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These are complete exercise solutions for Samacheer Kalvi 9th Science Chapter 10 — Chemical Bonding. Solve each question yourself first, then verify your answers below. Focus on Lewis structures, bond types, and electron configurations.
Key Concepts (Quick Recap)
- Chemical bond: Force of attraction holding atoms together in a molecule.
- Octet rule: Atoms tend to attain 8 electrons in their outermost shell (noble gas configuration).
- Ionic bond: Formed by electron transfer from metal to non-metal; involves electrostatic attraction between oppositely charged ions. Example: NaCl.
- Covalent bond: Formed by electron sharing between non-metals. Examples: H₂ (single bond), O₂ (double bond), N₂ (triple bond).
- Lewis dot structure: Representation of valence electrons as dots around the atomic symbol.
- Electrovalency: Number of electrons lost or gained to form an ionic bond.
- Covalency: Number of electron pairs shared to form covalent bonds.
- Properties of ionic compounds: High melting/boiling points, soluble in water, conduct electricity in molten/aqueous state, crystalline solid.
- Properties of covalent compounds: Low melting/boiling points, insoluble in water (generally), do not conduct electricity, exist as gases, liquids, or soft solids.
I. Choose the Best Answer (MCQs)
Q1. The bond formed by the transfer of electrons from one atom to another is: (a) Covalent bond (b) Ionic bond (c) Metallic bond (d) Hydrogen bond
Answer(b) Ionic bond An ionic bond (also called electrovalent bond) is formed by the complete transfer of one or more electrons from a metal atom to a non-metal atom, resulting in oppositely charged ions held by electrostatic attraction.
Q2. The number of valence electrons present in nitrogen atom is: (a) 3 (b) 5 (c) 7 (d) 8
Answer(b) 5 Nitrogen (atomic number 7) has the electron configuration 2, 5. The 5 electrons in the outermost shell are the valence electrons.
Q3. Which of the following has a double covalent bond? (a) H₂ (b) N₂ (c) O₂ (d) CH₄
Answer(c) O₂ Oxygen has 6 valence electrons and needs 2 more to complete its octet. Two oxygen atoms share two pairs of electrons, forming a double bond (O=O). H₂ has a single bond; N₂ has a triple bond; CH₄ has four single bonds.
Q4. The Lewis dot structure of sodium (Na) ion is: (a) Na with 1 dot (b) Na with 8 dots (c) Na⁺ with no dots (d) Na⁻ with 2 dots
Answer(c) Na⁺ with no dots Sodium (2, 8, 1) loses its one valence electron to form Na⁺ with the stable neon configuration (2, 8). The Na⁺ ion has zero valence electrons, so the Lewis structure shows no dots.
Q5. The nature of bond in CaO is: (a) Covalent (b) Ionic (c) Metallic (d) Coordinate
Answer(b) Ionic Calcium (metal, 2, 8, 8, 2) transfers 2 electrons to oxygen (non-metal, 2, 6), forming Ca²⁺ and O²⁻ ions held by electrostatic attraction — an ionic bond.
Q6. The covalency of carbon in methane (CH₄) is: (a) 1 (b) 2 (c) 3 (d) 4
Answer(d) 4 Carbon (2, 4) shares its 4 valence electrons with 4 hydrogen atoms, forming 4 covalent bonds. The covalency of carbon in CH₄ is 4.
Q7. Which of the following is an example of a coordinate covalent bond? (a) HCl (b) NH₃ (c) NH₄⁺ (d) H₂O
Answer(c) NH₄⁺ In NH₄⁺ (ammonium ion), the nitrogen atom donates both electrons to form a bond with H⁺. This is a coordinate covalent (dative) bond where both shared electrons come from one atom.
Q8. An element X with atomic number 11 forms an ionic compound with element Y of atomic number 8. The formula of the compound is: (a) XY (b) X₂Y (c) XY₂ (d) X₂Y₃
Answer(b) X₂Y X (Na, atomic number 11): 2, 8, 1 — loses 1 electron → X⁺ Y (O, atomic number 8): 2, 6 — gains 2 electrons → Y²⁻ To balance charges: 2X⁺ + Y²⁻ → X₂Y (Na₂O)
Q9. Which of the following compounds is insoluble in water? (a) NaCl (b) HCl (c) CCl₄ (d) KCl
Answer(c) CCl₄ Carbon tetrachloride (CCl₄) is a covalent compound and is insoluble in water. NaCl, HCl, and KCl are ionic compounds that dissolve in water.
Q10. The electronic configuration of a stable ion of magnesium is: (a) 2, 8, 1 (b) 2, 8, 2 (c) 2, 8 (d) 2, 8, 8
Answer(c) 2, 8 Magnesium (2, 8, 2) loses 2 electrons to form Mg²⁺ with the configuration 2, 8 (same as neon), achieving stable noble gas configuration.
II. Fill in the Blanks
1. The valency of chlorine in NaCl is 1. 2. The covalency of nitrogen in NH₃ is 3. 3. The bond formed by sharing of electrons is called a covalent bond. 4. The bond formed by transfer of electrons is called an ionic (electrovalent) bond. 5. In a coordinate bond, both shared electrons come from one atom. 6. The noble gas configuration is also called octet configuration. 7. Ionic compounds are generally soluble in water. 8. Covalent compounds have low melting and boiling points. 9. The number of electrons in the outermost shell of argon is 8. 10. An atom that loses electrons becomes a cation (positive ion).
III. True or False
1. Ionic compounds conduct electricity in solid state.
AnswerFalse — Ionic compounds conduct electricity only in molten state or in aqueous solution, not in solid state. In solid state, ions are fixed in the crystal lattice and cannot move.
2. Covalent compounds are generally soluble in organic solvents.
AnswerTrue — Covalent compounds dissolve in organic solvents (benzene, ether) but are generally insoluble in water.
3. A coordinate bond is a type of covalent bond.
AnswerTrue — In a coordinate (dative) bond, both shared electrons come from one atom, but once formed, it is identical to a covalent bond.
4. Metals lose electrons to form cations.
AnswerTrue — Metals have 1–3 valence electrons and tend to lose them to achieve noble gas configuration, forming positively charged cations.
5. Covalent compounds have high melting points.
AnswerFalse — Covalent compounds generally have low melting points because the intermolecular forces (van der Waals forces) between molecules are weak.
6. Sodium chloride is a covalent compound.
AnswerFalse — NaCl is an ionic compound formed by electron transfer from sodium to chlorine.
7. A triple bond is stronger than a single bond.
AnswerTrue — A triple bond involves sharing three pairs of electrons (e.g., N≡N), making it stronger than a single bond (one pair).
8. Hydrogen follows the duplet rule, not the octet rule.
AnswerTrue — Hydrogen needs only 2 electrons (duplet) in its outermost shell (K shell) to achieve stability, similar to helium.
9. Ionic bonds are non-directional.
AnswerTrue — Ionic bonds are electrostatic forces of attraction that act in all directions equally, making them non-directional.
10. In NaCl crystal, each Na⁺ is surrounded by 6 Cl⁻ ions.
AnswerTrue — In the NaCl crystal lattice, each Na⁺ ion is surrounded by 6 Cl⁻ ions and vice versa (coordination number = 6).
IV. Match the Following
Set 1: Bond Types and Examples
| Column A | Column B |
|---|
| 1. Ionic bond | (d) NaCl |
| 2. Single covalent bond | (a) H₂ |
| 3. Double covalent bond | (e) O₂ |
| 4. Triple covalent bond | (b) N₂ |
| 5. Coordinate bond | (c) NH₄⁺ |
Set 2: Elements and Valency
| Column A | Column B |
|---|
| 1. Sodium (Na) | (c) Electrovalency 1 |
| 2. Magnesium (Mg) | (d) Electrovalency 2 |
| 3. Aluminium (Al) | (e) Electrovalency 3 |
| 4. Carbon (C) | (a) Covalency 4 |
| 5. Nitrogen (N) in NH₃ | (b) Covalency 3 |
Set 3: Properties
| Column A | Column B |
|---|
| 1. High melting point | (b) Ionic compound |
| 2. Low melting point | (a) Covalent compound |
| 3. Conducts electricity in molten state | (b) Ionic compound |
| 4. Insoluble in water | (a) Covalent compound |
| 5. Crystalline solid | (b) Ionic compound |
V. Short Answer Questions
Q1. What is a chemical bond? Why do atoms form bonds?
Answer
A chemical bond is the force of attraction that holds two or more atoms together in a molecule. Atoms form bonds to achieve a stable electron configuration (noble gas configuration with 8 electrons in the outermost shell), which lowers their energy and makes them more stable.
Q2. State the octet rule.
Answer
The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their outermost shell, matching the electron configuration of the nearest noble gas. Hydrogen is an exception — it achieves a duplet (2 electrons).
Q3. Differentiate between ionic and covalent bonds.
Answer
| Feature | Ionic Bond | Covalent Bond |
|---|
| Formation | Transfer of electrons | Sharing of electrons |
| Atoms involved | Metal + Non-metal | Non-metal + Non-metal |
| Directional? | Non-directional | Directional |
| Melting point | High | Low |
| Solubility in water | Generally soluble | Generally insoluble |
| Electrical conductivity | Conducts in molten/aqueous state | Does not conduct |
| Examples | NaCl, CaO, MgCl₂ | H₂, H₂O, CH₄, CCl₄ |
Q4. What is a Lewis dot structure? Draw the Lewis dot structure of NaCl.
Answer
A Lewis dot structure is a representation of an atom or molecule where valence electrons are shown as dots around the atomic symbol.
NaCl formation:
- Na (2, 8, 1): Na with 1 dot → loses electron → Na⁺ (no dots)
- Cl (2, 8, 7): Cl with 7 dots → gains electron → Cl⁻ (8 dots)
- Na⁺ Cl⁻ held by electrostatic attraction
Q5. What is a coordinate covalent bond? Give an example.
Answer
A coordinate covalent (dative) bond is a type of covalent bond in which both shared electrons come from the same atom. The donor atom provides the electron pair; the acceptor atom provides the empty orbital.
Example: Formation of ammonium ion (NH₄⁺)
- NH₃ has a lone pair on nitrogen
- H⁺ has an empty orbital (no electrons)
- Nitrogen donates both electrons to form N→H bond
ResultNH₄⁺ (ammonium ion)
Q6. Why do ionic compounds have high melting points?
Answer
Ionic compounds have high melting points because the ions are held together by strong electrostatic forces of attraction (ionic bonds) in a regular crystal lattice. A large amount of energy is required to overcome these strong forces and break the lattice.
Q7. Why do covalent compounds have low melting points?
Answer
Covalent compounds have low melting points because the covalent bonds are between atoms within a molecule, but the intermolecular forces (van der Waals forces) between molecules are weak. Less energy is needed to separate the molecules.
Q8. Define electrovalency with an example.
Answer
Electrovalency is the number of electrons lost or gained by an atom to form an ionic bond. It equals the magnitude of charge on the ion.
Example: In NaCl, sodium loses 1 electron → electrovalency of Na = 1; chlorine gains 1 electron → electrovalency of Cl = 1.
Q9. Define covalency with an example.
Answer
Covalency is the number of electron pairs shared by an atom with other atoms to form covalent bonds.
Example: In CH₄, carbon shares 4 electron pairs with 4 hydrogen atoms → covalency of C = 4.
Q10. Write the electron configuration and valency of: (a) Na (b) Cl (c) Mg (d) O
Answer
| Element | Atomic No. | Electron Configuration | Valency |
|---|
| Na | 11 | 2, 8, 1 | 1 (loses 1e⁻) |
| Cl | 17 | 2, 8, 7 | 1 (gains 1e⁻) |
| Mg | 12 | 2, 8, 2 | 2 (loses 2e⁻) |
| O | 8 | 2, 6 | 2 (gains 2e⁻) |
VI. Detailed Answer Questions
Q1. Explain the formation of ionic bond in NaCl with Lewis dot structure.
Answer
Sodium (Na) has atomic number 11, electron configuration 2, 8, 1. It has 1 valence electron and tends to lose it to achieve the stable neon configuration (2, 8).
Chlorine (Cl) has atomic number 17, electron configuration 2, 8, 7. It has 7 valence electrons and tends to gain 1 electron to achieve the stable argon configuration (2, 8, 8).
Formation process: 1. Na → Na⁺ + e⁻ (sodium loses 1 electron) 2. Cl + e⁻ → Cl⁻ (chlorine gains 1 electron) 3. Na⁺ + Cl⁻ → Na⁺Cl⁻ (electrostatic attraction forms ionic bond)
Lewis dot structure:
Na• + :Ċl̈: → [Na]⁺ [:Ċl̈:]⁻
Na⁺ has no dots (lost its only valence electron). Cl⁻ has 8 dots around it (gained 1 electron, completing octet). The resulting NaCl forms a crystal lattice where each Na⁺ is surrounded by 6 Cl⁻ ions and each Cl⁻ is surrounded by 6 Na⁺ ions.
Q2. Explain the formation of covalent bonds in: (a) H₂ (b) O₂ (c) N₂
Answer
(a) H₂ (Single bond): Each hydrogen atom has 1 electron and needs 1 more to complete its duplet (K shell = 2 electrons).
H• + •H → H:H (or H—H)
Two hydrogen atoms share 1 pair of electrons, forming a single covalent bond.
(b) O₂ (Double bond): Each oxygen atom has 6 valence electrons and needs 2 more to complete its octet.
:Ö• + •Ö: → :Ö::Ö: (or O=O)
Two oxygen atoms share 2 pairs of electrons, forming a double covalent bond.
(c) N₂ (Triple bond): Each nitrogen atom has 5 valence electrons and needs 3 more to complete its octet.
:Ṅ• + •Ṅ: → :N:::N: (or N≡N)
Two nitrogen atoms share 3 pairs of electrons, forming a triple covalent bond. N₂ has the strongest bond among these.
Q3. Explain the formation of covalent bonds in water (H₂O) and methane (CH₄).
Answer
(a) H₂O: Oxygen (2, 6) has 6 valence electrons and needs 2 more. Each hydrogen (1) needs 1 more.
H• + :Ö: + •H → H:Ö:H (or H—O—H)
Oxygen shares 2 electrons, one with each hydrogen atom, forming 2 single covalent bonds. Oxygen also has 2 lone pairs.
(b) CH₄: Carbon (2, 4) has 4 valence electrons and needs 4 more. Each hydrogen (1) needs 1 more.
H
|
H—C—H
|
H
Carbon shares 1 electron with each of 4 hydrogen atoms, forming 4 single covalent bonds. The shape of CH₄ is tetrahedral.
Q4. Compare the properties of ionic and covalent compounds.
Answer
| Property | Ionic Compounds | Covalent Compounds |
|---|
| Formation | Transfer of electrons | Sharing of electrons |
| Bond type | Electrostatic attraction | Shared electron pair |
| Physical state | Solid crystalline | Gas, liquid, or soft solid |
| Melting/Boiling point | High | Low |
| Solubility | Soluble in water | Insoluble in water; soluble in organic solvents |
| Electrical conductivity | Conducts in molten state and aqueous solution | Does not conduct (no free ions) |
| Directionality | Non-directional | Directional |
| Examples | NaCl, CaO, MgCl₂ | H₂O, CH₄, CCl₄ |
Q5. Write the Lewis dot structures and explain the formation of: (a) MgCl₂ (b) CaO
Answer
(a) MgCl₂: Magnesium (2, 8, 2) loses 2 electrons → Mg²⁺ Each chlorine (2, 8, 7) gains 1 electron → 2Cl⁻
Mg•• + 2[:Ċl̈:] → [Mg]²⁺ 2[:Ċl̈:]⁻
One Mg²⁺ ion is held by 2 Cl⁻ ions by electrostatic attraction. The formula is MgCl₂.
(b) CaO: Calcium (2, 8, 8, 2) loses 2 electrons → Ca²⁺ Oxygen (2, 6) gains 2 electrons → O²⁻
Ca•• + :Ö: → [Ca]²⁺ [:Ö:]²⁻
Ca²⁺ and O²⁻ are held by ionic bond. The formula is CaO.
Common Mistakes
- Confusing ionic and covalent bonds — Ionic = transfer of electrons (metal + non-metal); Covalent = sharing of electrons (non-metal + non-metal).
- Writing wrong Lewis structures — Always count valence electrons first, then distribute to achieve octets.
- Forgetting that H₂O has 2 lone pairs on oxygen — Oxygen has 6 valence electrons; 2 form bonds with H, 4 remain as 2 lone pairs.
- Saying ionic compounds conduct electricity in solid state — They conduct only in molten or aqueous state when ions are free to move.
- Confusing electrovalency with covalency — Electrovalency = electrons lost/gained; Covalency = electron pairs shared.
- Thinking coordinate bonds are different from covalent bonds after formation — Once formed, a coordinate bond is identical to a covalent bond; the difference is only in the origin of the electron pair.
- Forgetting exceptions to the octet rule — Hydrogen follows the duplet rule (2 electrons); elements like P and S can have expanded octets.
Exam Checklist
- [ ] Definitions of ionic bond, covalent bond, and coordinate bond memorized
- [ ] Octet rule and its exceptions understood
- [ ] Lewis dot structures for Na, Cl, NaCl, H₂, O₂, N₂, H₂O, CH₄, NH₃ practiced
- [ ] Formation of ionic compounds: NaCl, MgCl₂, CaO — step by step
- [ ] Formation of covalent molecules: H₂, O₂, N₂, H₂O, CH₄ — step by step
- [ ] Comparison table: ionic vs covalent compounds
- [ ] Properties of ionic and covalent compounds
- [ ] Electrovalency and covalency calculations
- [ ] Electron configurations of elements (1–20) for quick bond prediction
