Class 9 Science Chemical Bonding Notes

Class 9 Science Chemical Bonding Notes

Chapter 7 — Chemical Bonding

The chapter focuses on why atoms combine and how chemical bonds produce stable substances.

1. Why do atoms form bonds?

Atoms combine because bonding can produce a more stable electronic arrangement.

The outermost electrons are particularly important in determining chemical behaviour.

Atoms may achieve greater stability by:

  • losing electrons;
  • gaining electrons;
  • sharing electrons.

2. Ionic bonding

An ionic bond results from electron transfer.

Typically:

  • one atom loses electron(s) → positive ion;
  • another gains electron(s) → negative ion.

The oppositely charged ions attract each other electrostatically.

For example, sodium can lose an electron while chlorine gains one, producing ions that attract to form sodium chloride.

3. Covalent bonding

In a covalent bond, atoms share electron pairs.

Depending on the number of shared pairs, bonds can be:

  • single;
  • double;
  • triple.

Covalent bonding allows atoms to achieve more stable electron arrangements without complete electron transfer.

4. Lewis structures

Lewis representations show valence electrons and shared electron pairs.

They are useful for understanding:

  • which electrons participate in bonding;
  • how many bonds an atom tends to form;
  • lone pairs;
  • overall molecular structure.

5. Bonding and properties

The type of bonding strongly influences properties.

Ionic substances generally consist of oppositely charged ions arranged in an extended structure, while covalent substances involve shared electron pairs between atoms.

Exam focus

Be able to distinguish:

Ionic bonding → electron transfer → ions → electrostatic attraction

from

Covalent bonding → electron sharing → molecules/structures

The chapter’s treatment should be learned through the specific examples and representations given in the PDF rather than treating all compounds as having identical properties.

Chapter 7 — Chemical Bonding

The chapter includes the octet rule and the electron-sea model of metallic bonding, including its connection with conductivity, malleability and ductility.

Questions

A. Multiple Choice Questions

1. Valence electrons are especially important in determining an atom’s:
a) Chemical behaviour
b) Nuclear size only
c) Neutron number
d) Mass number only

2. The octet rule states that many atoms tend to achieve:
a) One valence electron
b) Two valence electrons only
c) Eight valence electrons
d) Zero electrons

3. Ionic bonding primarily involves:
a) Electron sharing
b) Electron transfer
c) Neutron transfer
d) Proton sharing

4. Covalent bonding primarily involves:
a) Electron sharing
b) Proton transfer
c) Neutron loss
d) Electron destruction

5. In the electron-sea model, metal ions are:
a) Completely free to leave the metal
b) Arranged in a relatively fixed structure
c) Neutralised into atoms
d) Absent

6. The mobile electrons in a metal are called a:
a) Proton cloud
b) Sea of electrons
c) Neutron field
d) Valence shell

7. Metallic bonding is described as:
a) Directional and localised
b) Non-directional and delocalised
c) Limited to one pair of atoms
d) Based on proton transfer

B. Fill in the Blanks

8. An atom that loses an electron becomes a ______.

9. An atom that gains an electron becomes an ______.

10. Covalent bonds involve ______ of electrons.

11. The attraction between positive metal ions and delocalised electrons is called ______ bonding.

12. The mobile electrons in a metal form a “______ of electrons”.

C. True or False

13. Ionic bonding involves electron transfer.

14. Covalent bonding involves electron sharing.

15. Electrons in the electron-sea model are fixed to individual metal atoms.

16. Mobile electrons help explain the electrical conductivity of metals.

17. Metallic bonding is directional in the same way as a localised covalent bond.

D. Short Answer

18. What is the octet rule?

19. Distinguish between ionic and covalent bonding.

20. Explain the electron-sea model of metallic bonding.

21. Why are metals good conductors of electricity according to this model?

22. How does the electron-sea model help explain malleability and ductility?

Answers

A. MCQ Answers

  1. a) Chemical behaviour
  2. c) Eight valence electrons
  3. b) Electron transfer
  4. a) Electron sharing
  5. b) Arranged in a relatively fixed structure
  6. b) Sea of electrons
  7. b) Non-directional and delocalised

B. Fill in the Blanks

  1. cation
  2. anion
  3. sharing
  4. metallic
  5. sea

C. True/False

  1. True
  2. True
  3. False
  4. True
  5. False

D. Answers

  1. The octet rule states that many atoms tend to gain, lose or share electrons to achieve a stable arrangement of eight electrons in their valence shell. Hydrogen follows a two-electron arrangement.
  2. Ionic bonding involves transfer of electrons and electrostatic attraction between oppositely charged ions. Covalent bonding involves sharing of electrons between atoms.
  3. In the electron-sea model, positive metal ions form an ordered structure while valence electrons become delocalised and move throughout the metal. The attraction between these ions and mobile electrons holds the metal together.
  4. The electrons are mobile and can move through the metal when an electric field is applied, allowing electric current to flow.
  5. Metal ions can shift relative to one another while the delocalised electrons continue to maintain attraction throughout the structure. This allows metals to be hammered into sheets and drawn into wires.