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Ionic and Covalent Bonding Models

Learn and apply Ionic and Covalent Bonding Models in the published Chemistry course sequence.

  • GCE A-Level H2 Chemistry 9476-2027
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Ionic and Covalent Bonding Models: Orientation

Most bonding questions boil down to one move: name the structure, then name the attraction, then link to the property. This lesson gives you the mark-scheme definitions and the dot-and-cross workflow that starts almost every bonding answer.

Treat this as an extension of Atomic Structure (A Level), then use the Chemical Bonding hub to compare models across the topic.

Definitions (Must Know)

A. Ionic bonding

Ionic bonding is the electrostatic attraction between oppositely charged ions.

B. Covalent bonding

Covalent bonding is the electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.

C. Ionic lattice (giant ionic structure)

An ionic lattice is a giant structure of alternating ions held by ionic bonding in all directions.

D. Simple molecular substance

A simple molecular substance contains molecules held together by intermolecular forces.

E. Giant covalent structure (network)

A giant covalent structure is a network of atoms linked by covalent bonds throughout the structure.

F. Electrostatic attraction

Electrostatic attraction is the attraction between opposite charges.

G. Metallic bonding

Metallic bonding is the electrostatic attraction between a lattice of positive metal ions and delocalised electrons.

Detailed Explanations

A. Dot-and-cross diagrams (workflow)

  1. Decide whether it is ionic (lattice of ions) or covalent (molecules/network).
  2. Count valence (outer-shell) electrons for each atom/ion.
  3. Draw only outer electrons:
    • use different symbols (• and ×) for electrons from different atoms
  4. For ionic diagrams:
    • show the electron transfer result as ions in brackets with charges
    • show a full outer shell (octet) on ions where applicable
  5. For covalent diagrams:
    • show shared pairs between atoms (one dot + one cross per bond)
    • show lone pairs on the correct atom

Required ionic examples:

  • NaCl: one electron is transferred from Na to Cl; show [Na]⁺ and [Cl]⁻, with eight outer electrons around Cl⁻.
  • MgO: two electrons are transferred from Mg to O; show [Mg]²⁺ and [O]²⁻, with eight outer electrons around O²⁻.

Required covalent examples:

SpeciesShared pairs between the named atomsLone-pair check
H₂, Cl₂, HClone shared paireach Cl has three lone pairs
O₂two shared pairseach O has two lone pairs
N₂three shared pairseach N has one lone pair
CO₂two shared pairs in each C = Oeach O has two lone pairs
CH₄four C–H shared pairsno lone pair on C
C₂H₄two shared pairs between C atoms; four C–H shared pairsno lone pair on C
Dot-and-cross diagrams for sodium chloride and magnesium oxide showing bracketed ions, charges and transferred outer electrons
For ionic dot-and-cross diagrams, show only outer electrons, distinguish their origins, and place every ion in brackets with its charge.

B. What “electrostatic attraction” means (avoid the wrong definition)

  • Ionic bond: attraction between Na⁺ and Cl⁻ (opposite charges).
  • Covalent bond: attraction between a shared electron pair and both nuclei.

So “transfer” and “sharing” are formation stories. The bond itself is the attraction.

C. Metallic bonding

Metal atoms contribute outer electrons to a delocalised electron system. The remaining positive ions occupy lattice positions, and attraction between the ions and delocalised electrons acts throughout the structure. Because the electrons are mobile, metals conduct electricity in both solid and molten states.

D. Using bonding models to predict properties (exam chain)

Use a structure-first chain:

  1. Identify structure (ionic lattice / molecules / giant covalent).
  2. State the forces/bonds present (ionic bonding / intermolecular forces / covalent bonds).
  3. Link to the property asked (melting point / conductivity / solubility).

Mini example (1–2 sentences, exam style): Solid NaCl does not conduct because ions are fixed in the lattice (no mobile charge carriers). Molten NaCl conducts because ions are mobile.

Worked Examples

Modelled example 1

Construct the Magnesium Oxide Diagram

Core

Problem

Draw a dot-and-cross diagram for MgO.

Study the worked solution

  1. Show the electron transfer

    Method

    Transfer magnesium’s two outer electrons to oxygen.

    Reason

    Magnesium forms Mg²⁺ and oxygen completes its outer shell as O²⁻.

    Working

    Mg → Mg²⁺ + 2e⁻; O + 2e⁻ → O²⁻.

  2. Draw separate bracketed ions

    Method

    Place Mg²⁺ and O²⁻ in separate brackets with charges.

    Reason

    An ionic diagram shows transferred electrons and distinct ions, not a shared pair.

    Working

    [Mg]²⁺\ [O]²⁻

  3. Complete the oxide outer shell

    Method

    Show eight outer electrons around oxide, distinguishing their origins.

    Reason

    Six came from oxygen and two came from magnesium.

    Working

    Eight electrons around O²⁻: six of one symbol and two of the other.

Guided practice 2

Specify the Carbon Dioxide Diagram

About 6 min

Problem

Describe what a correct dot-and-cross diagram of CO₂ must show.

Try this before viewing the solution

Hints

Hint 1: satisfy the central atom
Carbon needs four shared electron pairs in total.
Hint 2: check each oxygen
After double bonding, each oxygen retains two lone pairs.
View solution step by step

  1. Set the bond pattern

    Method

    Place carbon between two oxygen atoms with a double bond to each.

    Reason

    Two C = O double bonds complete the outer shells of all three atoms.

    Working

    O = C = O

  2. Show shared-pair origins

    Method

    Use two shared pairs in each double bond, with one electron from each bonded atom per pair.

    Reason

    Dot-and-cross symbols distinguish electron origin without implying different electron types.

    Working

    Each C = O bond contains two dot-cross pairs.

  3. Add lone pairs

    Method

    Give each oxygen two lone pairs and carbon none.

    Reason

    This accounts for all valence electrons and completes every octet.

    Working

    Two lone pairs on each O; no lone pairs on C.

Common misconception 3

Identify the Mobile Charge Carrier

Find and correct the mistake

Learner claim

Asked why solid NaCl does not conduct electricity but molten NaCl does, a learner says, “Melting releases delocalised electrons that carry charge.” Identify the error and give the correct explanation.

Choose the mobile species

Charge carriers in molten NaCl

View solution step by step

  1. Explain the solid

    Method

    State that the ions are fixed in the ionic lattice.

    Reason

    Without mobile charged particles, the solid cannot carry current.

    Working

    Solid NaCl: ions present but not mobile.

  2. Explain the melt

    Method

    State that melting allows the ions to move.

    Reason

    Mobile Na⁺ and Cl⁻ ions carry charge through the liquid.

    Working

    Molten NaCl conducts by ion movement.

Examiner practice 4

Define Metallic Bonding and Explain Conduction

3 marks

Problem

Define metallic bonding and explain why copper conducts electricity as a solid. [3 marks]

Try this before viewing the solution

View solution step by step

  1. Name the two charged components

    1 mark

    Method

    Identify a lattice of positive ions and delocalised electrons.

    Reason

    A metallic-bond definition must specify what attracts what.

    Working

    Positive metal ions and delocalised electrons.

  2. Define the attraction

    1 mark

    Method

    State that metallic bonding is electrostatic attraction between those components.

    Reason

    The bond is the attraction, not either component by itself.

    Working

    Electrostatic attraction: positive-ion lattice ↔ delocalised electrons.

  3. Explain solid conduction

    1 mark

    Method

    State that delocalised electrons move through the solid lattice and carry charge.

    Reason

    The electrons remain mobile even when the positive ions occupy fixed lattice positions.

    Working

    Mobile electrons → electrical conduction.

Challenge 5

Transfer the Ionic Diagram to Aluminium Oxide

Minimal support

Problem

Describe how to draw a complete dot-and-cross diagram for one formula unit of Al₂O₃. Your description must include the number and charges of all ions and the origin of the outer electrons around each oxide ion.

Try this before viewing the solution

Hints

Hint 1: balance the charges
Two Al³⁺ ions give total charge 6 +; three O²⁻ ions give 6-.
Hint 2: track the transferred electrons
The two aluminium atoms lose six electrons in total, two to each oxygen atom.
View solution step by step

  1. Set the ion ratio

    Method

    Draw two aluminium ions and three oxide ions in separate brackets.

    Reason

    The 2:3 ratio balances 2(3 +) with 3(2-).

    Working

    2[Al]³⁺ and 3[O]²⁻.

  2. Track electron transfer

    Method

    Show each aluminium atom losing three electrons and each oxygen atom gaining two.

    Reason

    Six electrons lost by aluminium must equal six gained by oxygen.

    Working

    2 × 3 = 3 × 2 = 6 transferred electrons.

  3. Complete the oxide ions

    Method

    Show eight outer electrons around every O²⁻, with six oxygen symbols and two aluminium symbols.

    Reason

    Each oxide ion has a complete outer shell and preserves electron origin in the diagram.

    Working

    Three complete oxide octets; no shared electron pairs.

Mind Stretchers

Mind stretcher 1Extension

Explain why MgO has a higher melting point than NaCl.

Show Answer

Mark scheme:

  • Both are ionic lattices (giant ionic structures).
  • Mg²⁺ and O²⁻ have higher charges than Na⁺ and Cl⁻.
  • Higher charge (and typically smaller ions) gives stronger electrostatic attraction in the lattice.
  • Stronger attraction means more energy is needed to melt → higher melting point.