Class 9 · Advanced ScienceNCERT 2026 Revised Syllabus
Chapter 7: Chemical Bonding
Science Advanced
Octet rule and its exceptions (BF3, SF6, NO), duplet rule, Lewis dot structures, and metallic bonding explained by the electron sea model.
Find complete NCERT solutions for Class 9 Advanced Science, Science Advanced Chapter 7. Every question has a clear, step-by-step answer.
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Quick Check (Octet Rule)(8 questions)
1
What is meant by the octet rule?
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Solution:- Atoms tend to gain, lose, or share electrons.
- They aim to attain eight electrons in their valence shell.
- This achieves a stable, noble-gas-like configuration.Answer: Atoms tend to gain, lose, or share electrons to attain eight electrons in the valence shell.
2
Why does hydrogen not follow the octet rule?
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Solution:- Hydrogen's valence shell needs only two electrons, like helium.
- It follows a duplet rule rather than an octet rule.Answer: Hydrogen follows a duplet rule (2 electrons), not the octet rule.
3
Give one example each of a molecule with (a) incomplete octet (b) expanded octet (c) an odd number of electrons.
- a.(a) incomplete octet
- b.(b) expanded octet
- c.(c) an odd number of electrons
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Solution:(a) Incomplete octet: BF3 (boron trifluoride).
(b) Expanded octet: SF6 (sulphur hexafluoride).
(c) Odd electron molecule: NO (nitric oxide).Answer: (a) BF3 (b) SF6 (c) NO.
4
Why can boron form compounds with only six electrons around it?
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Solution:- Boron has only three valence electrons.
- It can form only three bonds.
- This leaves six electrons around it - an incomplete octet.Answer: Boron has only three valence electrons, so it forms three bonds leaving six electrons.
5
What is meant by a duplet configuration?
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Solution:A duplet configuration is a stable arrangement of only two electrons in the valence shell, as seen in hydrogen and helium.Answer: A duplet is a stable arrangement of two electrons in the valence shell.
6
Why is NO considered an exception to the octet rule?
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Solution:- NO has an odd total of eleven valence electrons.
- At least one atom (nitrogen) is left with an incomplete octet and an unpaired electron.Answer: NO has 11 valence electrons (odd), leaving nitrogen with an incomplete octet.
7
Draw the Lewis dot structure of BF3 and explain why boron does not complete its octet.
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Ans.
Solution:- Boron forms three single bonds with three fluorine atoms.
- It uses only six electrons around itself.
- Boron has just three valence electrons available for bonding.Answer: Boron uses only six electrons around it because it has only three valence electrons.
8
Assertion: SF6 violates the octet rule. Reason: Sulphur can accommodate more than eight electrons.
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Solution:- Assertion: SF6 violates the octet rule - correct.
- Reason: Sulphur can accommodate more than eight electrons - correct.
- The reason correctly explains the assertion.Answer: (A) Both assertion and reason are correct, and the reason explains the assertion.
Check Your Understanding (Metallic Bonding)(18 questions)
1
What is meant by the term ‘electron sea’ in metals?
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Solution:- Electron sea refers to the collection of delocalised, freely moving valence electrons.
- These surround the fixed positive metal ions in a metallic structure.Answer: The electron sea is the delocalised, freely moving valence electrons around positive metal ions.
2
What type of particles are in a fixed position in a metal according to the electron sea model?
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Solution:Positive metal ions are fixed in a regular, ordered arrangement within the metal's structure.Answer: Positive metal ions are fixed in a regular, ordered arrangement.
3
Define metallic bonding.
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Solution:Metallic bonding is the force of attraction between fixed positive metal ions and the surrounding sea of freely moving, delocalised electrons.Answer: Metallic bonding is the attraction between positive metal ions and the electron sea.
4
Why are metallic bonds called non-directional?
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Solution:- Electrons are shared collectively by all atoms.
- They are not localised between two specific atoms.
- Ions can shift position without breaking the bond.Answer: Electrons are shared collectively by all atoms, so the bond is non-directional.
5
Name two properties of metals explained by the electron sea model.
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Solution:- Electrical conductivity - explained by the mobile electron sea.
- Malleability (or ductility) - explained by the fixed positive ion structure.Answer: Electrical conductivity and malleability (or ductility).
6
Explain how the electron sea model accounts for electrical conductivity in metals.
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Solution:- Free electrons move throughout the metal.
- When an electric field is applied, they drift in one direction.
- This movement of electrons constitutes an electric current.Answer: Drifting free electrons under an electric field constitute an electric current.
7
How does the electron sea model explain thermal conductivity in metals?
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Solution:- Heated electrons gain energy and move faster.
- They transfer this energy through collisions to other electrons and vibrating ions.
- Heat spreads quickly.Answer: Fast-moving electrons transfer energy by collisions, spreading heat quickly.
8
Why can metals be beaten into thin sheets? Explain using the electron sea model.
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Solution:- Layers of positive ions can slide over one another without breaking.
- The mobile electron sea continues to hold the shifted ions together.
- This allows malleability.Answer: Sliding ion layers are held together by the mobile electron sea, allowing malleability.
9
What is meant by ductility? How is it explained by the electron sea model?
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Solution:- Ductility is the ability of a metal to be drawn into wires.
- Metal ions slide past each other.
- The mobile electron sea maintains the attraction holding them together.Answer: Ductility is drawing metals into wires; the electron sea holds sliding ions together.
10
How is metallic bonding different from covalent bonding?
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Solution:- Covalent bonds: localised electron sharing between two specific atoms.
- Metallic bonds: electrons delocalised and shared collectively among all atoms.Answer: Covalent = localised sharing between two atoms; metallic = delocalised sharing among all atoms.
11
Explain the structure of a metal according to the electron sea model.
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Solution:- A metal consists of fixed, regularly arranged positive metal ions.
- Surrounded by a 'sea' of freely moving, delocalised valence electrons.
- The electron sea holds the structure together.Answer: Fixed positive ions surrounded by a sea of delocalised valence electrons.
12
If electrons in a metal were not free to move, which property would be most affected? Explain.
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Solution:- Electrical conductivity would be most affected.
- Current flow depends entirely on the free movement of delocalised electrons.Answer: Electrical conductivity would be most affected.
13
Explain why metals do not break when hammered but instead change shape.
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Solution:- The non-directional metallic bond allows layers of ions to slide past one another.
- The electron sea continues to hold them together.
- This prevents breakage.Answer: Sliding ion layers held together by the electron sea prevent breakage.
14
Copper is used for electrical wiring, while rubber is not. Explain using the electron sea model.
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Solution:- Copper has free, delocalised electrons that enable current flow.
- Rubber has no free electrons or mobile charges.
- Rubber is therefore a poor conductor.Answer: Copper has free delocalised electrons; rubber has none, making it a poor conductor.
15
Why are metals generally good conductors of heat as compared to non-metals?
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Solution:- Metals have free electrons that quickly transfer thermal energy throughout the structure.
- Non-metals lack such mobile charge carriers.Answer: Metals conduct heat well because of their free, mobile electrons.
16
Assertion: Metals are good conductors of electricity. Reason: Metals contain free electrons that can move under an electric field.
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Solution:- Assertion: Metals are good conductors of electricity - correct.
- Reason: Metals contain free electrons that can move under an electric field - correct.
- The reason correctly explains the assertion.Answer: (A) Both assertion and reason are correct, and the reason explains the assertion.
17
Assertion: Metallic bonds are non-directional. Reason: Electrons in metals are localised between two atoms.
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Solution:- Assertion: Metallic bonds are non-directional - correct.
- Reason: Electrons in metals are localised between two atoms - wrong.
- Metallic electrons are delocalised, not localised.Answer: (D) Assertion correct, reason wrong.
18
Assertion: Metals are malleable. Reason: Layers of metal ions can slide while electrons continue to hold them together.
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Solution:- Assertion: Metals are malleable - correct.
- Reason: Layers of metal ions can slide while electrons continue to hold them together - correct.
- The reason correctly explains the assertion.Answer: (A) Both assertion and reason are correct, and the reason explains the assertion.