Edexcel iGCSE Chemistry -1.40 Dot-and-Cross Diagrams for Ionic Bonding- Study Notes- New Syllabus

Edexcel iGCSE Chemistry -1.40 Dot-and-Cross Diagrams for Ionic Bonding- Study Notes- New syllabus

Edexcel iGCSE Chemistry -1.40 Dot-and-Cross Diagrams for Ionic Bonding- Study Notes -Edexcel iGCSE Chemistry – per latest Syllabus.

Key Concepts:

1.40 draw dot-and-cross diagrams to show the formation of ionic compounds by electron transfer (only outer electrons need be shown)

Edexcel iGCSE Chemistry -Concise Summary Notes- All Topics

1.40 Dot-and-Cross Diagrams for Ionic Compounds

Dot-and-cross diagrams are used to show ionic bonding through electron transfer.

They show how a metal atom transfers electrons to a non-metal atom to form oppositely charged ions.

What Must Be Shown in the Diagram

  • Only outer (valence) electrons
  • Dots for one atom and crosses for the other
  • Electron transfer from metal to non-metal
  • Square brackets around each ion
  • The correct charge on each ion
  • Full outer shells after transfer

Inner shells are not drawn at GCSE level.

Why Electron Transfer Occurs

Atoms transfer electrons to achieve a full outer shell.

Metals lose electrons because they have few outer electrons.

Non-metals gain electrons because their outer shell is nearly full.

Result: Stable electronic configuration (like noble gases)

Group Trends (Exam Essential)

The number of electrons transferred depends on the group number.

  • Group 1 → Lose 1 electron → +1
  • Group 2 → Lose 2 electrons → +2
  • Group 3 → Lose 3 electrons → +3
  • Group 5 → Gain 3 electrons → −3
  • Group 6 → Gain 2 electrons → −2
  • Group 7 → Gain 1 electron → −1

Balancing Charges in Ionic Compounds

The total positive charge must equal the total negative charge.

For example:

Mg2+ and Cl− Need two Cl− to balance +2

Formula = MgCl2

Step-by-Step Method for Any Question

  1. Write electronic configurations.
  2. Identify how many electrons are lost/gained.
  3. Draw outer electrons only.
  4. Show electron transfer.
  5. Draw ions in brackets with charges.
  6. Check both ions have full outer shells.

Common Exam Mistakes

  • Forgetting brackets around ions
  • Not showing charges
  • Incorrect number of transferred electrons
  • Forgetting to balance ion ratio
  • Drawing inner shells

Example 1 (Conceptual)

Explain how a dot-and-cross diagram shows the formation of sodium chloride.

▶️ Answer / Explanation

Sodium has 1 outer electron.   

Chlorine has 7 outer electrons.

Sodium transfers 1 electron to chlorine.

[Na]+    [Cl]−

Chlorine shows 8 electrons (7 dots + 1 cross).

Both ions now have full outer shells.

Example 2 (Numerical)

Draw a dot-and-cross diagram for magnesium chloride.

▶️ Answer / Explanation

Magnesium loses 2 electrons.

Each chlorine gains 1 electron.

[Mg]2+    2[Cl]−

Each chloride ion shows 7 original dots + 1 cross.

Magnesium shows no outer electrons.

Important: There must be TWO chloride ions.

Example 3 (Hard )

Draw and explain the dot-and-cross diagram for aluminium oxide.

▶️ Answer / Explanation

Aluminium loses 3 electrons.

Oxygen gains 2 electrons.

To balance charges:

2Al → 2Al3+ 3O → 3O2−

Formula = Al2O3

Each oxygen shows 6 original dots + 2 crosses.

Each aluminium loses 3 electrons and is shown as Al3+.

Key Point: Charges must balance (+6 and −6).

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