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Ionic Bonds

Ionic bonding: electron transfer to form ions, electrostatic attraction in a giant lattice, and melting point vs conductivity explained clearly.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the formation of ions by electron loss/gain and that these ions usually have the electronic configuration of a noble gas
  • describe, including the use of ‘dot-and-cross’ diagrams, the formation of ionic bonds between metals and non-metals, e.g. NaCl; MgCl2
  • state that ionic materials contain a giant lattice in which the ions are held by electrostatic attraction, e.g. NaCl (candidates will not be required to draw diagrams of ionic lattices)
  • relate the physical properties (including electrical property) of ionic compounds to their lattice structure (see also 3.4(g)).

Strong ionic-bonding answers connect three ideas in order: electron transfer, formation of ions, and electrostatic attraction between oppositely charged ions.

1. Definition

Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions (cations and anions).

2. Key Ideas

  • Metals form positive ions by losing electrons.
  • Non-metals form negative ions by gaining electrons.
  • Ionic bonds form after electron transfer between a metal and a non-metal.
  • Ionic compounds form a giant ionic lattice (not molecules).
  • Ionic compounds conduct electricity only when ions are mobile (molten or aqueous).

3. Detailed Explanations

Quick Recall (ionic bonding checklist)
  • Metal loses electrons → cation. Non-metal gains electrons → anion.
  • Ionic bond = electrostatic attraction between oppositely charged ions.
  • Ionic compounds form a giant ionic lattice (not molecules).
  • Conducts only when ions are mobile (molten or aqueous).

Atoms tend to form ions to get a stable outer-shell arrangement (a full outer shell).

  • Cations (positive ions): metal atoms lose valence electrons.
    • Example: lithium (2.1) loses 1 electron → Li⁺ (2).
    • Example: aluminium (2.8.3) loses 3 electrons → Al³⁺ (2.8).
  • Anions (negative ions): non-metal atoms gain electrons.
    • Example: oxygen (2.6) gains 2 electrons → O²⁻ (2.8).
    • Example: chlorine (2.8.7) gains 1 electron → Cl⁻ (2.8.8).

See Electron Configuration if you are slow at shell counting.

B. How the Ionic Bond Forms (Electron Transfer)

Ionic bonds are formed between metal ions and non-metal ions after electrons are transferred from the metal to the non-metal.

The ions then attract by electrostatic attraction (+ attracts -).

Example: Sodium Chloride (NaCl)

  1. Sodium (2.8.1) transfers 1 valence electron to Chlorine (2.8.7).
  2. Na⁺ and Cl⁻ ions are formed.
  3. The oppositely charged ions attract each other strongly to form an ionic bond.

C. Giant Ionic Lattice (Not Molecules)

Ionic compounds exist as a giant ionic lattice: a repeating 3D arrangement of alternating positive and negative ions.

Do not write “molecules of NaCl”. The simplest ratio is a formula unit.

D. Properties of Ionic Compounds (What to Write in Exams)

Ionic compounds have these typical properties because of the strong ionic lattice.

PropertyObservationExplanation
Melting/Boiling PointsVery HighLarge amounts of energy are needed to overcome the strong electrostatic forces of attraction between ions.
Electrical Conductivity (Solid)Does not conductIons are held in fixed positions in the lattice and are not mobile.
Electrical Conductivity (Liquid/Aqueous)ConductsThe lattice breaks down; ions become mobile and can carry charge.
SolubilityMany are soluble in water, but some are insolubleSolubility depends on the ions present; use solubility rules (see Salts).
Conductivity keyword

Do not write “electrons move” for ionic conductivity. Write: “ions are mobile and carry charge”.

E. Dot-and-Cross Diagrams (NaCl and MgCl₂)

The syllabus can test ionic bonding using dot-and-cross diagrams.

  • Use dots for electrons from the non-metal and crosses for electrons transferred from the metal.
  • Show the ions in square brackets with the charge outside.
  • In the final ions, the non-metal ion has a full outer shell (usually 8 electrons at O-Level).
Dot-and-cross diagrams for sodium chloride and magnesium chlorideSodium chloride contains a bracketed sodium ion with charge plus one and chloride ion with charge minus one. Magnesium chloride contains a magnesium two-plus ion and two chloride one-minus ions. Each chloride outer shell has seven dots from chlorine and one cross transferred from the metal.Sodium chloride, NaClMagnesium chloride, MgCl₂Na+Cl−ו••••••Na loses 1 electron; Cl gains 1 electron.Oppositely charged ions attract in a giant lattice.Mg2+Cl−ו••••••Cl−ו••••••Mg loses 2 electrons; one goes to each Cl atom.Charge balance: Mg²⁺ + 2Cl⁻ → MgCl₂.• from chlorine × transferred from metal
Dot-and-cross ionic bonding: show only outer-shell electrons, place each ion in square brackets, and write its charge outside. A cross marks each electron transferred from the metal.

4. Common Mistakes

  • Saying ionic bonding is “sharing electrons” (that is covalent bonding). Ionic = electron transfer.
  • Saying solid ionic compounds conduct because “they have ions”. Ions are present but not mobile in the solid.
  • Writing “molecules of NaCl”. Use giant ionic lattice / formula unit.
  • Forgetting charge balance when writing formulas (e.g., writing MgCl instead of MgCl₂).
  • Writing conductivity explanations using “electrons flow” instead of “mobile ions”.

5. Exam Tips

Definition that scores

“Strong electrostatic attraction between oppositely charged ions.”

Formula writing shortcut

Write the ion charges, then balance to make total charge zero (e.g., Mg²⁺ needs two Cl⁻ → MgCl₂).

6. Worked Examples

Modelled example 1

Describe Ionic Bonding (Magnesium Chloride)

Core

Problem

Describe the formation of ionic bonds in magnesium chloride, MgCl₂.
Study the worked solution

  1. Form the magnesium ion

    Method

    Transfer both magnesium valence electrons away.

    Reason

    Magnesium has configuration 2.8.2 and reaches a full outer shell by losing two electrons.

    Working

    Mg → Mg²⁺ + 2e⁻; configuration 2.8.2 → 2.8.

  2. Form two chloride ions

    Method

    Give one transferred electron to each of two chlorine atoms.

    Reason

    Each chlorine atom needs one electron to change from 2.8.7 to 2.8.8.

    Working

    2Cl + 2e⁻ → 2Cl⁻.

  3. Define the bond

    Method

    State the force after the ions form.

    Reason

    Oppositely charged Mg²⁺ and Cl⁻ ions attract.

    Working

    Strong electrostatic attraction holds the ions in a giant ionic lattice.

Guided practice 2

Predicting a Formula From Group Numbers

About 5 min

Problem

Element X is in Group 2 and element Y is in Group 17. Predict the formula of their ionic compound.

Balance total positive and negative charge

Ion formed by X
Ion formed by Y
Neutral formula

Hints

Hint 1: write both ion charges
Group 2 forms X²⁺; Group 17 forms Y⁻.
Hint 2: make the total zero
One 2 + charge requires two 1- charges.
View solution step by step

  1. Predict the ions

    Method

    Use each group to predict electron loss or gain.

    Reason

    Group 2 atoms lose two electrons; Group 17 atoms gain one.

    Working

    Group 2 → X²⁺; Group 17 → Y⁻.

  2. Balance charges

    Method

    Use two Y ions for each X ion.

    Reason

    1(2 +) + 2(1-) = 0, so the formula unit is electrically neutral.

    Working

    X²⁺ + 2Y⁻ → XY₂.

  3. Write the simplest ratio

    Working

    XY₂.

Common misconception 3

Error Analysis (Fix the Formula)

Find and correct the mistake

Learner response

A student writes aluminium oxide as AlO because it contains aluminium and oxygen. Locate the first error, correct the formula and justify it using charges.

Test whether the proposed formula is neutral

Correct formula

View solution step by step

  1. Locate the first error

    Method

    Check the net charge of one Al³⁺ and one O²⁻.

    Reason

    3 + + 2- = 1 +, so AlO is not electrically neutral.

    Working

    Net charge of proposed AlO = +1.

  2. Find the smallest balanced totals

    Method

    Use the lowest common multiple of 3 and 2.

    Reason

    The smallest equal positive and negative charge totals are both 6.

    Working

    2 × Al³⁺ = 6 + and 3 × O²⁻ = 6-.

  3. Correct the formula

    Working

    Al₂O₃.

Examiner practice 4

Conductivity Explanation (Solid vs Molten)

4 marks

Examination question

Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. [4 marks]

Compare the charge carriers in both states

View solution step by step

  1. Describe solid sodium chloride

    2 marks

    Method

    State the position and mobility of its ions.

    Reason

    The ions are held in fixed positions in the giant lattice and cannot move to carry charge.

    Working

    Solid NaCl does not conduct.

  2. Describe molten sodium chloride

    2 marks

    Method

    State what melting changes.

    Reason

    The lattice breaks down sufficiently for ions to become mobile and carry charge.

    Working

    Molten NaCl conducts through moving ions.

Challenge 5

Identify the Bond Type

Minimal support

Evidence-to-model transfer

A metal atom transfers electrons to a non-metal atom. The resulting particles form a repeating lattice. Identify the bonding, name the particles and state the force holding the lattice together.

Build one connected explanation

Bonding
Particles formed
Holding force

Hints

Hint 1: follow the electrons
Electron transfer produces charged particles rather than neutral molecules.
Hint 2: name the lattice force
Ask what attraction acts between the resulting positive and negative particles.
View solution step by step

  1. Infer particle formation

    Method

    Connect electron transfer to ion formation.

    Reason

    The metal loses electrons to form cations; the non-metal gains them to form anions.

    Working

    Metal → cation; non-metal → anion.

  2. Name the bonding and force

    Working

    This is ionic bonding: strong electrostatic attraction between oppositely charged ions in a giant lattice.

7. Mind Stretchers

Mind stretcher 1: Two Compounds, Same Formula TrapExtension

Question: Two substances have formula NaCl. One is molten and conducts electricity. The other is solid and does not. Explain why the conductivity differs, even though the formula is the same.

Show Answer

Same formula means the same ions are present. Conductivity depends on whether ions are mobile.

Solid: ions fixed in the lattice → no conduction.
Molten: lattice broken; ions mobile → conduction.

Mind stretcher 2: Reverse Charge ReasoningExtension

Question: An ionic compound has formula X₂O₃ where oxygen is O²⁻. What charge must ion X have?

Show Answer

Total negative charge from 3 oxygen ions: 3 × (-2) = -6.

Total positive charge must be +6 from 2 X ions, so each X is + 3.

Final: X³⁺.

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz