Last updated: 29 September 2026
A crystal of sodium chloride may look like an ordinary white solid, but on the particle level it contains an enormous repeating arrangement of Na⁺ and Cl⁻ ions. Once you understand how those ions form and why they attract one another, many familiar properties of ionic compounds begin to make sense.
Ionic bonding therefore links several Chemistry ideas together. Electron arrangement explains how ions form; ion charges explain chemical formulae; and the giant ionic lattice explains melting point and electrical conductivity. Rather than memorising each fact separately, the aim is to see the whole chain of reasoning.
Try these five questions before reading the chapter. They are designed to uncover the ideas that ionic bonding builds upon.
- 5 short questions
- Instant feedback after every choice
- Explanations for every answer option
How ions form from atoms
An atom is electrically neutral because it contains equal numbers of positively charged protons and negatively charged electrons. During ordinary chemical reactions, the nucleus remains unchanged, but electrons may be transferred from one atom to another.
When the number of electrons is no longer equal to the number of protons, the particle has an overall electrical charge and is called an ion.
Ion formation changes electrons, not protons
When an atom becomes an ion, it gains or loses electrons. Its number of protons does not change. Changing the number of protons would produce a different element.
Metals form positive ions by losing electrons
Metals on the left-hand side of the Periodic Table usually have relatively few electrons in their outer shell. In the simple ionic compounds studied at O-Level, they lose these outer-shell electrons.
Consider sodium. A sodium atom has the electron arrangement:
2,8,1
When it loses its one outer-shell electron:
Na → Na⁺ + e⁻
The Na⁺ ion has electron arrangement 2,8. Because it contains 11 protons but only 10 electrons, it has an overall charge of +1.
A positively charged ion is called a cation.
Non-metals form negative ions by gaining electrons
Non-metals near the right-hand side of the Periodic Table often have nearly full outer shells. In many simple ionic compounds, they gain enough electrons to complete the outer shell.
Chlorine has electron arrangement:
2,8,7
It gains one electron:
Cl + e⁻ → Cl⁻
The Cl⁻ ion has electron arrangement 2,8,8. It contains 17 protons but 18 electrons, so it has an overall charge of −1.
A negatively charged ion is called an anion.
| Atom | Electron arrangement | Electron change | Ion formed | Ion arrangement |
|---|---|---|---|---|
| Sodium | 2,8,1 | Loses 1 electron | Na⁺ | 2,8 |
| Magnesium | 2,8,2 | Loses 2 electrons | Mg²⁺ | 2,8 |
| Aluminium | 2,8,3 | Loses 3 electrons | Al³⁺ | 2,8 |
| Oxygen | 2,6 | Gains 2 electrons | O²⁻ | 2,8 |
| Chlorine | 2,8,7 | Gains 1 electron | Cl⁻ | 2,8,8 |
Predicting common ion charges
For many main-group elements studied at O-Level, the group number helps us predict how many electrons an atom will lose or gain.
Common ion charges
| Group | Typical electron change | Common charge | Example |
|---|---|---|---|
| Group 1 | Loses 1 electron | +1 | Na⁺ |
| Group 2 | Loses 2 electrons | +2 | Mg²⁺ |
| Group 13 | Often loses 3 electrons | +3 | Al³⁺ |
| Group 16 | Gains 2 electrons | −2 | O²⁻ |
| Group 17 | Gains 1 electron | −1 | Cl⁻ |
What exactly is an ionic bond?
Once electron transfer has produced positive and negative ions, the two types of ion attract one another because they have opposite electrical charges.
Exam definition
An ionic bond is the strong electrostatic attraction between oppositely charged ions.
Notice an important distinction here. Electron transfer is not itself the ionic bond. Electron transfer produces the ions. The ionic bond is the electrostatic attraction that exists between those ions afterwards.
Formation of sodium chloride, NaCl
A sodium atom has one outer-shell electron, while a chlorine atom needs one more electron to complete its outer shell.
Sodium loses one electron:
Na → Na⁺ + e⁻
Chlorine gains that electron:
Cl + e⁻ → Cl⁻
The resulting Na⁺ and Cl⁻ ions have opposite charges and therefore attract one another strongly.
One Na⁺ ion has a charge of +1 and one Cl⁻ ion has a charge of −1. They therefore combine in a 1:1 ratio, giving the formula:
NaCl
Formation of magnesium chloride, MgCl₂
Magnesium has electron arrangement 2,8,2, so one magnesium atom loses two electrons:
Mg → Mg²⁺ + 2e⁻
Each chlorine atom gains only one electron. Two chlorine atoms are therefore needed to accept the two electrons lost by one magnesium atom.
Each of magnesium's two outer electrons is transferred to a different chlorine atom.
The electron changes can be represented as:
Mg → Mg²⁺ + 2e⁻
2Cl + 2e⁻ → 2Cl⁻
One Mg²⁺ ion has a charge of +2, while two Cl⁻ ions provide a total charge of −2:
+2 + 2(−1) = 0
The electrically neutral formula is therefore:
MgCl₂
An ionic compound is electrically neutral overall
The total positive charge and total negative charge must balance. The formula tells us the simplest whole-number ratio of ions required for the overall compound to have no net charge.
Why magnesium oxide is MgO, not Mg₂O₂
Magnesium forms Mg²⁺ ions while oxygen forms O²⁻ ions. One ion of each already balances the charges:
+2 + (−2) = 0
The simplest ratio is therefore 1:1, so the formula is MgO. Ionic formulae are written using the simplest whole-number ratio.
| Ions | Ratio needed | Charge check | Formula |
|---|---|---|---|
| Na⁺ and Cl⁻ | 1:1 | +1 + (−1) = 0 | NaCl |
| Mg²⁺ and Cl⁻ | 1:2 | +2 + 2(−1) = 0 | MgCl₂ |
| Mg²⁺ and O²⁻ | 1:1 | +2 + (−2) = 0 | MgO |
| Al³⁺ and O²⁻ | 2:3 | 2(+3) + 3(−2) = 0 | Al₂O₃ |
Drawing dot-and-cross diagrams for ionic compounds
Dot-and-cross diagrams help us keep track of outer-shell electrons during electron transfer. Electrons from one atom are represented using dots, while electrons from the other atom are represented using crosses.
The symbols do not represent different types of electron. All electrons are identical. The dots and crosses simply tell us where each electron originally came from.
How to draw the diagram correctly
1. Show the outer electrons
Show the correct number of valence electrons for each atom. Inner electron shells do not need to be shown in a simple dot-and-cross diagram.
2. Transfer electrons
Move electron symbols from the metal atom to the non-metal atom. No electrons should disappear or appear from nowhere.
3. Draw square brackets
Enclose each completed ion in square brackets.
4. Add the charges
Write the ion charge outside the bracket at the upper-right corner.
5. Check the outer shells
In the simple examples studied here, the resulting ions should have complete outer electron shells.
6. Check the ratio
MgCl₂ needs one Mg²⁺ ion and two Cl⁻ ions, while NaCl needs one ion of each type.
What should you notice in the NaCl diagram?
Before the transfer, sodium has one outer electron while chlorine has seven. The electron originally belonging to sodium is represented by a different symbol from chlorine's electrons.
After the transfer, Cl⁻ has eight outer electrons:
7 electrons originally from chlorine + 1 electron transferred from sodium
This is why the final chloride ion contains one dot-and-cross pair: one of the paired electrons came from sodium while the other originally belonged to chlorine.
What should you notice in the MgCl₂ diagram?
Magnesium starts with two outer electrons. One is transferred to the first chlorine atom and the other is transferred to the second chlorine atom.
Each Cl⁻ ion therefore has eight outer electrons:
7 electrons originally from chlorine + 1 electron transferred from magnesium
This also explains the formula MgCl₂: two chloride ions are needed because each chlorine accepts only one of magnesium's two outer electrons.
What a dot-and-cross diagram can and cannot show
| It shows | It does not show accurately |
|---|---|
| Outer-shell electrons | The actual size of each ion |
| Which electrons were transferred | The actual distances between ions |
| The charge on each ion | The whole three-dimensional crystal structure |
| The required ratio of ions | A literal pathway travelled by an electron |
Ionic compounds have giant ionic lattices
A dot-and-cross diagram focuses on only a few ions so that we can see how electrons are transferred. The actual solid is very different in scale.
Solid sodium chloride does not consist of separate Na⁺–Cl⁻ pairs. Instead, enormous numbers of Na⁺ and Cl⁻ ions are arranged in a regular, repeating three-dimensional structure known as a giant ionic lattice.
| Term | Meaning |
|---|---|
| Giant | The structure contains a very large number of ions. |
| Ionic | The particles are positively and negatively charged ions. |
| Lattice | The ions are arranged in a regular, repeating structure. |
Each positive ion is surrounded by negative ions, while each negative ion is surrounded by positive ions. Strong electrostatic attractions therefore act in all directions throughout the lattice.
NaCl is not a sodium chloride molecule
The formula NaCl gives the simplest ratio of Na⁺ ions to Cl⁻ ions in the lattice. Solid sodium chloride does not contain separate NaCl molecules.
How the giant ionic lattice explains physical properties
Once you know the structure, the important physical properties of ionic compounds can be explained rather than memorised.
Two questions are especially useful:
- How strongly are the ions attracted to one another?
- Are the charged ions free to move?
Why ionic compounds have high melting and boiling points
Oppositely charged ions in a giant ionic lattice are held together by strong electrostatic attractions. These attractions act throughout the structure.
A large amount of thermal energy is therefore needed to overcome enough of these attractions for the ions to move past one another.
Ionic compounds consequently usually have high melting and boiling points and are normally solids at room temperature.
A stronger examination explanation
Instead of writing only that ionic compounds have high melting points because their “bonds are strong”, state that there are strong electrostatic attractions between oppositely charged ions throughout the giant ionic lattice , so a large amount of energy is required to overcome these attractions.
Why solid ionic compounds do not conduct electricity
Electrical conduction requires charged particles that can move through a substance.
Solid sodium chloride certainly contains charged particles — Na⁺ and Cl⁻ ions — but the ions are held in fixed positions in the lattice. They can vibrate about their positions, but they cannot move through the solid and carry electrical charge.
The solid therefore does not conduct electricity.
Why molten ionic compounds conduct electricity
When an ionic compound melts, the ions gain enough energy to move out of their fixed positions. The rigid lattice arrangement breaks down and the ions become mobile.
These mobile positive and negative ions can carry electrical charge through the liquid, so the molten ionic compound conducts electricity.
Why aqueous ionic compounds can conduct electricity
When a soluble ionic compound dissolves in water, its ions separate from one another and become free to move through the solution.
The moving ions carry electrical charge, so the aqueous solution conducts electricity.
| State | Are ions present? | Can the ions move? | Conducts electricity? |
|---|---|---|---|
| Solid | Yes | No — fixed in the lattice | No |
| Molten | Yes | Yes | Yes |
| Aqueous | Yes | Yes | Yes |
Do not confuse ionic conduction with metallic conduction
In metals, electrical charge is carried by mobile delocalised electrons. In molten or aqueous ionic compounds, electrical charge is carried by mobile ions. The explanation is different.
Are all ionic compounds soluble in water?
No. Many ionic compounds are soluble in water, but some are not. Sodium chloride is soluble, for example, whereas silver chloride and barium sulfate are insoluble.
You should therefore avoid the blanket statement that “ionic compounds are soluble in water”. Solubility depends on the particular ions present.
Why ionic solids are brittle
Ionic crystals may be hard because the ions are strongly attracted to one another, but they are also brittle.
If a force shifts one layer of ions relative to another, ions with the same charge can become positioned next to one another. Like charges repel strongly, causing the lattice to split rather than bend.
Linking structure to property in examination answers
When an examination asks you to explain a physical property, a good answer should connect what you observe to the particles inside the substance.
Use this reasoning chain
Structure → particles → forces or movement → observable property
| Property | Strong explanation |
|---|---|
| High melting point | Strong electrostatic attractions act between oppositely charged ions throughout the giant ionic lattice, so a large amount of energy is required to overcome these attractions. |
| Does not conduct when solid | The charged ions are fixed in position and cannot move through the solid to carry electrical charge. |
| Conducts when molten | The ions are free to move through the liquid and carry electrical charge. |
| Conducts when aqueous | The dissolved ions are mobile and can carry electrical charge through the solution. |
Common mistakes in ionic bonding
Electron transfer is the ionic bond
Electron transfer forms the ions. The ionic bond is the electrostatic attraction between the resulting oppositely charged ions.
NaCl exists as individual molecules
Sodium chloride forms a giant ionic lattice. NaCl represents the simplest Na⁺:Cl⁻ ratio of 1:1.
Solid NaCl contains no charged particles
It contains Na⁺ and Cl⁻ ions. It fails to conduct because those ions are fixed in position, not because charges are absent.
Electrons conduct through molten NaCl
The moving charge carriers are Na⁺ and Cl⁻ ions, not freely moving electrons.
Dots and crosses are different electrons
They are merely different symbols used to identify where the electrons originally came from.
MgCl₂ contains one chloride ion
One Mg²⁺ ion requires two Cl⁻ ions to balance the total positive charge of +2.
Ion formation changes the number of protons
Ion formation involves the gain or loss of electrons. The nucleus remains unchanged.
All ionic compounds dissolve in water
Some ionic compounds are soluble, while others are insoluble. Solubility depends on the particular compound.
Ionic bonding checklist
Before moving on, make sure you can explain the topic as one connected story rather than as isolated facts.
Ion formation
- I can explain how losing electrons forms positive ions.
- I can explain how gaining electrons forms negative ions.
- I know that the number of protons does not change.
Ionic bonding
- I can define an ionic bond correctly.
- I can distinguish electron transfer from electrostatic attraction.
- I can explain why ionic compounds are neutral overall.
Formulae and diagrams
- I can determine formulae such as NaCl, MgCl₂ and Al₂O₃.
- I can draw brackets and ion charges correctly.
- I can show electron transfer using dots and crosses.
Structure and properties
- I can describe a giant ionic lattice.
- I can explain high melting and boiling points.
- I can explain conductivity in solid, molten and aqueous states.
These questions go beyond simple recall. Use ion charges, electron arrangements and structure–property reasoning to work out each answer.
- 10 application questions
- Common misconceptions tested
- Full explanations for every option
Questions students often ask about ionic bonding
Once the basic model is clear, a few deeper questions naturally follow. These help sharpen the difference between the simplified diagrams we draw and the actual ionic structure.
Is electron transfer the ionic bond?
No. Electron transfer produces positive and negative ions. The ionic bond is the strong electrostatic attraction between those oppositely charged ions.
Why do we say sodium chloride has the formula NaCl if there are no NaCl molecules?
NaCl tells us the simplest ratio of Na⁺ ions to Cl⁻ ions in the giant ionic lattice. There is one Na⁺ ion for every Cl⁻ ion overall.
Do dots and crosses represent different types of electron?
No. All electrons are identical. The two symbols simply show which atom each electron originally came from.
Why do the dot-and-cross diagrams show only outer-shell electrons?
Ionic bonding involves transfer of outer-shell, or valence, electrons. Showing only these electrons makes the transfer much clearer.
Why does molten sodium chloride still contain Na⁺ and Cl⁻?
Melting changes the arrangement and mobility of the ions. It does not convert them back into neutral atoms.
Does dissolving sodium chloride in water turn the ions back into atoms?
No. Sodium chloride separates into Na⁺ and Cl⁻ ions in water. The ions retain their electrical charges.
Why does sodium chloride solution conduct electricity?
The dissolved Na⁺ and Cl⁻ ions are free to move through the solution. Their movement carries electrical charge.
Are all ionic compounds soluble in water?
No. Many ionic compounds are soluble, but some are insoluble. Sodium chloride is soluble, whereas silver chloride and barium sulfate are insoluble.
Do all ions have a complete outer shell?
The complete-outer-shell model works well for simple main-group ions commonly studied at O-Level, such as Na⁺, Mg²⁺, O²⁻ and Cl⁻. It is an introductory model rather than a universal rule for every ion.
Why are ionic crystals brittle if the ionic attractions are strong?
If layers shift, ions with the same charge may become aligned. Strong repulsion between like charges then causes the lattice to crack.
Ionic bonding becomes much easier when you follow the ideas in order: electrons are transferred, ions form, opposite charges attract, and the ions assemble into a giant ionic lattice. That lattice then explains the characteristic properties of ionic compounds. The key is therefore not to memorise ion formation, bonding and properties as separate facts, but to see how each idea leads naturally to the next.