Lesson 8: TS Class 10 Physical Science Lesson 8: Chemical Bonding - Important Q&A, and Objective Bits

⚛️ Telangana Class 10 Physical Science — Lesson 8

Chemical Bonding

Complete Chapter Guide: Octet Rule, Lewis Dot Structures, Ionic & Covalent Bonding, VSEPR Theory Molecular Geometries, Solved Examples & Part-B MCQs

📌 Essential Chemical Bonding Concepts

1. Octet Rule

Atoms gain, lose, or share electrons to attain 8 electrons in their outer shell (stable ns² np⁶ configuration):

Valence Electrons = 8 (Stable Octet)

2. Ionic vs Covalent Bond Criterion

Electronegativity Difference (ΔEN):

ΔEN > 1.9 → Ionic  |  ΔEN < 1.9 → Covalent

3. VSEPR Repulsion Order

Repulsion between electron pairs around the central atom decreases in the order:

LP - LP > LP - BP > BP - BP

📐 Summary of VSEPR Molecular Geometries & Bond Angles

The Valence Shell Electron Pair Repulsion (VSEPR) Theory determines the 3D shape of covalent molecules based on the repulsion between electron pairs surrounding the central atom:

VSEPR Molecular Shapes At A Glance

1. Linear (180°)

Example: BeCl₂, CO₂
Bond Pairs: 2 | Lone Pairs: 0
Central Atom: Be / C

2. Trigonal Planar (120°)

Example: BF₃, BCl₃
Bond Pairs: 3 | Lone Pairs: 0
Central Atom: Boron (B)

3. Tetrahedral (109°28')

Example: CH₄ (Methane)
Bond Pairs: 4 | Lone Pairs: 0
Central Atom: Carbon (C)

4. Pyramidal (107°)

Example: NH₃ (Ammonia)
Bond Pairs: 3 | Lone Pairs: 1
Central Atom: Nitrogen (N)

5. Bent / V-Shaped (104°30')

Example: H₂O (Water)
Bond Pairs: 2 | Lone Pairs: 2
Central Atom: Oxygen (O)

Key VSEPR Principle: Presence of lone pairs on the central atom causes stronger repulsion, compressing the bond angle below the ideal tetrahedral value of 109°28'.

Comparison: Ionic vs. Covalent Compounds

Property	Ionic Compounds	Covalent Compounds
Mode of Formation	Complete transfer of electrons from metal to non-metal.	Sharing of valence electron pairs between non-metals.
Melting & Boiling Points	Very High (Strong electrostatic forces between ions).	Relatively Low (Weak intermolecular forces).
Physical State	Crystalline solids at room temperature.	Gases, liquids, or soft solids.
Electrical Conductivity	Conduct electricity in molten or aqueous state (not in solid).	Generally non-conductors of electricity.
Solubility	Soluble in polar solvents like Water; insoluble in organic solvents.	Soluble in non-polar organic solvents (Benzene, CCl4).

📝 Short Answer Questions (2 Marks & 4 Marks)

QUESTION 1 - 2 MARKS

Q1. What is the Octet Rule? Why are Noble Gases chemically inert?

Answer:

• Octet Rule: Atoms of elements tend to gain, lose, or share electrons in order to acquire 8 electrons in their outermost (valence) shell to achieve chemical stability.
• Inertness of Noble Gases: Noble gases (except Helium which has a stable duplet 1s²) already possess 8 electrons in their valence shell (ns² np⁶). Being completely stable, they do not react or form chemical bonds easily.

QUESTION 2 - 2 MARKS

Q2. Explain the formation of an Ionic Bond in Sodium Chloride (NaCl) using electron transfer.

Answer:

• Sodium (Na, Z = 11): Configuration 2, 8, 1. Loses 1 valence electron to form a stable cation:
  Na → Na⁺ + e^-
• Chlorine (Cl, Z = 17): Configuration 2, 8, 7. Gains 1 electron to complete its octet and form an anion:
  Cl + e^- → Cl^-
• The strong electrostatic force of attraction between opposite ions Na⁺ and Cl^- forms the Ionic Bond in NaCl.

QUESTION 3 - 4 MARKS

Q3. Explain the formation of a Nitrogen molecule (N₂) with Lewis Dot Structures and triple bond.

Answer:

• Nitrogen Atom (N, Z = 7): Electronic configuration = 2, 5. Each Nitrogen atom needs 3 electrons to complete its valence octet.
• Electron Sharing: Two Nitrogen atoms share 3 pairs of valence electrons (total 6 electrons) between them.
• Bonding Result: A Triple Covalent Bond (≡) is formed between the two Nitrogen atoms:
  :N ••• N:  →  :N ≡ N:
• The molecule contains 1 triple bond and 1 lone pair on each Nitrogen atom.

🧮 VSEPR Theory Explanations & Essay Questions

4 MARKS - VSEPR BOND ANGLE DEVIATION

Q4. Explain why Ammonia (NH₃) has a bond angle of 107° and Water (H₂O) has 104°30' instead of the ideal tetrahedral angle of 109°28'.

1. Methane (CH₄): Central Carbon has 4 Bond Pairs and 0 Lone Pairs. Repulsion is equal, resulting in a perfect Tetrahedral shape with 109°28' angle.

2. Ammonia (NH₃): Central Nitrogen has 3 Bond Pairs and 1 Lone Pair.

• LP - BP repulsion is stronger than BP - BP repulsion.

• The single lone pair pushes the N-H bonds closer, reducing the bond angle from 109°28' to 107° (Trigonal Pyramidal).

3. Water (H₂O): Central Oxygen has 2 Bond Pairs and 2 Lone Pairs.

• Strong LP - LP repulsion pushes the two O-H bond pairs even closer.

• This further compresses the bond angle down to 104°30' (Bent / V-shaped).

✓ Conclusion: Increasing number of lone pairs increases repulsion, decreasing the bond angle.

6 MARKS - ESSAY ON CHEMICAL BONDING

Q5. Define Chemical Bond. Differentiate between Ionic and Covalent Bonds with respect to mechanism, electron movement, and characteristic properties of compounds formed.

Answer:

• Chemical Bond: The attractive force that holds atoms or ions together to form a stable chemical entity (molecule or compound).

1. Ionic Bond (Electrovalent Bond):
   • Formed by complete transfer of one or more valence electrons from a metal atom to a non-metal atom.
   • Involves formation of positive cations and negative anions.
   • Ionic compounds have high melting points, exist as crystalline solids, and conduct electricity in liquid/dissolved states.
2. Covalent Bond:
   • Formed by mutual sharing of electron pairs between non-metal atoms.
   • Can be single (-), double (=), or triple (≡) covalent bonds depending on whether 1, 2, or 3 pairs of electrons are shared.
   • Covalent compounds have lower melting points, exist as gases/liquids/soft solids, and are non-conductors of electricity.

🎯 PART-B Objective Bits Bank (10 MCQs)

BIT 1

1. A chemical bond formed by the complete transfer of electrons from one atom to another is called:

• Covalent Bond
• Ionic Bond
• Metallic Bond
• Hydrogen Bond

✓ Correct Option: B (Ionic Bond)

BIT 2

2. What is the 3D molecular shape of Methane (CH₄) according to VSEPR Theory?

• Linear
• Trigonal Planar
• Tetrahedral
• Pyramidal

✓ Correct Option: C (Tetrahedral - Bond angle 109°28')

BIT 3

3. The bond angle in a Water molecule (H₂O) is:

• 180°
• 120°
• 107°
• 104°30'

✓ Correct Option: D (104°30' due to 2 lone pairs)

BIT 4

4. How many valence electrons are present in an Oxygen atom (Z = 8)?

• 2
• 4
• 6
• 8

✓ Correct Option: C (6 valence electrons: 2, 6)

BIT 5

5. A triple covalent bond (≡) is present in which of the following molecules?

• H₂
• O₂
• N₂
• Cl₂

✓ Correct Option: C (Nitrogen N₂ molecule)

BIT 6

6. The VSEPR theory was originally proposed by:

• Sidgwick and Powell
• Lewis and Kossel
• Niels Bohr
• Linus Pauling

✓ Correct Option: A (Sidgwick and Powell, later developed by Gillespie and Nyholm)

BIT 7

7. Which type of compound is soluble in non-polar organic solvents like CCl₄ and Benzene?

• Ionic Compounds
• Covalent Compounds
• Metallic Salts
• Inorganic Acids

✓ Correct Option: B (Covalent Compounds)

BIT 8

8. What is the number of lone pairs present on the central Nitrogen atom in an Ammonia (NH₃) molecule?

• 0
• 1
• 2
• 3

✓ Correct Option: B (1 Lone Pair)

BIT 9

9. The bond angle in a linear Beryllium Chloride (BeCl₂) molecule is:

• 90°
• 109°28'
• 120°
• 180°

✓ Correct Option: D (180°)

BIT 10

10. An ionic bond is formed between a metal and a non-metal when their Electronegativity Difference (ΔEN) is:

• Zero
• Less than 1.9
• Greater than 1.9
• Exactly 0.5

✓ Correct Option: C (Greater than 1.9)