# S2: Models of bonding and structure

> IB Chemistry HL · IB Chemistry HL 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u2-overview/
> Weight: n/a

This unit explores how atoms bond to form compounds and how their arrangement creates different structures, explaining nearly all physical properties of chemical substances.

**Prerequisites:** [Unit 1: Atomic structure and periodicity](https://www.owlsprep.com/study/ib-chemistry-hl-u1-overview/)

## Learning objectives

- Distinguish between the four main types of chemical bonding and relate bulk structure to physical properties
- Predict molecular geometry and orbital hybridization using VSEPR theory
- Classify and compare the strength of different intermolecular forces
- Analyze crystal and unit cell structures for ionic, metallic and covalent solids
- Apply advanced HL concepts of bond order, delocalization and solid-state chemistry to explain material properties

## 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](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ionic-bonding-and-structure/) — Learn how ionic bonds form, the properties of ionic compounds and their giant ionic lattice structure.
- [Covalent bonding and Lewis structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-covalent-bonding-and-lewis-structures/) — Understand covalent bond formation, and how to draw Lewis structures for molecules and polyatomic ions.
- [VSEPR theory and basic hybridization](https://www.owlsprep.com/study/ib-chemistry-hl-u2-vsepr-theory-and-basic-hybridization/) — Predict molecular geometry from electron domains and learn core concepts of orbital hybridization.
- [Intermolecular forces](https://www.owlsprep.com/study/ib-chemistry-hl-u2-intermolecular-forces/) — Classify London dispersion, dipole-dipole and hydrogen bonding, and relate their strength to physical properties.
- [Metallic bonding and alloy structure](https://www.owlsprep.com/study/ib-chemistry-hl-u2-metallic-bonding-and-alloy-structure/) — Explore the metallic bonding model, properties of metals and how alloys form from mixed metal structures.
- [Crystal lattice structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-crystal-lattice-structures/) — Compare the four main types of crystalline solid and their characteristic bulk properties.
- [AHL: Covalent bond order and electron delocalization](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-covalent-bond-order-and/) — Study how bond order relates to bond length and energy, and learn the basics of electron delocalization.
- [AHL: Advanced hybridization and delocalized pi systems](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-advanced-hybridization-and-delocalized/) — Extend hybridization concepts to conjugated molecules and explain delocalized pi bonding systems.
- [AHL: Solid state and unit cells](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ahl-solid-state-and-unit/) — Calculate density, mass and ionic radius from unit cell dimensions for different crystalline lattices.

## Common pitfalls

- **Wrong:** Confusing intermolecular forces with intramolecular covalent/ionic bonds
  - Why it fails: This leads to incorrect predictions of melting/boiling point trends for molecular vs ionic compounds
  - Correct: Always confirm what force you are comparing: breaking intermolecular forces does not break covalent bonds
- **Wrong:** Assuming all compounds with polar bonds are polar molecules
  - Why it fails: Symmetrical molecules can cancel out individual bond dipoles, resulting in a non-polar overall molecule
  - Correct: Check molecular geometry after identifying polar bonds to determine net molecular polarity
- **Wrong:** Forgetting to convert units for AHL unit cell density calculations
  - Why it fails: Edge lengths are usually given in picometers, but density requires units of cm to get correct results in g/cm³
  - Correct: Always convert edge length to cm before calculating unit cell volume for density

## Cheatsheet

| Concept / Formula | Key Summary |
| --- | --- |
| Lattice energy (ionic bond strength) | $\text{lattice energy} \propto \frac{q_1 q_2}{r}$ where $q$ = ion charge, $r$ = 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 = $sp$, steric 3 = $sp^2$, steric 4 = $sp^3$ |
| Unit cell density formula | $\rho = \frac{Z \times M}{N_A \times a^3}$ where $Z$ = formula units per cell, $a$ = 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.

- [Ionic bonding and structure (first sub-topic of this unit)](https://www.owlsprep.com/study/ib-chemistry-hl-u2-ionic-bonding-and-structure/)
- [Covalent bonding and Lewis structures](https://www.owlsprep.com/study/ib-chemistry-hl-u2-covalent-bonding-and-lewis-structures/)
- [VSEPR theory and basic hybridization](https://www.owlsprep.com/study/ib-chemistry-hl-u2-vsepr-theory-and-basic-hybridization/)

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From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-hl-u2-overview/