Class 12 Chemistry Chapter 7 Notes:The p Block Elements | Important Questions & PYQs

 Class 12 Chemistry – Chapter 7 The p Block Elements Notes

1. p-Block Elements

• Elements in Groups 15 to 18

• Valence shell configuration: ns² np¹–⁶

• Show variable oxidation states

• Mostly non-metals, metallic character increases down the group

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2. Group 15 Elements (N, P, As, Sb, Bi)

General Characteristics

• Oxidation states: –3, +3, +5

• Stability of +5 state decreases down the group (inert pair effect)

• Atomic size increases down the group

• Electronegativity decreases down the group

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Nitrogen (N₂)

• Exists as diatomic molecule

• Triple bond (N≡N) → very high bond energy

• Hence least reactive at room temperature

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Ammonia (NH₃)

• Prepared by Haber’s process

• Conditions:

  • Moderate temperature (~700 K)

  • High pressure (~200 atm)

  • Fe catalyst with promoters

• Basic in nature

• Forms complex with Cu²⁺ (deep blue colour)

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Hydrides of Group 15

• General formula: EH₃

• Shape: Pyramidal

• Bond angle order:
  NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃

• Reducing character increases down the group

• BiH₃ is strongest reducing agent

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Phosphorus

• Exists as P₄ molecule

• Shows allotropy:

  • White phosphorus

  • Red phosphorus

White vs Red Phosphorus

• White: reactive, poisonous, glows in dark

• Red: stable, non-poisonous, polymeric

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3. Oxides & Oxoacids of Phosphorus

• PCl₅ on heating → PCl₃ + Cl₂

• H₃PO₄ is tribasic

• H₃PO₃ undergoes disproportionation

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4. Group 16 Elements (O, S, Se, Te, Po)

General Characteristics

• Oxidation states: –2, +2, +4, +6

• Metallic character increases down the group

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Dioxygen (O₂)

• Paramagnetic

• Strong oxidising agent

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Ozone (O₃)

• Bent molecule

• Resonance hybrid

• Strong oxidising agent

• Quantitative estimation: KI titration

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Sulphur

• Exists mainly as S₈

• Shows allotropy:

  • Rhombic sulphur

  • Monoclinic sulphur

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Sulphur Dioxide (SO₂)

• Angular molecule

• Acts as reducing agent

• Bleaches temporarily

• Detected by:

  • Acidified K₂Cr₂O₇ (green colour)

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Sulphuric Acid (H₂SO₄)

• Prepared by Contact Process

• Key step:

$$2S{O}_2+O_2\rightleftharpoons 2S{O}_3$$

• Catalyst: V₂O₅

• Ka₁ » Ka₂ (second ionisation weak)

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5. Group 17 Elements (Halogens)

General Characteristics

• Oxidation states: –1, +1, +3, +5, +7

• Strong oxidising agents

• Coloured due to absorption of visible light

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Fluorine – Anomalous Behaviour

• Highest electronegativity

• Only –1 oxidation state

• Forms only one oxoacid (HOF)

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Oxidising Power

$$F_2>C{l}_2>B{r}_2>I_2$$

• Due to high hydration enthalpy of F⁻

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Chlorine

• Bleaches by oxidation

• Used in:

  • Disinfecting water

  • Manufacture of PVC

• Poisonous gases from Cl₂:

  • Phosgene (COCl₂)

  • Tear gas

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Interhalogen Compounds

• More reactive than halogens

• Types: XY, XY₃, XY₅, XY₇

• Example: ICl, ClF₃, IF₇

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6. Group 18 Elements (Noble Gases)

General Characteristics

• Valence shell complete

• Monoatomic

• Chemically inert (except Xe, Kr)

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Xenon Compounds

• XeF₂ → linear

• XeF₄ → square planar

• XeF₆ → distorted octahedral

• XeO₃, XeOF₄ also known

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Why Helium is Used in Diving

• Very low solubility in blood

• Prevents decompression sickness

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7. Important Trends (Exam Favourite)

Bond Strength

$$F_2<C{l}_2>B{r}_2>I_2$$

Acid Strength

$$HF<HCl<HBr<HI$$

Basic Strength (Group 15 Hydrides)

$$Bi{H}_3<Sb{H}_3<As{H}_3<P{H}_3<N{H}_3$$

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8. Environmental Chemistry

• CFCs destroy ozone layer

• SO₂ causes acid rain

• Ozone depletion via free radicals

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9. Key Exam Points

✔ Pentahalides are more covalent than trihalides
✔ NH₃ shows H-bonding, PH₃ does not
✔ O₃ is strong oxidant
✔ F₂ shows anomalous behaviour
✔ Xe forms stable compounds

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What are p-block elements?
   Elements belonging to Groups 15 to 18 with valence shell configuration ns²np¹–⁶.

2. Why do p-block elements show variable oxidation states?
   Because both ns and np electrons participate in bonding.

3. Why is nitrogen less reactive at room temperature?
   Due to strong N≡N triple bond with very high bond energy.

4. What is inert pair effect?
   The tendency of ns² electron pair to remain inert, reducing stability of higher oxidation states down the group.

5. Which hydride of Group 15 is the strongest reducing agent?
   BiH₃.

6. Why is white phosphorus more reactive than red phosphorus?
   Because it has strained P₄ tetrahedral structure.

7. Why is ozone a strong oxidising agent?
   Because it readily decomposes to give nascent oxygen.

8. Why does SO₂ act as a reducing agent?
   Because sulphur is in +4 oxidation state and can be oxidised further.

9. Which halogen has maximum oxidising power?
   Fluorine (F₂).

10. Why are noble gases generally inert?
    Because they have completely filled valence shells.

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📝 Long Answer Questions

1. Describe general characteristics of Group 15 elements.

Group 15 elements include N, P, As, Sb and Bi.

• Valence shell configuration: ns²np³

• Oxidation states: –3, +3 and +5

• Stability of +5 oxidation state decreases down the group due to inert pair effect

• Atomic size increases down the group

• Electronegativity decreases down the group

• Hydrides have pyramidal shape

• Reducing character of hydrides increases down the group

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2. Explain allotropy of phosphorus and compare white and red phosphorus.

Phosphorus shows allotropy mainly in two forms: white and red.

White Phosphorus

• Exists as P₄ molecule

• Very reactive and poisonous

• Glows in dark

• Insoluble in water

Red Phosphorus

• Polymeric structure

• More stable and non-poisonous

• Does not glow in dark

• Less reactive

Red phosphorus is safer and more commonly used.

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3. Describe Group 17 elements and anomalous behaviour of fluorine.

Group 17 elements are called halogens.

• Oxidation states: –1, +1, +3, +5, +7

• Strong oxidising agents

• Exist as diatomic molecules

• Coloured due to absorption of visible light

Fluorine shows anomalous behaviour because:

• Highest electronegativity

• Small size

• Only –1 oxidation state

• Forms only one oxoacid (HOF)

📝 PYQs (Previous Years’ Questions – CBSE Board Oriented)

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🔹 1 Mark PYQs

1. What is inert pair effect?

2. Why is nitrogen less reactive than phosphorus?

3. Name the strongest reducing hydride of Group 15.

4. Which allotrope of phosphorus glows in dark?

5. Why is ozone a powerful oxidising agent?

6. Which halogen has the highest oxidising power?

7. Name one compound formed by xenon.

8. Why are noble gases monoatomic?

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🔹 2 Mark PYQs

1. Why does stability of +5 oxidation state decrease down Group 15?

2. Write two differences between white and red phosphorus.

3. Why is SO₂ a reducing agent?

4. Why does fluorine show anomalous behaviour?

5. Why does NH₃ show hydrogen bonding but PH₃ does not?

6. Write two uses of chlorine.

7. What are interhalogen compounds? Give one example.

8. Why is helium used in deep-sea diving?

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🔹 3 Mark PYQs

1. Explain trends in acidic strength of hydrogen halides.

2. Describe preparation, properties and uses of ammonia.

3. Explain allotropy of sulphur.

4. Describe Contact process for manufacture of sulphuric acid.

5. Explain bleaching action of chlorine.

6. Explain oxidising nature of halogens with trend.

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🔹 5 Mark PYQs

1. (a) Describe general characteristics of Group 15 elements.
   (b) Explain inert pair effect.

2. (a) Compare white and red phosphorus.
   (b) Write two uses of phosphorus compounds.

3. (a) Describe preparation and properties of ozone.
   (b) Why is ozone thermodynamically unstable?

4. (a) Explain anomalous behaviour of fluorine.
   (b) Write properties of interhalogen compounds.

5. (a) Write general characteristics of noble gases.
   (b) Describe structure of XeF₂ and XeF₄.

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Group 15 trends & inert pair effect
✔ White vs red phosphorus
✔ Ozone & sulphur dioxide
✔ Contact process (H₂SO₄)
✔ Fluorine anomaly
✔ Interhalogen compounds
✔ Xenon compounds (structure-based)

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❓ FAQs (Concept Clarity)

1. Why does basic strength of Group 15 hydrides decrease down the group?
   Due to decrease in bond angle and lone pair availability.

2. Why does NH₃ show hydrogen bonding but PH₃ does not?
   Because nitrogen is more electronegative than phosphorus.

3. Why is H₂SO₄ called a dibasic acid in practice?
   Because second ionisation (Ka₂) is much weaker than first.

4. Why is helium used in deep-sea diving?
   Because of its very low solubility in blood.

5. Why does fluorine form only one oxoacid?
   Because it cannot expand its octet due to absence of d-orbitals.

6. 
   

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

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