Nature of Ionic Bonding
A concise revision guide to ionic bonding, electron transfer, giant ionic lattice structure, ionic bonding strength and ionic radii for Cambridge International AS & A Level Chemistry.
- 3.2.1
- 3.2.2-a
- 3.2.2-b
- 3.2.2-c
What these spec points say
- 3.2.1 define ionic bonding as the electrostatic attraction between oppositely charged ions (positively charged cations and negatively charged anions)
- 3.2.2-a describe ionic bonding including the example of sodium chloride
- 3.2.2-b describe ionic bonding including the example of magnesium oxide
- 3.2.2-c describe ionic bonding including the example of calcium fluoride
The Sodium Chloride Giant Ionic Lattice
Choose a step to watch sodium and chloride ions build the cubic lattice, see each ion’s six neighbours, extend the lattice and melt it.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Ions and Electron Transfer
Before you start, check the GCSE ideas about ions and electron transfer that this page builds on.
What Is Ionic Bonding?
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions formed by electron transfer.
In sodium chloride, a sodium atom transfers one electron to a chlorine atom. Sodium becomes a positive ion, Na+, and chlorine becomes a negative ion, Cl–.
The attraction between the Na+ cation and the Cl– anion is an ionic bond.
Key idea: Ionic bonding is not the transfer of electrons itself. The bond is the electrostatic attraction between the oppositely charged ions after transfer has occurred.
![Dot-and-cross diagram of a sodium atom transferring its 3s electron to a chlorine atom, forming [Na]⁺ (2,8) and [Cl]⁻ (2,8,8)](https://www.onlinelearningsystem.net/wp-content/uploads/2026/10/t3-cd-01-dotcross-nacl-v2.svg)
Formation of sodium and chloride ions by electron transfer
Sodium loses one outer electron to form Na+, while chlorine gains one electron to form Cl–.
Quick Check: Predicting the Ions Formed
Answer each quick question on the ions formed and what the ionic bond is.
Ionic Bonding in Magnesium Oxide and Calcium Fluoride
Cambridge names three ionic compounds. Sodium chloride is described above; the same reasoning works for compounds where the ions carry different charges, provided you count the electrons carefully.
| Compound | Electron transfer | Ions formed | Ratio in the lattice | Melting point / °C |
|---|---|---|---|---|
| Sodium chloride, NaCl | Each Na atom (2,8,1) gives one electron to one Cl atom (2,8,7) | Na+ (2,8) and Cl– (2,8,8) | 1 : 1 | 801 |
| Magnesium oxide, MgO | Each Mg atom (2,8,2) gives both outer electrons to one O atom (2,6) | Mg2+ (2,8) and O2- (2,8) | 1 : 1 | 2852 |
| Calcium fluoride, CaF2 | Each Ca atom (2,8,8,2) gives one electron to each of two F atoms (2,7) | Ca2+ (2,8,8) and two F– (2,8) | 1 : 2 | 1418 |
Magnesium oxide has the same lattice arrangement as sodium chloride (each ion surrounded by six of the opposite charge).
Its ions, however, carry twice the charge and are smaller, so the electrostatic attraction is much stronger and its melting point far higher.
In calcium fluoride there are twice as many fluoride ions as calcium ions, so each Ca²⁺ is surrounded by eight F⁻ and each F⁻ by four Ca²⁺.
Dot-and-cross diagrams of MgO and CaF2: outer-shell electrons only, with the ion charges in square brackets.
Exam sentence: Magnesium oxide has a higher melting point than sodium chloride because Mg2+ and O2- carry twice the charge and are smaller than Na+ and Cl–.
The electrostatic attraction between the ions is therefore stronger, so more energy is needed to overcome it.
Quick Check: Electrons Transferred in Aluminium Oxide
Count the electrons transferred and the arrangement of each ion in Al₂O₃.
Giant Ionic Lattice Structure
Ionic solids, such as sodium chloride, contain a giant ionic lattice. This means the ions are arranged in a repeating, ordered three-dimensional structure.
In sodium chloride, Na+ and Cl– ions alternate in a regular pattern. Each ion is attracted to neighbouring ions with the opposite charge.
The electrostatic forces act in all possible directions through the lattice, so ionic bonding is not limited to one isolated pair of ions.
| Term | Meaning | Exam focus |
|---|---|---|
| Giant | A very large repeating structure containing many ions. | Avoid describing NaCl as a small molecule. |
| Lattice | A regular, repeating arrangement of particles. | Refer to a three-dimensional ionic lattice. |
| Ionic bond | Electrostatic attraction between oppositely charged ions. | Attractions act in all directions. |
Giant ionic lattice structure of sodium chloride
The alternating arrangement of Na+ and Cl– ions produces a regular cubic lattice in sodium chloride.
Quick Check: Lattice or Molecule?
In each round, choose the one accurate statement about ionic lattices.
Watch: sodium chloride against magnesium oxide
Larger singly charged ions against smaller doubly charged ones. Which melts higher, and by how much?
Ionic Bonding Strength
Ionic bonds become stronger when the ions are smaller and/or carry greater charges.
Smaller ions allow oppositely charged ions to get closer together, increasing electrostatic attraction. Higher ionic charges also increase the attraction between ions.
This is why magnesium oxide, MgO, has a much higher melting point than sodium chloride, NaCl. The ions in MgO, Mg2+ and O2-, are smaller and more highly charged than Na+ and Cl–.
| Compound | Ions involved | Ion size | Ion charge | Relative melting point |
|---|---|---|---|---|
| NaCl | Na+ and Cl– | Larger | ±1 | 801 °C |
| MgO | Mg2+ and O2- | Smaller | ±2 | 2852 °C |
Exam focus: When comparing ionic bonding strength, refer to both charge and ionic radius where relevant.
Quick Check: Explain a Melting Temperature Difference
Write a short explanation, then compare it with the mark points and the model answer.
Quick Check: Rank the Melting Temperatures
Put the six compounds in order of melting temperature, highest first.
Ionic Radii
Positive ions, called cations, are smaller than their parent atoms. This is often because an entire outer electron shell is lost and the remaining electrons are pulled in more strongly by the nucleus.
Negative ions, called anions, are larger than their parent atoms. Extra electrons are added while the number of protons stays the same, so the nuclear attraction is spread over more electrons.
| Ion | Li+ | Na+ | K+ | Rb+ |
|---|---|---|---|---|
| Ionic radius / nm | 0.060 | 0.095 | 0.133 | 0.148 |
| Ion | N3- | O2- | F– | Na+ | Mg2+ | Al3+ |
|---|---|---|---|---|---|---|
| No. of electrons | 10 | 10 | 10 | 10 | 10 | 10 |
| No. of protons | 7 | 8 | 9 | 11 | 12 | 13 |
| Ionic radius / nm | 0.171 | 0.140 | 0.136 | 0.095 | 0.065 | 0.050 |
Key idea: In an isoelectronic series, the ions have the same number of electrons. As the number of protons increases, the attraction for those electrons increases and the ionic radius decreases.
In an isoelectronic series more protons give a smaller ion, and smaller, more highly charged ions attract more strongly.
Quick Check: Ions with 18 Electrons
Count the protons and electrons in each ion, then drag the terms into place.
Quick Check: The Whole Page in Five Statements
In each round, pick the one accurate statement to review everything on this page.
Common Exam Points
For ionic bonding and structure questions, exam answers usually need precise language. The most important phrases are oppositely charged ions, strong electrostatic attraction, giant ionic lattice and attraction in all directions.
Do not call ionic compounds molecules
NaCl is normally described as a giant ionic lattice, not as individual NaCl molecules.
Separate electron transfer from bonding
Electron transfer forms ions. The ionic bond is the electrostatic attraction between those ions.
Use charge and radius in explanations
Greater charge and smaller ionic radius generally produce stronger ionic attractions and higher melting points.
Frequently Asked Questions
Use these quick answers to check the core language needed for ionic bonding and structure questions.
What is ionic bonding?
Ionic bonding is the strong electrostatic force of attraction between oppositely charged ions formed by electron transfer.
Is ionic bonding the same as electron transfer?
No. Electron transfer forms positive and negative ions. The ionic bond is the electrostatic attraction between those oppositely charged ions.
What is a giant ionic lattice?
A giant ionic lattice is a regular, repeating three-dimensional arrangement of ions held together by electrostatic attractions in all directions.
Why does MgO have a higher melting point than NaCl?
MgO contains Mg2+ and O2- ions, which are more highly charged and smaller than Na+ and Cl– ions. This creates stronger electrostatic attractions, so more energy is needed to melt MgO.
Why are cations smaller and anions larger than their parent atoms?
A cation has lost electrons, often a whole shell, so the remaining electrons are pulled in more strongly. An anion has gained electrons with no extra protons, so the nuclear attraction is spread over more electrons and the ion is larger.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
Keep this note — free
Save your progress across every Cambridge topic. A free account remembers which topics you have covered, saves your question scores, and syncs across your phone and laptop.
- Track every topic you have finished
- Keep your practice-question scores
- No payment, no card, free forever
Already registered? Log in