Chemical Bonding
Introduction
Chemical bonding is the force of attraction between atoms that holds them together in a compound. In ICSE Class 10 Chemistry, you study three types of bonds: electrovalent (ionic), covalent, and coordinate (dative) bonds, with emphasis on electron dot diagrams.
Electrovalent (Ionic) Bond
Formed by the complete transfer of electrons from one atom to another.
- Metal atoms lose electrons to form positive ions (cations).
- Non-metal atoms gain electrons to form negative ions (anions).
- Electrostatic attraction between oppositely charged ions forms the bond.
Examples
| Compound | Electron transfer | Ions formed | Properties |
|---|---|---|---|
| NaCl | Na → Na⁺ + e⁻; Cl + e⁻ → Cl⁻ | Na⁺ Cl⁻ | High MP, conducts electricity in molten/aqueous state |
| MgCl₂ | Mg → Mg²⁺ + 2e⁻; 2Cl + 2e⁻ → 2Cl⁻ | Mg²⁺ 2Cl⁻ | Soluble in water |
| CaO | Ca → Ca²⁺ + 2e⁻; O + 2e⁻ → O²⁻ | Ca²⁺ O²⁻ | High MP, used in cement |
Covalent Bond
Formed by the sharing of electrons between two or more non-metal atoms.
Single Covalent Bond (Shared pair = 1)
| Molecule | Electron dot structure | Bond |
|---|---|---|
| H₂ | H:H | H−H |
| Cl₂ | :Cl̈: + :Cl̈: → :Cl̈:Cl̈: | Cl−Cl |
| CH₄ | C shares 4 electrons with 4 H atoms | C−H (4 bonds) |
Double Covalent Bond (Shared pairs = 2)
Example: O₂ = O::O or O=O
Triple Covalent Bond (Shared pairs = 3)
Example: N₂ = N:::N or N≡N
Examples
| Molecule | Type | Number of shared pairs |
|---|---|---|
| H₂ | Single | 1 |
| O₂ | Double | 2 |
| N₂ | Triple | 3 |
| NH₃ | Single (3 bonds + 1 lone pair on N) | 3 |
| CCl₄ | Single (4 bonds) | 4 |
Coordinate (Dative) Bond
Formed when one atom donates both electrons of a shared pair to another atom that accepts them. Represented by an arrow (→).
Examples
| Ion | Formation | Structure |
|---|---|---|
| H₃O⁺ | H₂O + H⁺ → H₃O⁺ | O donates lone pair to H⁺ |
| NH₄⁺ | NH₃ + H⁺ → NH₄⁺ | N donates lone pair to H⁺ |
Distinguishing Coordinate Bonds
- In H₃O⁺: O has 3 bonds (2 covalent + 1 coordinate) and a positive charge.
- In NH₄⁺: N has 4 bonds (3 covalent + 1 coordinate) and a positive charge.
Comparison of Bond Types
| Property | Electrovalent (Ionic) | Covalent | Coordinate |
|---|---|---|---|
| Electron transfer | Complete transfer | Sharing | One-sided sharing |
| Melting point | High | Low to moderate | Moderate |
| Solubility in water | Usually soluble | Usually insoluble | Variable |
| Electrical conductivity | Conducts when molten/aqueous | Does not conduct (except some) | Depends on ionisation |
| Examples | NaCl, MgCl₂, CaO | H₂, O₂, CH₄, NH₃ | H₃O⁺, NH₄⁺ |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Drawing only 6 electrons around N in NH₃ | N needs 8 electrons (octet) — 3 bonds + 1 lone pair |
| Forgetting lone pairs in electron dot diagrams | Count total valence electrons and distribute evenly |
| Confusing coordinate bond with covalent bond | In coordinate, BOTH shared electrons come from ONE atom |
| Writing NaCl as a molecule (NaCl) in gaseous state | NaCl exists as ions only in solid/aqueous — as Na⁺Cl⁻ lattice |
ICSE Exam Focus
This chapter carries 6–8 marks. Key topics: electron dot diagrams, properties of ionic vs covalent compounds, coordinate bond identification.
Marks Blueprint: Electrovalent bond examples — 2 marks, Covalent bond diagrams — 3 marks, Coordinate bond — 2 marks, Comparison — 1 mark.
Self-Test Questions
-
Draw the electron dot structure of NaCl and explain how the bond is formed.
-
Draw the electron dot structure of NH₃ and CH₄.
-
Explain the formation of a coordinate bond in NH₄⁺ and H₃O⁺.
-
Differentiate between ionic and covalent compounds.
-
Why does NaCl conduct electricity in aqueous solution but not in the solid state?
-
Draw the electron dot structure of O₂ and N₂. How many shared pairs are present?
