Unit Overview
S2: Models of bonding and structure
IB Chemistry HLΒ· 5 min read π n/a
1. Unit at a glance
We progress from core strong bonding types (ionic, covalent) to 3D molecular shape, weaker intermolecular forces, metallic bonding and extended crystal structures. The final three additional higher level (AHL) sub-topics expand on core concepts for HL students only, covering complex bonding and solid-state calculations. The overall learning arc connects bonding at the atomic level to observable bulk properties of materials.
Below are all sub-topics in this unit, ordered from core SL to AHL HL content:
Ionic bonding and structure
Learn how ionic bonds form, the properties of ionic compounds and their giant ionic lattice structure.
β β β± 15 min
Covalent bonding and Lewis structures
Understand covalent bond formation, and how to draw Lewis structures for molecules and polyatomic ions.
β β β± 18 min
VSEPR theory and basic hybridization
Predict molecular geometry from electron domains and learn core concepts of orbital hybridization.
β β β β± 20 min
Intermolecular forces
Classify London dispersion, dipole-dipole and hydrogen bonding, and relate their strength to physical properties.
β β β β± 17 min
Metallic bonding and alloy structure
Explore the metallic bonding model, properties of metals and how alloys form from mixed metal structures.
β β β± 12 min
Crystal lattice structures
Compare the four main types of crystalline solid and their characteristic bulk properties.
β β β β± 15 min
AHL: Covalent bond order and electron delocalization
Study how bond order relates to bond length and energy, and learn the basics of electron delocalization.
β β β β± 14 min
AHL: Advanced hybridization and delocalized pi systems
Extend hybridization concepts to conjugated molecules and explain delocalized pi bonding systems.
β β β β β± 18 min
AHL: Solid state and unit cells
Calculate density, mass and ionic radius from unit cell dimensions for different crystalline lattices.
β β β β β± 20 min
2. Common Pitfalls
Wrong move:
Confusing intermolecular forces with intramolecular covalent/ionic bonds
Why:
This leads to incorrect predictions of melting/boiling point trends for molecular vs ionic compounds
Correct move:
Always confirm what force you are comparing: breaking intermolecular forces does not break covalent bonds
Wrong move:
Assuming all compounds with polar bonds are polar molecules
Why:
Symmetrical molecules can cancel out individual bond dipoles, resulting in a non-polar overall molecule
Correct move:
Check molecular geometry after identifying polar bonds to determine net molecular polarity
Wrong move:
Forgetting to convert units for AHL unit cell density calculations
Why:
Edge lengths are usually given in picometers, but density requires units of cm to get correct results in g/cmΒ³
Correct move:
Always convert edge length to cm before calculating unit cell volume for density
3. Quick Reference Cheatsheet
Concept / Formula | Key Summary |
|---|---|
Lattice energy (ionic bond strength) | where = ion charge, = interionic distance |
VSEPR core rule | Electron domain geometry is determined by steric number (number of bonding + lone pair domains) |
Intermolecular force strength order | Ion-dipole > hydrogen bonding > dipole-dipole > London dispersion forces |
Bond order relationship | Higher bond order = shorter bond length = higher bond dissociation energy |
Hybridization by steric number | Steric 2 = , steric 3 = , steric 4 = |
Unit cell density formula | where = formula units per cell, = edge length |
Alloy vs pure metal properties | Alloys are typically harder and stronger than pure metals due to disrupted crystal lattices |
What's Next
We recommend working through this unit in order, starting with the core sub-topic on ionic bonding below. If you are studying IB Chemistry SL, you can skip the final three AHL sub-topics and proceed directly to the next unit after completing the core crystal structures sub-topic. HL students should complete all sub-topics in order before moving on.