrogen and atomic-hydrogen torches for welding.
5. Hydrides
When dihydrogen combines with other elements it forms hydrides. Based on bonding, hydrides are of three types.
| Type | Formed with | Nature | Examples |
|---|---|---|---|
| Ionic (saline) | s-block metals (Group 1, 2) | Crystalline, conduct on melting, liberate H₂ at anode | NaH, CaH₂ |
| Covalent (molecular) | p-block non-metals | Volatile, low m.p./b.p., poor conductors | CH₄, NH₃, H₂O, HF |
| Metallic (interstitial) | d- and f-block metals | Non-stoichiometric, metallic, conduct heat/electricity | TiH₁.₇₃, PdH₀.₆ |
Note: Elements of groups 7, 8, and 9 (the middle of the d-block) generally do not form hydrides - this is called the hydride gap.
6. Water - Structure and Properties
A water molecule (H₂O) has a bent (angular) shape with an H–O–H bond angle of about 104.5° and two lone pairs on oxygen. Oxygen is sp³ hybridised.
[DIAGRAM: Bent H₂O molecule - central O with two O–H bonds at 104.5°, two lone pairs on O, dipole pointing from H toward O.]
Because oxygen is far more electronegative than hydrogen, water is a highly polar molecule and an excellent solvent. Extensive hydrogen bonding gives water its high boiling point, high specific heat, high surface tension, and the unusual fact that ice is less dense than liquid water.
Amphoteric Nature
Water can act both as an acid and as a base (it is amphoteric):
- As an acid with NH₃: H₂O + NH₃ → NH₄⁺ + OH⁻
- As a base with H₂S: H₂O + H₂S → H₃O⁺ + HS⁻
7. Hard and Soft Water
Soft water lathers readily with soap. Hard water does not give lather easily because it contains dissolved calcium and magnesium salts.
| Type of hardness | Cause | Removal method |
|---|---|---|
| Temporary hardness | Bicarbonates of Ca²⁺ and Mg²⁺ | Boiling; Clark’s method (slaked lime) |
| Permanent hardness | Chlorides and sulphates of Ca²⁺ and Mg²⁺ | Washing soda; ion-exchange resins; Calgon |
Softening Methods
- Clark’s method: calculated slaked lime removes temporary hardness: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O.
- Washing soda: Na₂CO₃ removes both kinds: CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl.
- Calgon method: sodium hexametaphosphate Na₆P₆O₁₈ forms a soluble complex with Ca²⁺/Mg²⁺.
- Ion-exchange (permutit): hydrated sodium aluminium silicate exchanges Na⁺ for Ca²⁺/Mg²⁺.
8. Hydrogen Peroxide (H₂O₂)
Hydrogen peroxide is a pale-blue, syrupy liquid (almost colourless when dilute) and an important oxidising and bleaching agent.
Preparation
- Laboratory: acidified hydrogen peroxide from barium peroxide - BaO₂·8H₂O + H₂SO₄ → BaSO₄ + H₂O₂ + 8H₂O.
- Industrial: 2-ethylanthraquinol is oxidised by air (auto-oxidation), then H₂O₂ is extracted with water.
Structure
H₂O₂ has a non-planar, open-book structure. The two O–H bonds lie in different planes joined at the O–O bond, like the two pages of a half-open book.
[DIAGRAM: Open-book structure of H₂O₂ - O–O single bond as the spine, each O carrying one H; dihedral angle 111° in gas phase (90.2° in solid).]
- O–O bond length: 147.5 pm; O–H bond length: 95 pm
- Dihedral angle: 111.5° in the gas phase (90.2° in the solid)
Properties and Uses
- Acts as both an oxidising and reducing agent in acidic and basic media.
- Decomposes on standing: 2H₂O₂ → 2H₂O + O₂ - stored in coloured plastic bottles, away from light.
- Used as a mild antiseptic, hair bleach, in restoring old paintings, and as a propellant.
9. Heavy Water (D₂O)
Heavy water is deuterium oxide, D₂O, where ordinary hydrogen is replaced by its heavier isotope deuterium. It is obtained by the prolonged electrolysis of ordinary water.
- It is denser (1.106 g/cm³) and has a higher boiling point (101.4 °C) than ordinary water.
- Its main use is as a moderator in nuclear reactors, where it slows down fast neutrons.
- It is also used to prepare deuterated compounds for studying reaction mechanisms.
10. Hydrogen as a Fuel
Dihydrogen releases a large amount of energy per gram on combustion and produces only water as the by-product, making it a clean, pollution-free fuel.
- Energy released on combustion is about 143 kJ/g - far higher than petrol or LPG by mass.
- Used in hydrogen fuel cells, which convert chemical energy directly into electricity: 2H₂ + O₂ → 2H₂O + electricity.
- 5% dihydrogen blended with CNG (called Hydrogen-CNG) is used as a cleaner automobile fuel.
Important: The main challenges are the safe storage and transport of a highly inflammable gas.
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Weightage in Board & Entrance Exams
| Exam | Typical Weightage | Most-Tested Areas |
|---|---|---|
| CBSE Board (Class 11) | 4–6 marks | Hydrides, H₂O₂ structure & reactions, hard/soft water, heavy water |
| JEE Main | 1 question | H₂O₂ structure, redox behaviour, hydrides classification |
| NEET | 1–2 questions | Isotopes, hydrides, H₂O₂ as oxidising/reducing agent, hard water |
[TABLE: Question-type split - VSA (1 mark): isotopes, definitions, formulae; SA (2–3 marks): preparation of H₂/H₂O₂, hardness removal; LA (5 marks): structure of H₂O₂, classification of hydrides with examples.]
Important Definitions
| Term | Definition |
|---|---|
| Dihydrogen | The diatomic molecular form of hydrogen, H₂ - most abundant element in the universe |
| Isotopes of hydrogen | Protium (₁H¹), deuterium (₁H²), tritium (₁H³) - same protons, different neutrons |
| Ionic hydride | Hydride of s-block metals containing the H⁻ ion, e.g. NaH, CaH₂ |
| Interstitial hydride | Non-stoichiometric hydride of d/f-block metals, e.g. PdH₀.₆ |
| Hard water | Water containing dissolved Ca²⁺ and Mg²⁺ salts that does not lather with soap |
| Temporary hardness | Hardness due to bicarbonates of Ca/Mg, removed by boiling |
| Permanent hardness | Hardness due to chlorides/sulphates of Ca/Mg, removed by washing soda |
| Hydrogen peroxide | H₂O₂ - pale-blue liquid with an open-book structure; oxidising and reducing agent |
| Heavy water | Deuterium oxide, D₂O - used as a moderator in nuclear reactors |
| Calgon | Sodium hexametaphosphate, used to soften water by forming soluble complexes |
Solved Examples
Example 1
Why is hydrogen placed separately in the periodic table?
Answer: With configuration 1s¹ it resembles both alkali metals (one valence electron, forms H⁺) and halogens (one electron short of a noble gas, forms H⁻ and a diatomic molecule), but matches neither completely - so it is given a unique isolated position.
Example 2
Write the reaction for the laboratory preparation of dihydrogen.
Answer: Granulated zinc with dilute HCl: Zn + 2HCl → ZnCl₂ + H₂↑. Dilute H₂SO₄ may also be used.
Example 3
How is temporary hardness of water removed by Clark’s method?
Answer: A calculated amount of slaked lime is added, precipitating insoluble carbonate: Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O.
Example 4
Give the structure of hydrogen peroxide and state its dihedral angle in the gas phase.
Answer: H₂O₂ has a non-planar open-book structure with the two O–H bonds in different planes joined at the O–O bond. The dihedral angle is 111.5° in the gas phase (90.2° in the solid).
Example 5
Why is hydrogen peroxide stored in coloured plastic bottles?
Answer: H₂O₂ decomposes in the presence of light: 2H₂O₂ → 2H₂O + O₂. Coloured bottles cut out light and the smooth plastic surface avoids catalytic decomposition, so it stays stable.
Example 6
Distinguish between ionic and covalent hydrides with one example each.
Answer: Ionic hydrides form with reactive s-block metals, contain the H⁻ ion, and are crystalline solids that conduct on melting - e.g. NaH. Covalent hydrides form with p-block non-metals, are volatile with low melting points, and are poor conductors - e.g. CH₄.
Important Questions for Board Exams
1-Mark Questions (VSA)
- Name the three isotopes of hydrogen.
- What is the hybridisation of oxygen in a water molecule?
- Write the formula of Calgon.
- Why is heavy water used in nuclear reactors?
- Name the by-product formed when hydrogen is burnt as a fuel.
2–3-Mark Questions (SA)
- Explain why hydrogen resembles both alkali metals and halogens.
- Distinguish between temporary and permanent hardness of water and give one method to remove each.
- Describe the industrial preparation of dihydrogen by the water-gas shift reaction.
- Why does H₂O₂ act both as an oxidising and a reducing agent? Give one example of each.
5-Mark Questions (LA)
- Classify hydrides into three types based on bonding, giving their characteristics and examples.
- Draw and explain the structure of hydrogen peroxide and discuss its preparation and uses.
- Explain hard and soft water. Describe Clark’s method, washing-soda method, and the ion-exchange method of softening water.
Quick Revision Points
- Hydrogen (1s¹) has a unique position - resembles both Group 1 and Group 17
- Three isotopes: protium ₁H¹, deuterium ₁H², tritium ₁H³ (radioactive)
- Lab prep of H₂: Zn + 2HCl → ZnCl₂ + H₂; pure H₂ by electrolysis of water
- Hydrides: ionic (NaH), covalent (CH₄, H₂O), metallic/interstitial (PdH₀.₆); hydride gap in groups 7–9
- Water: bent, sp³, bond angle 104.5°, polar, amphoteric, extensive H-bonding
- Temporary hardness = bicarbonates (boiling/Clark’s); permanent = chlorides/sulphates (washing soda, Calgon, ion-exchange)
- H₂O₂: open-book structure, dihedral angle 111.5° (gas); oxidising + reducing agent; stored in coloured bottles
- Heavy water D₂O: moderator in nuclear reactors; denser and higher b.p. than H₂O
- Hydrogen as fuel: ~143 kJ/g, only water as by-product, used in fuel cells and H-CNG
Next Chapter: Chapter 9 - The s-Block Elements
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Why Hydrogen Stands Alone
The simplest atom resembles two different groups, so it gets its own seat at the top of the table.
Most abundant element in the universe; only a small fraction of Earth’s crust by mass.
- Like alkali metals: 1 valence e⁻, forms H⁺, +1 state
- Like halogens: forms H⁻, diatomic H2, high ionization enthalpy
- Fits neither cleanly → placed separately
The Three Isotopes
Same protons, different neutrons, so identical chemistry but different mass-based properties.
Neutrons = mass no. − atomic no. (H atomic no. = 1). Tritium t½ ≈ 12.3 yr.
- Protium ~99.98% abundance; deuterium is stable ‘heavy hydrogen’
- Only tritium is radioactive (β-emitter)
- Same 1s1 config ⇒ same chemical properties
Making Dihydrogen (H2)
Lab prep is a reactive metal in acid; industry uses steam over hot coke.
Use granulated (impure) zinc; never copper, never concentrated HNO3.
- Zinc is amphoteric: Zn + 2NaOH → Na2ZnO2 + H2↑
- Water gas: C + H2O →(1270 K) CO + H2
- Water-gas shift: CO + H2O →(cat.) CO2 + H2 (CO2 scrubbed)
Why H2 Is Unreactive
One of the strongest single bonds makes H2 sluggish at room temperature until heated.
Atomic and nascent hydrogen are far more reactive (bond already broken).
- Lightest gas, colourless, odourless, slightly soluble in water
- Highest combustion energy per unit mass → clean fuel, burns to water
- Reacts (on heating) with O2, halogens and metals
Three Families of Hydrides
The partner’s position in the periodic table decides which hydride type forms.
Exception: BeH2 and MgH2 are covalent/polymeric, not ionic.
- Ionic/saline: contain H⁻; NaH + H2O → NaOH + H2↑; give H2 at anode when molten
- Covalent: electron-deficient B2H6 · precise CH4 · rich NH3, H2O
- Metallic: interstitial, non-stoichiometric (LaH2.87); ‘hydride gap’ in mid d-block
The Water Molecule
A bent, highly polar molecule whose hydrogen bonding gives it unusual properties.
Lone-pair repulsion compresses the angle below the ideal 109.5°.
- Extensive H-bonding → high boiling point and high specific heat
- Ice is less dense than water, so it floats
- Heavy water D2O is used as a moderator in nuclear reactors
Hard Water & Softening
Dissolved Ca2⁺ and Mg2⁺ salts stop soap from lathering; the anion sets the cure.
The bicarbonate vs chloride/sulphate split is the most common exam trap.
- Temporary: removed by boiling [Ca(HCO3)2 → CaCO3↓ + H2O + CO2] or Clark’s lime
- Permanent: washing soda (Na2CO3), Calgon, or ion-exchange
- Calgon = sodium hexametaphosphate; boiling does NOT remove permanent hardness
Hydrogen Peroxide: Dual Redox
Oxygen sits exactly halfway, so H2O2 can act as both oxidant and reductant.
Each H = +1 ⇒ two O must total −2 ⇒ −1 each.
- As oxidant: 2I⁻ + H2O2 + 2H⁺ → I2 + 2H2O; black PbS → white PbSO4
- As reductant (vs KMnO4): releases O2 and decolourises the permanganate
- Open-book non-planar structure; O–O bond ≈ 1.48 Å
Storing & Rating H2O2
It slowly disproportionates, so it is stabilised and rated by the O2 it can release.
’20 volume’ = 1 mL solution gives 20 mL O2 at STP.
- Stored in wax-lined / dark plastic bottles to slow decomposition
- Urea or phosphoric acid added as a stabiliser
- Pure H2O2 is a pale-blue syrupy liquid
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Frequently Asked Questions
Hydrogen has a 1s1 configuration, so it resembles Group 1 alkali metals by having one valence electron and forming H+, but it also resembles Group 17 halogens by forming H- and existing as a diatomic gas with high ionization enthalpy. Because it fits neither group cleanly, it is placed separately at the top of the table.
They are protium (1 proton, 0 neutrons, about 99.98 percent abundance), deuterium or heavy hydrogen (1 neutron), and tritium (2 neutrons). All share the same 1s1 configuration so their chemistry is identical, but only tritium is radioactive, a beta-emitter with a half-life of about 12.3 years.
In the lab, granulated zinc is reacted with dilute sulphuric acid: Zn + H2SO4 gives ZnSO4 and H2 gas. Industrially, water gas is made by passing steam over hot coke at about 1270 K (C + H2O gives CO + H2), and the water-gas shift reaction (CO + H2O over a catalyst gives CO2 + H2) increases the hydrogen yield.
Temporary hardness is caused by dissolved bicarbonates of calcium and magnesium and can be removed simply by boiling or by Clark’s lime method. Permanent hardness is caused by chlorides and sulphates of calcium and magnesium, is not removed by boiling, and needs washing soda, Calgon, or ion-exchange resins.
In H2O2 oxygen is in the intermediate -1 oxidation state, halfway between 0 and -2, so it can either gain or lose electrons. It acts as an oxidant, for example converting iodide to iodine, and as a reductant, for example decolourising acidified KMnO4 while releasing oxygen.