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1.6.2Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Ionic Bonding and Structures — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 18 of 36 Unit quiz

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Part 1 of 6Forming Ionic Bonds

1.6.2 Ionic Bonding and Structures

An ionic bond is the electrostatic attraction between oppositely charged ions.

Electron transfer
  • A metal atom gives electrons to a non-metal atom, forming a cation and an anion with full outer shells.
Ion charges
  • Group 1, 2 and 3 metals form 1+, 2+ and 3+ ions, while groups 15, 16 and 17 form 3−, 2− and 1− ions.
Dot-and-cross diagram
  • Show only the outer shell, use dots for one atom and crosses for the other, and put each ion in square brackets with its charge.
Example
  • Magnesium transfers two electrons to oxygen to give Mg²⁺ and O²⁻, both with the neon configuration 1s² 2s² 2p⁶.

Part 2 of 6Formulae of Ionic Compounds

Formulae of Ionic Compounds
Balance charges
  • The total positive charge must equal the total negative charge, giving a neutral compound.
Simple examples
  • Na⁺ and Cl⁻ give NaCl, Mg²⁺ and Cl⁻ give MgCl₂, and Al³⁺ and O²⁻ give Al₂O₃.
Compound ions
  • Learn NH₄⁺, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻ and PO₄³⁻, and use brackets when more than one is needed, as in Mg(OH)₂.
Empirical formula
  • The formula of an ionic compound is the simplest whole-number ratio of ions in the lattice.

Part 3 of 6Giant Ionic Lattices

1
Each ion in sodium chloride is surrounded by six oppositely charged ions, so the coordination number is 6:6.
2
The Na⁺ and Cl⁻ ions alternate in all three directions to give a cubic crystal.
3
Magnesium oxide has the same arrangement, with Mg²⁺ and O²⁻ ions instead.
4
In caesium chloride the larger Cs⁺ ion fits eight Cl⁻ neighbours, giving a coordination number of 8:8.
5
The type of lattice depends on the relative sizes of the positive and negative ions.

Part 4 of 6Properties

Properties
Melting and boiling
  • High, because many strong electrostatic attractions between ions must be broken.
Conduction
  • Poor as solids but good when molten or aqueous because the ions can move.
Brittleness
  • Shifting layers puts like charges next to each other, causing repulsion and fracture.
Solubility
  • Often soluble in water but insoluble in non-polar solvents.
CompoundIonsCoordination
NaClNa⁺, Cl⁻6:6
CsClCs⁺, Cl⁻8:8
MgOMg²⁺, O²⁻6:6

Part 5 of 6Worked Example

Question Deduce the formula of aluminium sulfate, containing Al³⁺ and SO₄²⁻ ions.

  1. Find the lowest common multiple of the charges: 3 and 2 give 6.
  2. Six positive charges need two Al³⁺ ions.
  3. Six negative charges need three SO₄²⁻ ions, written in brackets.

Answer Al₂(SO₄)₃

Part 6 of 6Exam Checklist & Tips

1.6.2 – Key Points for the Exam

  • Cation A positively charged ion formed by loss of electrons.
  • Anion A negatively charged ion formed by gain of electrons.
  • Coordination number The number of oppositely charged ions surrounding each ion in a lattice.
  • Ionic bond Electrostatic attraction between oppositely charged ions.
  • Dot-and-cross diagram A diagram using dots and crosses to show where outer electrons came from.

Common Mistakes

  • Drawing a shared pair in an ionic dot-and-cross diagram instead of transferring electrons.
  • Forgetting square brackets and charges around each ion.
  • Writing the formula of aluminium oxide as AlO instead of Al₂O₃.
  • Describing NaCl as individual molecules.

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