Ion and Ionic Bonds
2.4 Ion and Ionic Bonds
1. Formation of Positive and Negative Ions
1.1 Positive Ions (Cations)
- Definition: Positively charged ions formed when an atom loses one or more electrons.
- Usually formed by metallic elements.
- Reason for formation:
- Metals have few electrons in their outer shell (1–3).
- They lose electrons to achieve a full outer shell (stable noble gas configuration).
- Examples:
- Sodium: Na → Na⁺ + e⁻
- Magnesium: Mg → Mg²⁺ + 2e⁻
- Aluminium: Al → Al³⁺ + 3e⁻
1.2 Negative Ions (Anions)
- Definition: Negatively charged ions formed when an atom gains one or more electrons.
- Usually formed by non-metallic elements.
- Reason for formation:
- Non-metals have nearly full outer shells (5–7 electrons).
- They gain electrons to achieve a full outer shell.
- Examples:
- Chlorine: Cl + e⁻ → Cl⁻
- Oxygen: O + 2e⁻ → O²⁻
- Nitrogen: N + 3e⁻ → N³⁻
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
2. Giant Lattice Structure of Ionic Compounds
2.1 Structure
- Ionic compounds form a giant lattice structure:
- Regular, repeating 3D arrangement.
- Alternating positive and negative ions.
- Each positive ion is surrounded by negative ions, and vice versa.
- Example: Sodium chloride (NaCl)
- Na⁺ surrounded by 6 Cl⁻ ions.
- Cl⁻ surrounded by 6 Na⁺ ions.
- The arrangement maximises attraction and minimises repulsion between ions.
2.2 Strength
- Strong electrostatic forces between oppositely charged ions.
- Forces act in all directions (non-directional bonding).
3. Definition of Ionic Bond
- Ionic bond: A strong electrostatic attraction between oppositely charged ions.
- Formed through transfer of electrons from a metal atom to a non-metal atom.
4. Formation of Ionic Bonds with Dot-and-Cross Diagrams
4.1 Sodium and Chlorine (NaCl)
- Sodium atom: 2,8,1
- Chlorine atom: 2,8,7
- Sodium loses 1 electron → Na⁺ (2,8)
- Chlorine gains 1 electron → Cl⁻ (2,8,8)
- Ionic bond forms between Na⁺ and Cl⁻.
Dot-and-cross:
- Sodium’s electrons represented as dots.
- Chlorine’s electrons represented as crosses.
- After transfer, both ions have full outer shells.
4.2 Magnesium and Oxygen (MgO)
- Magnesium: 2,8,2 → loses 2 electrons → Mg²⁺ (2,8)
- Oxygen: 2,6 → gains 2 electrons → O²⁻ (2,8)
- Ionic bond forms between Mg²⁺ and O²⁻.
4.3 Calcium and Fluorine (CaF₂)
- Calcium: 2,8,8,2 → loses 2 electrons → Ca²⁺
- Each fluorine: 2,7 → gains 1 electron → F⁻
- One Ca²⁺ bonds with two F⁻ ions.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
5. Properties of Ionic Compounds in Terms of Structure and Bonding
5.1 High Melting and Boiling Points
- Reason:
- Giant lattice held by strong electrostatic forces.
- Large amounts of energy needed to break bonds.
- Example:
- NaCl melting point ≈ 801°C.
- MgO melting point ≈ 2852°C (higher because Mg²⁺ and O²⁻ have higher charges → stronger bonds).
5.2 Electrical Conductivity
- Solid state:
- Ions fixed in lattice → cannot move.
- Do not conduct electricity.
- Molten state:
- Lattice breaks → ions free to move → conducts electricity.
- Aqueous solutions:
- Ionic compounds dissolve → ions free to move and carry charge.
5.3 Solubility
- Many ionic compounds are soluble in water.
- Polar water molecules attract ions and pull them apart.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
6. Summary Table – Ionic Bonding
| Feature | Description | Example |
|---|---|---|
| Type of particles | Positive and negative ions | Na⁺, Cl⁻ |
| Bond type | Strong electrostatic attraction | NaCl |
| Structure | Giant lattice | NaCl, MgO |
| Melting/boiling points | High | NaCl: 801°C |
| Conductivity (solid) | Poor | NaCl solid |
| Conductivity (molten/aqueous) | Good | NaCl solution |
| Solubility in water | Often soluble | NaCl, KBr |
7. Exam Tips
- Always state: Ionic bond = strong electrostatic attraction between oppositely charged ions.
- Use correct electron configurations when explaining bond formation.
- For dot-and-cross diagrams:
- Show transfer clearly.
- Indicate charges on ions with square brackets.
- Compare melting points of different ionic compounds based on ion charges and sizes.