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1.4.2Physical Chemistry

Representing Ionic Bonding — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 11 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Drawing Rules

Dot-and-cross diagrams show which electrons move, not their exact position.

1
Draw the outer shell of each atom, using dots for electrons from one atom and crosses for those from the other.
2
Show one or more electrons moving from the metal atom to the non-metal atom.
3
Redraw the ions with square brackets around each and the charge at the top right.
4
The metal ion is drawn without its former outer electrons, so its next inner full shell becomes the outer shell.
5
The non-metal ion shows eight electrons in its outer shell, including the transferred ones, with brackets and a negative charge.

Part 2 of 4Examples

Sodium chloride
  • Sodium chloride Na gives one electron to Cl;
    • the diagram shows [Na]⁺ with no outer electrons and [Cl]⁻ with eight electrons in the outer shell.
Magnesium oxide
  • Magnesium oxide Mg gives two electrons to O;
    • the result is [Mg]²⁺ with its former outer electrons removed and [O]²⁻ with a full outer shell of eight.
Calcium chloride
  • Calcium chloride Ca gives one electron to each of two Cl atoms, giving [Ca]²⁺ and two [Cl]⁻ ions so the charges balance.
Lithium oxide
  • Lithium oxide Two Li atoms each give one electron to one O atom, giving two [Li]⁺ ions and one [O]²⁻ ion.
Aluminium oxide
  • Aluminium oxide Two Al atoms each give three electrons and three O atoms each gain two, so the ions are [Al]³⁺ and [O]²⁻ in the ratio 2 to 3.

Part 3 of 4Checking Your Diagram

Checking Your Diagram
Charge balance
  • Charge balance The total positive charge must equal the total negative charge, which fixes the number of each ion.
Electron count
  • Electron count The electrons lost by the metal must equal the electrons gained by the non-metal.
Brackets
  • Brackets Square brackets and charges are essential because they show that the particles are ions.
Lattice view
  • Lattice view A diagram of a few ions in a grid shows the ionic lattice, with alternating positive and negative ions.
Common error
  • Common error Drawing the electrons as if they were shared between the two atoms would show a covalent bond,
    • which is not what happens in ionic compounds.

Part 4 of 4Exam Checklist & Tips

1.4.2 – Key Points for the Exam

  • Dot-and-cross diagram A diagram using dots and crosses to show the outer electrons of different atoms.
  • Electron transfer The movement of electrons from a metal atom to a non-metal atom to form ions.
  • Charge balance The requirement that total positive and negative charge in a compound is equal.
  • Outer shell The highest-energy occupied shell, containing the electrons that take part in bonding.

Common Mistakes

  • Leaving out the brackets and charges on the ions.
  • Showing the metal ion with its original outer electrons still present.
  • Drawing shared pairs between the ions instead of showing transfer.
  • Forgetting to draw two chloride ions for each Ca²⁺ ion.

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