CG Board Class 9 Science Chapter 8 Solutions: Chemical Bonding (रासायनिक आबंधन) – 2026-27

(i) When sodium reacts with chlorine:**

• (a) Each sodium atom accepts an electron
• (b) Each chlorine atom accepts an electron
• (c) Each sodium atom loses seven electrons
• (d) Each chlorine atom loses seven electrons

(ii) A sodium atom and a sodium ion:

• (a) Are chemically same
• (b) Have the same number of protons
• (c) Form covalent bonds
• (d) Have the same number of electrons

(iii) An ionic bond is formed when:

• (a) The combining atoms accept electrons
• (b) The combining atoms lose electrons
• (c) One atom loses and the second gains electrons
• (d) When two metals react

(iv) Which of the following elements loses two electrons in order to attain inert gas configuration similar to argon?

• (a) magnesium
• (b) sodium
• (c) calcium
• (d) sulphur

(v) Double bonds are found in which of the following molecules:

• (a) N₂
• (b) C_2H₂
• (c) Cl₂
• (d) CCl₄

(i) Sodium atom …………. an electron to attain the electronic configuration of the element ……….**

(ii) Two atoms of nitrogen share ……………… pairs of electrons to form a nitrogen molecule.

(iii) The number of electrons in the outermost shells of inert gases is …………………. but in case of helium it is ……………………..

(iv) Chlorine molecule has ……………… bond but magnesium chloride has …………………. bond.

(v) Ionic compounds are usually …………………….. in water but covalent compounds are …………………… in water.

An Ionic bond (or Electrovalent bond) is formed.

• Explanation: When one atom loses electrons, it forms a positively charged cation, and when another atom gains those electrons, it forms a negatively charged anion. The oppositely charged ions attract each other strongly through electrostatic forces of attraction to form an ionic compound.

Nitrogen (₇N) has electronic configuration 2, 5. Two nitrogen atoms share 3 pairs of electrons (6 electrons total) to complete their octets: :N · · · + · · · N: → :N ≡ N:.

Argon (₁₈Ar) has an electronic configuration of 2, 8, 8. Since its outermost shell already contains a complete octet (8 electrons), it is highly stable and inert. It has no tendency to gain, lose, or share electrons, so it exists as a monoatomic gas (Ar) and does not form Ar₂.

Element X (Calcium, Z=20) loses 2 valence electrons to form X²⁺ ion (2, 8, 8).

• Element Y (Oxygen, Z=8) gains 2 electrons to form Y^2- ion (2, 8).
• An Ionic bond is formed by complete transfer of 2 electrons.
• Formula: XY (or CaO).
• Electron dot transfer: X · · + · · Y ( · · ) · · → X²⁺ + [ · · Y ( · · ) ( · · ) · · ]^2-.

Valence electrons (electrons in the outermost shell) dictate chemical reactivity and bond formation. Atoms gain, lose, or share only their valence electrons to attain a stable inert gas electronic configuration (octet). They determine the valency and the nature of bonding (ionic or covalent).

(i) Number of covalent bonds in NH₃ / :**

3 single covalent bonds (formed between Nitrogen and 3 Hydrogen atoms).

• (ii) Why Sodium Chloride is not a molecule / :

Sodium chloride is an ionic compound that exists as a 3D crystal lattice made up of an aggregate of oppositely charged Na^+ and Cl^- ions held by electrostatic forces, rather than existing as discrete single molecules.

1. Water (H_2O) / :

• Type of Bond / : Covalent bond / .
• Structure: Oxygen shares 1 electron with each of the two Hydrogen atoms (H-O-H).

1. Nitrogen (N₂) / :

• Type of Bond / : *Covalent bond (Triple bond) / .
• Structure: Two N atoms share 3 pairs of electrons (:N ≡ N:).

1. Magnesium Oxide (MgO) / :

• Type of Bond / : Ionic bond / .
• Structure: Mg²⁺ + [ · · O ( · · ) ( · · ) · · ]^2- (Mg transfers 2 electrons to O).

1. Calcium Oxide (CaO) / :

• Type of Bond / : Ionic bond / .
• Structure: Ca²⁺ + [ · · O ( · · ) ( · · ) · · ]^2- (Ca transfers 2 electrons to O).

• Properties of Ionic Compounds / :

1. Generally soluble in water.
2. High melting and boiling points due to strong electrostatic attraction.
3. Conduct electricity in aqueous solution or molten state.

• Properties of Covalent Compounds / :

1. Generally insoluble in water.
2. Low melting and boiling points.
3. Do not conduct electricity as they do not form free ions.

Valency is determined by the number of valence electrons present in the outermost shell.

• If valence electrons ≤ 4, Valency = Number of valence electrons (e.g., Sodium with 2, 8, 1 has valency 1).
• If valence electrons > 4, Valency = 8 – Number of valence electrons (e.g., Chlorine with 2, 8, 7 has valency 8 – 7 = 1).

(i) Electronic Configuration & Valency / :**

• (ii) Type of Compounds Formed / :

All three are non-metals. They complete their octet by sharing electrons, so they form covalent compounds when reacting among themselves or with other non-metals. (Note: Oxygen can also form ionic compounds with reactive metals like Ca or Mg).

• Ionic Compounds / :
• Examples: NaCl (Sodium chloride), MgCl₂ (Magnesium chloride).
• Reason / : Formed by electron transfer between metals and non-metals.
• Covalent Compounds / Molecules / /:
• Examples: CCl₄ (Carbon tetrachloride), H_2O (Water), O₂ (Oxygen molecule), CH₄ (Methane).
• Reason / : Formed by sharing of valence electrons between non-metallic atoms.

(i) Mg₂:**

• (ii) MgCl₂:
• (iii) Cl₂: