Dot-and-Cross Diagrams

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Use dot-and-cross diagrams to illustrate ionic bonding, covalent bonding, and coordinate bonding (also called dative covalent bonding)
  2. Draw dot-and-cross diagrams for any compounds mentioned in sections 3.4 and 3.5 of the syllabus
  3. Draw dot-and-cross diagrams for species with atoms that have an expanded octet (more than 8 electrons in their outer shell)
  4. Draw dot-and-cross diagrams for species with an odd number of electrons

What Are Dot-and-Cross Diagrams?

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Dot-and-cross diagrams are simple drawings that show how electrons are arranged in the outer shells of atoms when they form compounds. They help us understand how atoms bond together.

Key Features of Dot-and-Cross Diagrams

  • Only outer shell electrons are shown – these are called valence electrons (the electrons in the outermost shell of an atom)
  • Electrons from one atom are shown as dots (•) and electrons from another atom are shown as crosses (×)
  • This makes it easy to see which electrons originally came from which atom
  • For ions (charged particles), we use square brackets [ ] around the ion with the charge written at the top-right corner

1. Ionic Bonding

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What Is Ionic Bonding?

Ionic bonding occurs when a metal atom transfers one or more electrons to a non-metal atom. This creates two ions (charged particles):

  • Cation – a positively charged ion (the metal loses electrons)
  • Anion – a negatively charged ion (the non-metal gains electrons)

The ionic bond is the strong attraction between these oppositely charged ions.

Why Do Atoms Form Ionic Bonds?

Atoms want to achieve a noble gas configuration – this means having a full outer shell of electrons (usually 8 electrons, or 2 for hydrogen and helium). This makes them stable.

Drawing Ionic Dot-and-Cross Diagrams

Step-by-step method:

  1. Draw the outer shell electrons of each atom before bonding (use dots for one atom, crosses for the other)
  2. Show the transfer of electrons from the metal to the non-metal using arrows
  3. Draw the ions formed, using square brackets [ ] around each ion
  4. Write the charge at the top-right of each bracket
  5. The metal ion usually has no outer electrons shown (they've all been transferred)
  6. The non-metal ion has a full outer shell

Examples of Ionic Compounds

Example 1: Sodium Chloride (NaCl)

  • Sodium (Na) has 1 valence electron (shown as ×)
  • Chlorine (Cl) has 7 valence electrons (shown as •)
  • Sodium transfers its 1 electron to chlorine
  • Result: [Na]⁺ has no outer electrons and [Cl]⁻ has 8 electrons (7 dots + 1 cross)

Example 2: Magnesium Oxide (MgO)

  • Magnesium (Mg) has 2 valence electrons (shown as ××)
  • Oxygen (O) has 6 valence electrons (shown as ••••••)
  • Magnesium transfers both electrons to oxygen
  • Result: [Mg]²⁺ has no outer electrons and [O]²⁻ has 8 electrons (6 dots + 2 crosses)

Example 3: Calcium Fluoride (CaF₂)

  • Calcium (Ca) has 2 valence electrons
  • Each Fluorine (F) has 7 valence electrons
  • Calcium transfers 1 electron to each of two fluorine atoms
  • Result: [Ca]²⁺ and two [F]⁻ ions, each with 8 electrons (7 dots + 1 cross)

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