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

The atomic numbers of potassium and chlorine are 19 and 17 respectively

• (a) Write their electronic configuration.
• (b) In how many ways can the two elements achieve inert gas configuration? Describe.
• (c) Depict the ionic bond in potassium chloride using Lewis symbols.
• (d) What will be the charges on potassium and chloride ions in potassium chloride?

The atomic numbers of lithium and fluorine are 3 and 9 respectively. Use electron dot structures to show the ionic bond formation between lithium and fluorine.

• Electronic configuration of Lithium (₃Li) = 2, 1
• Electronic configuration of Fluorine (₉F) = 2, 7
• Lithium loses 1 electron to form Li^+ (configuration 2 like Helium).
• Fluorine accepts 1 electron to form F^- (configuration 2, 8 like Neon).
• Reaction: Li · + · F ( · · ) ( · · ) · → Li^+ + [ · · F ( · · ) ( · · ) · · ]^- or LiF.

What is the valency of oxygen? What is the valency of potassium? Explain.

The atomic number of oxygen is 8, with configuration (2, 6). It accepts or shares 2 electrons to complete its octet, so its valency is 2.

The atomic number of potassium is 19, with configuration (2, 8, 8, 1). It loses 1 valence electron to attain stable octet, so its valency is 1.

The number of electrons in the M shell of an element is 7 and its valency is 1. What will be the electronic configuration of the ion of this element?

• K shell = 2, L shell = 8, M shell = 7 (Total 17 electrons, Element = Chlorine).
• Since its valency is 1, it gains 1 electron to complete its octet and forms an anion.
• Electronic configuration of the ion (Cl^-) = 2, 8, 8.

You know that aluminium has three electrons in its valence shell which it loses to form Al³⁺. Can you name the element that has four electrons in its outermost shell?

Carbon (C) or Silicon (Si). Carbon has atomic number 6 with electronic configuration (2, 4).

Draw the electron dot structure of ethane (C_2H₆).

• Ethane contains two carbon atoms and six hydrogen atoms.
• Each carbon shares 1-1 electron with three hydrogen atoms and 1 electron with the second carbon atom.
• Structure: H_3C-CH₃ where all bonds are single covalent bonds.

Draw the electron dot structure of any molecule which has a double bond.

In an oxygen molecule (O₂), each oxygen atom has 6 valence electrons and shares 2 pairs of electrons with the other oxygen atom to complete its octet, forming a double bond (O=O).

The atomic number of chlorine is 17

• (a) Write its electronic configuration.
• (b) Explain the formation of chlorine molecule using electron dot structures.

Electronic configuration of Chlorine (₁₇Cl) = 2, 8, 7.

Each chlorine atom needs 1 electron to complete its octet. Two chlorine atoms share 1 pair of electrons between them to form a single covalent bond: · Cl ( · · ) ( · · ) · + · Cl ( · · ) ( · · ) · → :Cl ( · · ) ( · · ) · · Cl ( · · ) ( · · ): or Cl-Cl.

Write down the differences between ionic and covalent compounds.

Explain why ionic compounds have high melting and boiling points.

In ionic compounds, oppositely charged cations and anions are held together by very strong electrostatic forces of attraction in a 3D crystal lattice. A large amount of thermal energy is required to break these strong electrostatic interactions, resulting in high melting and boiling points.

(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: