Ionic Bonding
CIE IGCSE Chemistry· 2.4· 22 min read
1. 1. What is Ionic Bonding (Core)★★☆☆☆⏱ 5 min
Ionic Bond
The strong electrostatic force of attraction between positively charged cations and negatively charged anions, formed when electrons are transferred from metal atoms to non-metal atoms.
Example:
Sodium (Group 1 metal) loses 1 outer electron to chlorine (Group 17 non-metal), forming Na⁺ and Cl⁻ ions that are attracted to each other.
Ionic bonding only occurs between metals (left of the periodic table) and non-metals (right of the periodic table). Metals lose electrons to form positive cations, while non-metals gain those electrons to form negative anions. This electron transfer gives both ions a full outer shell of electrons, matching the stable noble gas electron configuration.
Explain how ionic bonding forms between lithium and fluorine.
- 1
Lithium is a Group 1 metal with 1 outer electron; fluorine is a Group 17 non-metal with 7 outer electrons.
- 2
Lithium transfers its single outer electron to fluorine.
- 3
Lithium now has a full outer shell, forming a +1 cation (Li⁺). Fluorine gains the electron, has a full outer shell, forming a -1 anion (F⁻).
- 4
The strong electrostatic attraction between Li⁺ and F⁻ is the ionic bond holding the compound together.
Exam tip:
Always specify ionic bonding is electrostatic attraction between oppositely charged ions in exam answers, not just 'attraction' to get full marks.
2. 2. Dot-Cross Diagrams for Ionic Compounds (Core)★★★☆☆⏱ 6 min
Dot-Cross Diagram
A simplified diagram showing the arrangement of outer shell electrons in atoms, ions or molecules, where electrons from one atom are drawn as dots and electrons from another as crosses.
For ionic compounds, every ion must be drawn inside square brackets, with the overall charge of the ion written in the top right corner outside the brackets. You do not need to draw lines between ions, as ionic bonds are electrostatic attractions not shared electron pairs.
Draw the dot-cross diagram for sodium chloride (NaCl).
- 1
Draw the Na⁺ ion: outer shell is empty (since it lost its 1 outer electron), enclose in square brackets, write +1 outside the top right corner.
- 2
Draw the Cl⁻ ion: outer shell has 8 electrons: 7 original (drawn as crosses) and 1 gained from Na (drawn as a dot).
- 3
Enclose the Cl⁻ ion in square brackets, write -1 outside the top right corner.
- 4
Place the two ions next to each other with no connecting line between them.
Exam tip:
Mark the transferred electron as a different symbol (dot vs cross) to clearly show which atom it came from, this guarantees you full marks for diagram questions.
3. 3. Predicting Ionic Compound Formulae (Core)★★★☆☆⏱ 5 min
All ionic compounds are electrically neutral, meaning the total positive charge from cations equals the total negative charge from anions. You can use the 'swap and drop' method to find the correct formula quickly.
Find the formula of aluminium oxide.
- 1
Identify ion charges: aluminium is Group 13, so charge = +3 (Al³⁺); oxygen is Group 16, so charge = -2 (O²⁻).
- 2
Swap the numerical values of the charges to become subscripts for the opposite ion: Al gets subscript 2, O gets subscript 3.
- 3
Write the formula: Al₂O₃.
- 4
Check neutrality: 2*(+3) + 3*(-2) = 6 - 6 = 0, so the formula is correct.
Exam tip:
If your subscripts share a common factor, simplify them: e.g. Mg₂O₂ simplifies to MgO, as ionic formulae are always written in the smallest whole number ratio.
4. 4. Extended Only: Advanced Ionic Bonding Concepts★★★★☆Extended only⏱ 6 min
Ionic Lattice
A giant regular 3D structure of alternating positive and negative ions held together by strong ionic bonds.
Example:
Sodium chloride forms a cubic lattice with equal numbers of Na⁺ and Cl⁻ ions.
The strength of an ionic lattice depends on two factors: 1) ion charge: higher charge ions create stronger electrostatic attraction, leading to higher melting points; 2) ionic radius: smaller ions pack closer together, so attraction is stronger, leading to higher melting points.
Explain why magnesium oxide (MgO) has a much higher melting point than sodium chloride (NaCl).
- 1
Identify ion charges: Mg²⁺ and O²⁻ in MgO; Na⁺ and Cl⁻ in NaCl.
- 2
Ions in MgO have double the charge of ions in NaCl, so electrostatic attraction between MgO ions is much stronger.
- 3
More energy is required to overcome these stronger ionic bonds, so MgO has a higher melting point.
Extended candidates also need to write balanced half-equations for ion formation, which show the gain or loss of electrons (written as e⁻) by an atom.
Write the half-equations for the formation of ions in magnesium chloride (MgCl₂).
- 1
Magnesium loses 2 electrons to form a Mg²⁺ cation (oxidation):
- 2
- 3
Two chlorine atoms each gain 1 electron to form 2 Cl⁻ anions (reduction):
- 4
- 5
Check that total charge is equal on both sides of each half-equation to confirm it is balanced.
Exam tip:
For oxidation half-equations (electron loss), electrons are written on the right of the arrow. For reduction half-equations (electron gain), electrons are written on the left of the arrow.
5. Common Pitfalls
Wrong move:
Describing ionic bonding as just the transfer of electrons
Why:
Electron transfer forms the ions, but the ionic bond itself is the electrostatic attraction between the charged ions.
Correct move:
First explain electron transfer to form ions, then define the ionic bond as the electrostatic attraction between oppositely charged ions.
Wrong move:
Drawing lines between ions in ionic dot-cross diagrams
Why:
Lines represent shared covalent electrons, not electrostatic ionic attractions.
Correct move:
Draw each ion separately in square brackets with charges, no connecting lines between ions.
Wrong move:
Writing unbalanced ionic formulae e.g. AlO instead of Al₂O₃
Why:
Ionic compounds are always neutral, so total positive charge must equal total negative charge.
Correct move:
Use the swap and drop method, then check that the sum of all ion charges equals zero.
Wrong move:
Writing ion charges inside square brackets in dot-cross diagrams
Why:
The charge is a property of the whole ion, not just the electrons inside the shell.
Correct move:
Write the charge in the top right corner outside the square brackets surrounding the ion.
Wrong move:
Ignoring ionic radius when explaining melting point differences (Extended)
Why:
Both ion charge and ionic radius affect the strength of electrostatic attraction in the lattice.
Correct move:
Mention both factors if relevant: e.g. LiF has a higher melting point than KF because Li⁺ is smaller than K⁺, so attraction is stronger.
6. Quick Reference Cheatsheet
Concept | Core Requirement | Extended Requirement |
|---|---|---|
Ionic Bond Definition | Electrostatic attraction between oppositely charged ions, formed by electron transfer from metal to non-metal | Same as Core, plus link bond strength to ion charge and radius |
Dot-Cross Diagrams | Draw outer shell electrons, square brackets, charge outside brackets | Same as Core, plus diagrams for multiply charged ion compounds |
Ionic Formula | Predict neutral formula using swap and drop method | Same as Core, plus formulae for transition metal ion compounds |
Ion Formation | Identify which atom loses/gains electrons | Write balanced half-equations for ion formation |
Lattice Properties | Recall high melting points, conduct when molten/dissolved | Explain effect of ion charge and radius on melting point |
7. Frequently Asked
Do I need to draw inner electron shells in ionic dot-cross diagrams?
For CIE IGCSE, you only need to draw outer shell electrons for both ions, unless explicitly asked to show inner shells in the question. Always put ions inside square brackets with the charge written outside the brackets.
How do I find the charge of an ion for formula prediction?
Group 1 metals form +1 ions, Group 2 form +2, Group 13 form +3; Group 15 non-metals form -3, Group 16 form -2, Group 17 form -1. Transition metal charges are always given in the question (e.g. copper(II) = Cu²⁺).
Going deeper
What's Next
Now that you have mastered ionic bonding, move on to covalent bonding, the next key type of chemical bonding in the CIE IGCSE Chemistry 0620 syllabus. Practice past paper questions on ionic bonding to consolidate your knowledge, focusing on dot-cross diagram and formula prediction questions that appear frequently in both Core and Extended papers. For Extended candidates, make sure you practice half-equation writing and lattice strength explanation questions, which are common in Paper 4 structured assessments. You can also revise periodic table group trends to predict ion charges faster and more accurately.